Dan Raymer's Aircraft Design Blog Archive

Here you will find the archives of Dan Raymer's Aircraft Design Blog, where anyone can post questions, comments, and suggestions. Dr. Raymer will try to answer them all (be patient - this isn't my only job!). Visitors are welcome to reply to each other as well. Dr. Raymer will eventually review all postings, and reserves the right to edit, shorten, or delete them.

(Postings below by Dr. Raymer are copyright C 2009, D. Raymer, All Rights Reserved, and may not be published, re-posted or used in any other way without written permission. All information below is posted without any guarantee or warrantee as to its correctness, accuracy or relevance and is used at readers’ own risk. No liability of any sort shall be assumed by the authors, webmaster, or Conceptual Research Corporation for the use of any information herein.)

Aircraft Design Questions & Comments

Geir Simonsen Says: December 4th, 2007 at 2:58 pm
Hi Mr.Raymer, I am wondering if the design of wings is limited to a horisontal line no matter what. Are wings that have a bend or say looks like an; A (with a lesser angle out from the body and lesser angle in general) an absolute no-go no matter the width surface of the body?

Dan Raymer Says: December 5th, 2007 at 11:23 am
I’m not sure if you are asking about “horizontal” as seen from the front, or a straight leading and trailing edge as seen from above - so I’ll try to answer both!

From the front, the angle is of course called “dihedral” and is important to rolling and directional stability. Normally the dihedral is about 0 to 5 degrees or so. Too little doesn’t provide roll stability, but too much makes the plane roll from side to side in gusty winds. This was one of the great insights of the Wright brothers. Extreme amount of dihedral, up or down, would create a lot of problems. Also, the lift of the wing depends upon the projected area - in other words, the area that you see from above the airplane. If there is a lot of dihedral you get less projected area so less lift.

If your question was about straight lines in top view - straight lines are easier to build and easier to put control surfaces into. Also, the wing gets less lift if it is swept so the best sweep for a low speed airplane is zero. For a high speed airplane, sweep reduces drag (why? see my book!), but there is a “best” angle so if the sweep changes for no good reason from root to tip, the drag will probably be higher. There have been some airplanes built with a lot of sweep near the roots and less sweep near the tip. This is mostly to get better low-speed lift and control from the wingtips.

Geir Simonsen Says: December 5th, 2007 at 1:47 pm
Hi, thank you for clarifying. I find both answers very useful. I was thinking of as seen from the front basically. I guess you could say the Bird of Prey model from Boeing would have issues with the roll as you describe. WHile on the subject i have heard that forward swept wings gives greater lift at low speeds as well.

Yes i know i should get your book.

Dan Raymer Says: December 5th, 2007 at 6:36 pm
Forward sweep gives more lift mostly because the wing tips don’t stall as quickly. Unfortunately, when the wing is swept forward there is a problem with the flaps. They have a lot more sweep (draw a picture) and the extra lift you get from flaps depends on the sweep of the flap hingeline - more is bad, zero is best. So, for takeoff and landing you actually get less lift from a forward swept wing (assuming flaps are to be used, which is normally the case).

Yet another “great idea” bites the dust. Dang- this airplane design stuff is tricky!

Geir Simonsen Says: December 6th, 2007 at 3:07 am
Yes indeed it is :). I refuse to throw my revolutionary new brainchild to the fire, so i will leave the flaps out of the design. It is infact easier for a manufacturer and safer as there are lesser parts. However i am reading between the lines that maybe the the two options equalize in performance and so manufacturers go for the conventional comprehension of airplane controlling as to not confuse the pilots. And now i am at a point in creativity that begs the physics of the world to not present another challenge by leaving the flaps out :)

Thank you so much for answering, it's in fact incredibly cool to be chatting with an experienced real designer :)

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Sreenivas Rao Says: December 13th, 2007 at 6:30 am
Is any military or civil aircraft having cocured or cobonded wing. Are they in the production and flying. Can I get more details.

Dan Raymer Says: December 13th, 2007 at 9:59 pm
A true co-cured wing would have all the pieces like skins, spars, and ribs put into a mold while “wet” - ie., before any curing is done. “Wet” can mean literally wet for liquid resin composites, or just un-cured pre-preg. The whole thing then gets cured together, probably in an autoclave, and comes out as truly one piece structurally. No fasteners, no glues.

A co-bonded structure mixes some already-cured parts like stiffeners with uncured parts like skins, and cures and bonds the whole thing together simultaneously. A secondary-bonded structure cures the parts separately then bonds them together. These have no fasteners, but lots of “glue” - we say “bonding agent” because it sounds more high-tech.

You can also cure parts separately then “bond” them together using additional composite material, like patches over the joint, then cure it all together. The BQM-167 target drone does it this way, and while they’ve been shot down with Aim-9’s and Aim-120’s, and have speared into the ground, they’ve never broken along those joints.

I’m not sure if any production aircraft has large parts like wings that are truly and only co-cured. I think the B-2 wing is co-bonded on one side and mechanically fastened on the other (it is really hard to co-cure or co-bond both top and bottom of a wing).

I believe the F-35 wing was supposed to be fully co-cured but they backed off to a co-cure/co-bond mix (smaller parts are co-cured then the whole thing is co-bonded together). The Boeing 787 has gone this way too, as I understand. Or, maybe these are actually co-cured smaller parts then secondary-bonded together. I’m not sure, and to some extent the line between these is fuzzy and a matter of opinion. Companies want to say they are co-curing rather than just “gluing stuff together!”

Sreenivas Rao Says: December 16th, 2007 at 4:46 am
I heard from my friends that Typhoon aircraft made few wings with cocured technology. But are they continuing this concept in there production line, I am not sure. Inspection and quality control of cocured wings is a difficult task. Any further comments and details please from your end.

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Geir Simonsen Says: January 17th, 2008 at 4:09 am
Me again :) I wonder if you know of any open source computer flightcontrol systems available for the homebuilders

Dan Raymer Says: February 21st, 2008 at 6:30 pm
I'm not aware of such a system but it sounds like a nice idea. Does anybody else know about one?

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Frank Shoemaker Says: January 18th, 2008 at 2:58 pm
Some thoughts: I was reading last night a book about non-standard aircraft. In it was mentioned Burt Rutan’s quickie. I was aware of the airplane but had never seen it pictured or read its specs. 480lbs. gross, 18hp, 125mph top speed and a 16 foot wingspan. What I was aware of the airplane was that someone’s opinion of the engine was that its best use was as a boat anchor.

I have wanted a very light airplane for very long time. Back in the 80’s I wanted to buy Mark Brown’s Starlight. (150mph, 42hp, 500lbs gross, 250lbs empty, wood wings, and a composite fuselage). So, last night as I was sitting in bed waiting to fall asleep I was thinking about the problem of building a very small airplane. The DA-150 is an interesting RC model airplane engine. 16.5hp, 8.5 lbs. (It would make a lousy boat anchor.) Doing the math in my head 15 goes into 450 30 times. So the lbs/hp is about 30:1.

Peter Garrison in his technicalities article entitled Coolness Factor (publish in Flying mag. end of ‘95 ish) said that all successful single engine airplanes have a wing loading times the power loading (lbs/hp*lbs/sq.ft. wing area) of between 200 and 250. 240 divided by 30 equals 8. So I would need a wing loading of 8 lbs/sq ft. 480/8=60 60 square ft of wing area would make a nice flying airplane with 16.5 hp and a gross of 480 lbs. Wing span, the more the better. A 20 ft span would get you just short of a 7:1 aspect ratio. Probably too short of a wing span, 24 ft would be better, almost a 10:1 AR ratio.

A conventional layout should get you conventional results. Low wing, prop up front, fixed gear, and it should qualify as an LSA too. Materials: wood construction wouldn’t be that bad. You could get the wood precut using laser cutting technology. I have a friend that has a Go board ( made out of laminated bamboo. The strips were about 1 inch wide and planed 3/16 in. thick. Hmmm, spar cap made out of bamboo. I was thinking about a square box beam that would double as a torque box for the wing. I wouldn’t mind a metal or composite airplane either. It should be possible to build it with an empty weight of less than 200lbs. That would give you a useful load of 280lbs. A 180 lb pilot (me) would be able to load 100lbs and go.

Dan Raymer Says: February 21st, 2008 at 6:30 pm
I'd love to see such a plane. A small manned airplane isn't that far from a really large RC airplane - have you seen the monstrous Hughes H-4 model? I'd go for a ride on it!

The Wing-Loading-Times-Power-Loading parameter has been used for a long time and is a reasonable first-guess for good performance, but is no substitute for actually doing performance calculations. So be careful out there.

nasupa Says: February 21st, 2008 at 11:22 pm
I’m very interested in flying wing aircraft.It’s very beautiful. How can we design it? How do we find the wing area?What is the airfoil for that design? And what is reflex airfoil? What is it for?

Dan Raymer Says: February 22nd, 2008 at 11:13 am
Flying wings are great. I designed and flew RC flying wing models for years, then suggested the flying wing configuration for a stealth bomber project at Rockwell in 1977. Everyone thought I was nuts, and nobody else was talking about flying wings back then (you can see my design here).

While we always start out with the usual wing sizing method (find W/S and Wo, solve for S), often we find we have to make the wing bigger to get everything to fit in. For bombers this is not such a problem since bombs are small and heavy and don’t need toilets and headroom. For transports, it is worse. This problem led to the Blended Wing Body concept which has a bulged center section.

The reflexed airfoils sometimes used on flying wings are to give it pitch stability. The other approach is to sweep the wing a lot and twist the tip leading-edge down (that’s how I stabilized my RC models). Either approach is an aerodynamic penalty, so for modern “real” flying wing designs we use an optimal airfoil and twist, and let the computer stabilize the resulting design.

There is a lot more on flying wings in my textbook.

nasupa Says: March 11th, 2008 at 9:27 pm
Is it enough to use 2D CFD program to simulate flowing across an aircraft drawing model?. I mean, I have a CFD program and it can simulate only in 2 dimensions flowing.So I will slice my model in to many pieces and test for each cross section.Is this work?

Dan Raymer Says: March 24th, 2008 at 10:52 am
This will ignore the air flowing around the wing tips, which causes drag due to lift and also loses about 10% of your lift.

Sunilraj Says: March 15th, 2008 at 10:14 pm
I am a lover of aircrafts and planning for design an ultralight carrying 2 persons,actually i am a technical person in TV broadcast,give me some guidance where to start??

Dan Raymer Says: March 24th, 2008 at 10:52 am
My books, my website, my shortcourse (come to LA in June).

Eliyar Says: March 23rd, 2008 at 1:41 pm
When I was in High school I joined the team of aircraft make, and when the game end I got 2nd all of those students! Since that day started I interested the Air planes, study aircraft, and made some airplanes. In these days I drew some of the aircrafts.but I really don’t know who’d have to show!?Is that possible to send to you see!?

Dan Raymer Says: March 24th, 2008 at 10:52 am
Hello and congratulations to you. It is nice to hear from a young person so interested in airplanes. However, I cannot review your designs both for legal reasons, and because if I start doing that I will be flooded with such designs from enthusiastic would-be airplane designers. Sorry, and best of luck.

Technology, Inventions, and Ideas

(those interested in this category may wish to see Dr. Raymer's Advice to Inventors)

Thomas Bambrick Says: March 27th, 2008 at 6:43 am
One lad was asking if you would look at designs earlier but do you know of any company that would perform that kind of work if someone wanted a design double checked. And advice on becoming an aircraft design engineer i would be quite interested in that too, its always been the dream.

Dan Raymer Says: March 31st, 2008 at 8:42 am
I am often asked if I can review a design or help to sell a design concept to a big company or government agency. My company (Conceptual Research Corporation) does aircraft design, analysis, and optimization work for hire. Usually, contracts are to develop a vehicle concept from the customer’s design requirements and top-level concept ideas. However, CRC can do a design review on a concept already prepared by the customer, and can advise on design issues and improvements as well as assist in the presentation of the design to potential customers. For more information click here.

These design studies and reviews are paid contracts. I get many offers to obtain stock or a share of a project in exchange for working for free, and I must always turn that down. I’m in business with bills to pay, mouths to feed, etc… This is my only job. Also, kindly do not send me your airplane drawings without appropriate proprietary agreements in place. I can’t even look at them without exposing myself to various legal liabilities.

A final thought – nobody at another company or a government agency will be interested in an aircraft design that is done by outsiders. Designs are done in response to specific requirements that have been studied for years. We don’t just draw a cool airplane and go try to sell it. Even though I wrote the aircraft design book that is sitting on their desks, if I went to Boeing or Lockheed or Airbus and said “here is my latest design concept - why don’t you buy the rights from me and build it?” they would just laugh. It doesn’t work that way.

Thomas Bambrick Says: March 31st, 2008 at 1:23 pm
As for your final thought, its a pity thats the way things are. If I recall my history Kelly Johnson designed the F-104 and just presented to the air force and they realised it met their needs. You can understand it to degree but you never know when they could be passing up on something brillant either.

Dan Raymer Says: March 31st, 2008 at 1:40 pm
Kelly Johnson ran the Lockheed Skunkworks, and the F-104 design was created to fulfill a well-known need of the Air Force for a fast interceptor to shoot down Soviet nuclear bombers before they could get within range of our cities. In fact, the Air Force paid for its development.

Don’t confuse new aircraft designs with new technologies. If someone comes up with a whole new technology that offers significant improvements in performance or cost, and can really show that it works, then the aviation community will beat a proverbial path to his door.

John Charnley Says: September 9th, 2008 at 3:55 am
Imagine if you will, a fuselage that is completely hollow and open at both ends, wings, tail etc being ‘conventional’. At low airspeeds would there be an advantage or disadvantage to the fuselage being an open tube ? Im thinking perhaps reduced drag. What Im kicking around is an idea for a lightweight, long range (relatively) electricially driven unmanned aircraft.

Dan Raymer Says: January 13th, 2009 at 12:44 pm
Sorry, but weight and internal drag would kill this idea. Perhaps you can think of a way to accelerate the air inside, making the whole thing into a ducted fan or jet. But, you’d still need some place to put the people and cargo.

On a related note, I recently did something similar for – a pumpkin race. You take a pumpkin and put wheels on it, and race it down a hill. Everyone has a similar pumpkin and uses the same type of wheels, so the only possible differences would be luck, and aerodynamics. Decorations are encouraged, so people cut eyes, noses, etc… I cut huge holes in the front and back allowing most of the air to pass through the pumpkin rather than go around it. I won.

Arash Says: September 22nd, 2008 at 1:59 am
I am interested in terrain following flight capability in an aircraft, I want to ask you what special considerations should be taking account in preliminary design for estimating T/W & W/S; Is it relating to flight path or not?

Dan Raymer Says: January 13th, 2009 at 12:44 pm
Yes, there is a strong connection between W/S and terrain following capability. T/W is less important, as long as you have enough to fly at the desired speed. What is important is “ride quality” which is a measure of the vertical gust loads the plane and crew have to endure. This was a big factor in the design of the B-1. Basically you need to use a 6-DOF model to determine the loads based on a gust model and the aircraft characterisitcs including flexibility. Not an easy task!

Raymer's Design Books

(see "Aircraft Design: A Conceptual Approach" and "Dan Raymer's Simplified Aircraft Design for Homebuilders".

Todd Ashcraft Says: December 28th, 2007 at 11:00 am
Just wanted to say Congratulations on a well written book, “Simplified Aircraft Design….” My personal favorite: footnote on page 113, “…stupid drawing of a marble in a bowl.” I don’t think I’ve cracked up reading an aero book before. Oh, and it was a helpful informative text as well.

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Anonymous Says: December 7th, 2007 at 6:57 pm
Hello Mr Raymer, I am an aerospace engineering student and I have just read your ‘Simplified…’ book and am wondering whether it is worth buying the ‘Conceptual…’ book as well. I am currently involved in a project designing a 2-seater composite training aircraft. I am only asking because there are lots of design books out there that are geared towards larger aircraft, and most that assume you are using metal.On that thought, will you be writing a new book on composite aircraft?

Dan Raymer Says: December 11th, 2007 at 1:01 pm
Is it worth buying my book Aircraft Design: A Conceptual Approach? OF COURSE! Buy an extra copy in case you can’t find the first one. Buy it for your mom, and your friends, and your mom’s friends, and your friends’ moms, and…….

Seriously, though…. my “big book” is used for senior and graduate-level aircraft design classes all around the world and is widely used in industry and government. It covers metal as well as composite aircraft, and covers big to little, slow to fast, and normal to weird. My philosophy is that the differences in the design process are far less important than the similarities.

As to writing a book on composite aircraft - if you mean a book on the conceptual design of just composite aircraft, then no. I think my “big book” covers that already. If you mean a book on the structural design of composite aircraft, or just structural design in general, I’ve toyed with the idea of a “Simplified Structural Design for Homebuilders” as a companion to “Simplified Aircraft Design for Homebuilders” but haven’t decided to do it yet. There are a few other projects in the queue. We’ll see.

Shane Akpeti Says: January 27th, 2008 at 6:31 pm
Hello Dr. Raymer; I think your book is really great ! I have two question for you:

(1) In your 4th edition in Chapter 3 “Sizing” for the examples with the ASW alot of the calculated figures for the determination of wo etc for the trade studies for range and payload etc do not seem to match exactly the figures when i calculate them. Is this just an issue of significant figures or decimal places ?

(2) During Initial Sizing and you are iterating toward a solution where wo guess = wo calculated , you are looking for the results that are the closes in value to each other ?

P.K.R (kamal) Says: February 5th, 2008 at 11:24 am
I’m not Dr Raymer, but since I’ve just passed through the same chapter and questions, I might be able to help you.

(1) I also calculated the results to cross-check with the example. I’m guessing that, since it is only a sketch calculation, it will not exactly match the number in the book - it is just an estimate. I guess it doesn’t really make a difference, since Wo will depend on many things as the ACFT is designed, and it’ll change as the real thing is built. That’s why the author says that it is important to keep those number in mind throught the desing process, otherwise you can end up with an aircraft much heavier than it was initially estimated.

(2) Yes, you are looking toward values that match. But for that you gotta have a reasonable estimate. For instance, if the final value after the iteration process is around 113000 lb but you start the guess with 50000 lb, it is most likely that you will end up “running away” from the real value, to other impossible values like positive or negative infinity. It happened to me on the 3.4 question, but then I tried to use higher values for the Wo (it was a trade study for a pure turbojet ASW) and ended up converging towards a reasonable value.

I don’t know if I answered correctly, but I tried

P.K.R (kamal) Says: February 5th, 2008 at 11:30 am
Just one more thing: by “running away” from the real value, I actually meant that if the Wo guess is too far down from the converged result (as it was with me) your iterations will not converge to an answer, but rather to very weird values. When you notice they are not varying within certain limits, then you probably will need to try some other guess - in my case, higher guesses, since I was calculating for the same ASW aircraft with less economical engines. If the guess is reasonable, it will vary less and less for each iteration, until it becomes more or less stationary.

Shane Akpeti Says: February 5th, 2008 at 2:44 pm
Thanks !! Kamal ! i really appreciate the response as your explanations makes perfect sense to me. One other question i would like to shoot your way:

In the conceptual design examples for fixed engine sizing on page 693 for single seat aerobatic; Dr. Raymer stated that he varied W4/Wo until wo(cal) = Wo Drawn

I have been racking my brain to understand how i could varry w4/wo

Thanks !!! its not easy to get such technical help !

P.K.R (kamal) Says: February 6th, 2008 at 4:33 pm
I’d be happy to try to answer that, Shane, but I’m devoting a lot of time to each chapter, and I’m still on chapter 4!

Let’s hope someone can provide you an explanation about it, as I’ll take a long time yet to get there…

Dan Raymer Says: February 21st, 2008 at 6:30 pm
Replies to the last several postings:

Q - Chapter 3 “Sizing” …calculated figures for the determination of Wo etc for the trade studies for range and payload etc do not seem to match exactly the figures when I calculate them.

DPR – There are a few numerical typos, where numbers from an earlier edition of the book were left in a few places. These make good exercises for students – find all the places where Raymer is stupid!

Q - During Initial Sizing … iterating toward a solution where Wo guess = Wo calculated, you are looking for the results that are the closest in value to each other ?

DPR – Just iterate until they are off by a small amount, say .01% if a computer is doing the work, ~1% if a human is doing it.

Q - In the conceptual design example… page 693 … varied W4/Wo until Wo(cal) = Wo Drawn

DPR – W4/Wo was calculated on page 591 as .906, and from that, Wf/Wo was calculated and then sizing was performed on page 692. This gave a weight that was a little too large (ie., over the 1200 lbs we’d hoped to have to get our performance). I just guessed slightly lower values of W4/Wo until I found that if it equals .925, you get 1200 lbs when you do the sizing. Then on page 693 I work backwards to find out the range that you would get. This is just one way to do this – you can also modify a little sizing program using these equations so that it reduces range by small amounts until Wo-guess equals Wo-calculated.

Shane Akpeti Says: March 1st, 2008 at 5:54 am
Page 589 Fig 19.4 (Sizing Matrix Plot) When optimizing T/W & W/S for the 3 performance criteria , the solution is the point where 2 contraint lines cross , but why did u choose that spot shown in the book, there is another spot where two lines cross which looks like it would also be a lower weight ?

Dan Raymer Says: March 24th, 2008 at 10:52 am
Look again – that other spot violates the S-takeoff line.

ismail sapmaz Says: February 27th, 2008 at 1:26 am
Thank you for your book. It is great. I dont know any specialist about aircraft design. But your book show my way to me. I want to ask how I can find alfa zero lift values of airfoils. Is there any databank for it. Thank you.

Dan Raymer Says: February 27th, 2008 at 7:18 am
Alpha-zero lift can be seen on the plot of airfoil Cl versus Alpha, as the location where the curve crosses the X axis. For a cambered airfoil that will be slightly on the negative side.

Airfoil data is available in many places - probably the best online resource is at UIUC (here).

ismail sapmaz Says: March 4th, 2008 at 10:57 am
I calculated aileron span according to your book ( simplified aircraft design for homebuilders). I wanted wing span to be 32 feet. And I calculated aileron 12.8 feet. Do I must divide by 2 for sharing two wings? Son one wing will has 6.4 feet aileron? Help me

Dan Raymer Says: March 24th, 2008 at 10:52 am
Yes, that is the sum of the aileron spans, ratioed to the total wing span. This assumes that the ailerons go almost to the wing tips like on normal designs.

Charles Simpson Says: April 16th, 2008 at 1:43 pm
Great book…. theory is great, but real world experience can’t be topped. I’ve working my way thru your DR-1 example, and I’m having trouble following where you got the numbers in the equation for dalpha OL on page 706.

dalpha OL = -(.9)/(.95)(Pi)*(5.3)(1)(1)*Kf*de = -.8*de

I follow everything up to there and after it. but as the old cartoon goes “then a miracle happens” the .9 is the tip loss factor and Kf should be .008 rad-1 / .12 deg-1. But where did the .95, Pi, 5.3 1, & 1 come from?

Dan Raymer Says: June 19th, 2008 at 7:51 am
dalpha Zero Lift comes from equation 16.17 (4th edition), using figure 16.6 (and note a typo in the 1st printing where t/c is mislabeled f/c in the figure).

John Kuo Says: May 9th, 2008 at 1:23 pm
I was originally taught the equal transition time for airfoil’s lift generation and then read it was wrong and then learned that lift is generated more to the pressure differences between the upper and lower surface of a wing, and because of that it explains how any surface can be made to fly, eg, a block of wood, piece of paper, an upside-down wing. Sluggishly, but will still fly.

In that case if we put a large enough scale under a wing in flight inside a wind tunnel, would the scale read the same weight as the wing, less, or zero? I’m not sure if a wing floats by pushing air downwards or getting “sucked” up, or a mixture of both, and how does that explain the high AoA stall because even though the airflow on the top surface has separated there should still be enough positive pressure below the wing to “push” it up, right? But in reality a normal wing will loose all lift after an AoA of 15~20 degrees, I just can’t see why.

A second question is in regard to pod position on a commercial jetliners. The question asks why are the pods usually below the wing on pylons instead of over the wing because supposedly that should increase airflow over the top and will increase lift. I searched your Conceptual Approach text and found 2 references to it and both says it’s mainly because of maintenance with the second (If I remember correctly) also mentioning the noise, are those the only reasons why pods are usually positioned below the wing? Then how is the A-40 Albatros explained? Won’t that design disrupt the airflow to the engines and cause it to stall at high AoA? Why aren’t the engines raised like the Honda Jet or the Rutan Proteus?

Dan Raymer Says: June 19th, 2008 at 7:51 am
(Reply to John Kuo question #1) The “equal transition time” explanation of lift is a useful visualization tool and is approximately, although not exactly correct. You can show in a wind tunnel that the air going over the top of the cambered airfoil does not exactly meet up with the air that went underneath, and instead slightly lags. Still, it is close and is one way of understanding how the pressure differential is created in situations like cars where you don’t have an obvious downwash behind the vehicle yet lift is created.

For wings and such, the airflow coming off the back has a noticeable downward direction due to the angle of attack and/or the shape of the airfoil. The lift created is equal to the downward momentum imparted on this air, but that doesn’t explain how lift is transmitted to the wing itself. For that, you need to look at pressures and yes, the averaged pressures over the top of the wing will be lower than atmospheric, and the averaged pressures underneath the wing will be higher. It is this pressure difference that actually lifts the wing, but the pressure difference is caused by the deflection of the air downward. This accelerates the air over the top of the wing, and per Bernoulli’s famous equation, reduces the pressure. You can see this in the effects of flaps. While they sit at the trailing edge of the wing, when they are deflected they change the pressures over the whole wing including around the leading edge.

BTW, wings don’t lose all lift at AoA of 15-20 degrees. They lose about half of their lift in a fully-developed stall, but for most airplanes the pilot loses control so the flight is over. In the X-31 and other airplanes that can be controlled at extreme angle of attack, there is still a lot of lift.

(Reply to John Kuo question #2) There is a possible theoretical advantage in putting engine pods above the wing. It is as you say – the jet increases airflow velocity over the wing which gives more lift “for free.” This arrangement increases the likelihood of unfavorable interference, messing up the lift distribution on the wing and thus increasing drag due to lift. But, if carefully designed this may be avoidable (or not!). And yes, maintenance access is a nightmare, especially for removing an engine. How do you get a lift under the engine?

Zachary Adams Says: July 14th, 2008 at 1:20 pm
I am analysing a all-moving control canard aircraft and had a couple of questions regarding the analysis of downwash and trim:

1. I’m assuming that since the canard is all moving the downwash angle on the inner part of the wing, and the upwash on the outer part of the wing will change significantly depending on the trimmed CLtotal flown. Is there any way to halfway accurately analyze the affect of the downwash on trim and stability at all of the various CLtotals? (I’m assuming by hand probably not….)

2. Eq 16.32 and 16.33 (in your extremely well written conceptual design book.) seem to omit the effect of airfoil camber on CL. To account for the effect of zero angle of attack lift can the equations be modified by simply adding the zero angle of attach lift Coefficient of the horizontal tail (or canard) to eq 16.32 and the zero angle of attack lift Coefficient of the Wing to eq. 16.33? Or is it better to assume that the angle of attack terms are the effective angles of attack measured from the zero lift actual angle of attack? (which could be increasingly confusing with the addition of wing incidence into the equations.) Further I’m guessing that the zero angle of attack lift (and the zero lift angle of attack)in a three dimensional flow is different than that of a two dimensional flow, so how would this effect be accounted for?

Dan Raymer Says: August 28th, 2008 at 5:32 am
Downwash behind the canard can be estimated using DATCOM charts (see my textbook), as can the wing’s upwash effect on the canard. Upwash outboard of the canard is fairly minor usually.

Don’t forget that in this entire chapter, the angles of attack are with respect to the zero-lift angle, so the camber effect is automatically included in lift. The effect of airfoil camber on CL is nearly constant with angle of attack so it disappears when you take the derivative.

Nasupa Says: July 28th, 2008 at 1:54 am
I have noticed that many homebuilt models have ‘flat plate tail’.Its tail looks like just flat plate structure not any designed airfoil. Suppose I would like to change my tail from NACA to those flat tail,how can I do it?.Does it affect other parameters?. What is advantages of that design?.I think the structure would be more simplier because I use H tail but the structure is quite complex.

Dan Raymer Says: August 28th, 2008 at 5:32 am
Flat plate airfoils are used for one reason only – they are easy to build. They have more drag, and are less effective as tails. I don’t have any numbers as to how much more/less. Wild guess - 20% (anybody have a better number?)

Graham Fyfe Says: August 20th, 2008 at 1:43 am
I am reading Simplified Aircraft Design. My interest is in seaplanes. Do you know a text which would be suitable to read in parallel with your book?

Dan Raymer Says: August 28th, 2008 at 5:32 am
My big textbook has a subchapter on flying boats and seaplanes. I don’t know of a dedicated book on the subject. A lot of early NACA reports covered them – see the NACA report server.

David K Says: October 5th, 2008 at 7:08 pm
I have purchased your text book (2cd printing), “Simplified A/C design for homebuilders” book as well as your RDS student software. I am working on a design study of a vintage WWII era aircraft. As a part of the review I also conducted a cost analysis. The problem I encountered was that when I used the DAPA IV formulas in the text, and I used the same numbers in the RDS software, the results were VERY different. (data deleted)…Aircraft is the Seversky/Republic P-35A.

Dan Raymer Says: January 13th, 2009 at 12:44 pm
You must be confusing my books – as of yet there is only the first edition of “Simplified….Homebuilders.” If you have the second edition of “Aircraft Design: A Conceptual Approach” and got the current version of the RDS software then yes, the answers are different. Inflation was factored in the new edition. Buy the current book, please. But, those cost equations cannot be expected to tell you anything about pre-WWII prop-powered fighters.

Fernando Maffeis Says: November 26th, 2008 at 5:36 am
How can I calculate the d value of Oswald Efficiency Factor of Fórmula 1/(1+d)?

Conceptual Design is a great book, but sometimes it is confused, especially for me that do not have english as a mother language, many technical terms is too complicated and have no translation on my language(Portuguese), so sometimes I’ve get lost on the book having to reread to understand. But this problem is my English.

Dan Raymer Says: January 13th, 2009 at 12:44 pm
First, thank you for the kind words! Sorry about the book being in English, but it is my only language. In Portuguese I can only say “hi”, “thank you” and “I don’t understand.” As to your question – I’m sorry, but I don’t recognize that formula as part of Oswald’s Efficiency Factor. Please give me the equation number (and the edition of the book).

Fernando Maffeis Says: November 26th, 2008 at 10:56 am
Reading your book I localized three forms to calculate e (Oswald Factor). To calculate straight wing, sweept wings (>30º) and biplane wings. But, how can I calculate trapezoidal and less then 30º sweept wings?

Dan Raymer Says: January 13th, 2009 at 12:44 pm
Oswald’s factor is an old and crude approximation. These formulas don’t provide “full coverage” for all possible wing geometries. If you simply must use “e” to analyze a wing with, say, 28 degrees of sweep, try plotting “e” versus sweep for 30 and above, and mark the zero sweep point, and graphically connect them. Probably the zero sweep value is good up to about 15 degrees of sweep. But this would be crude. Instead, use the Leading Edge Suction Method described later in that chapter. It is much better, and not too difficult to use.

Nashi Says: January 6th, 2009 at 2:21 pm
can anyone tell me that low price edition of book aircraft design conceptual approach (4th edition) is available in market or not? im from Pakistan and i wana buy this book if not plz let me know if someone has its soft copy

Dan Raymer Says: January 13th, 2009 at 12:44 pm
There is no paperback. You might find a used copy on (but why would anybody sell such a masterpiece? Joke.)

RDS Aircraft Design Software

(see RDS for information about RDS)

Pat Says: December 24th, 2007 at 10:33 am
I have purchased the student version of the software. I am attempting to load but it does not seem to be working correctly. I have Windows on my computer so I have followed the instructions for the RDS icon and the associated properties. The program does not run when I select the icon. I have selected the install file and I get a completely blue screen with a small white square in the upper right corner. Is this response correct? If not please advise.

Dan Raymer Says: December 27th, 2007 at 4:53 pm
The RDS-Student CD-ROM was produced with the files in a subdirectory, but the Install program expects that they are in the CD root directory. This is my fault - sorry - but is easy to work around. This is described at the RDS Tips page. And, please accept my apologies!

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Geir Simonsen Says: January 1st, 2008 at 4:47 pm
Hi, I got the big book, and its brilliant ! Easy to read and understand.

I also got the Rds-student, and while i wouldnt daresay that it is bad i sorely miss the direct input to graphics that have become the standard these days. Are there any plans for a “standard” windows app in the works?

Dan Raymer Says: January 1st, 2008 at 5:32 pm
RDS is over 30,000 lines of code. There is really no such thing as “conversion” to Windows - you have to basically start over with a Windows GUI then copy in the “action” code one piece at a time. I started working this over five years ago and got pretty far down the road, but I’m afraid I outsmarted myself. I had the brilliant idea of doing all the user input and output using Internet Explorer in local mode (no internet connectivity required), so that input sreens would look and act like a web page. It was working pretty well, when Micro$oft released a new version of Internet Explorer and all the stuff that was working, stopped working. At first I tried to fix it, then realized that I and the RDS users would be at the mercy of IE - existing versions of RDS would stop working whenever IE was changed. So, all that work went into the trash. Bad day for Dan.

I set it aside and decided to focus on an all-new Design Layout Module. It is now done, and I’m working on some other RDS features that have been requested by users, plus additional cleanup, etc… When that is all done, I’m going to grit my teeth and start - again - on porting RDS to a native Windows environment.

As you know, the current RDS does work very well in Windows, and has friendly input and output, is mouse-driven, etc… For those who haven’t tried it, it IS NOT an old-style DOS program with typed-in commands and such. True, there is no universal Undo, but it is always easy to reenter the right value if you mess up. In the Design Layout Module, there is an Undo, not of the last keystroke, but of the last design change. In fact, you can step back 9 times to undo design changes. Very nice! In the next RDS-DOS release, there will be a similar undo of any changes made in analysis module input arrays.

When will RDS-Win be released? Sorry, but no promises. It will not be just a few months, though. It may be a year or more. I hope it is before the end of the decade!

BTW, I get offers to help with the Windows conversion. Generally, it is somebody looking for paid work, and I do not subcontract programming of RDS. I tried it three times, and I won’t try it again. If any Windows experts out there would like to offer advice for the fun of it I’ll be grateful and will give credit appropriately.

Thomas Bambrick Says: March 27th, 2008 at 2:28 am
I really enjoy your books, have both Conceptual approach and design for hom builders. Two very interesting books.

I bought RDS student about a year ago in hope of using it for my final year project unfortunately my computer decided to slowly give way after 4 years faithful uni service. It just wasn’t able to handle all the processing required. I recently got around to upgrading but I seem to be having trouble installing on my Vista machine. Do you know if this is a standard problem and if there is a solution.

Dan Raymer Says: March 27th, 2008 at 4:37 am
Thanks for the kind words. Concerning running RDS on a Vista PC, yes, this is an issue. The problem and workaround can be seen here.

Itai Kaufman and Alon Tsin Says: December 28th, 2008 at 2:53 am
We are now studying your RDS-STUDENT software, and we have encountered a problem: After we complete the Design Layout Module, Aerodynamic Module ,Weight Estimation Module and Propulsion Module we are now unable to use the Aircraft Data File Module to process our results. In each of the modules we have saved the data in a file, and into the Aircraft Data File (which we created previously). Despite that, the Aircraft Data File doesn’t show any values (all the values are set to zero), although when we save it it doesn’t warn you that there are missing modules.

Dan Raymer Says: January 13th, 2009 at 12:44 pm
This is probably a bad installation or an old bug – not sure which, but download the latest version here.

Other Aircraft Design Topics

Cameron Says: December 11th, 2007 at 3:59 pm
I am a private pilot who wants to design and build his own airplane. I do have a mechanical engineering background, but I’ll be the first to admit that I have a lot to learn before I really get started.

I do have a concept in mind (its nothing unusual) and I was thinking of starting with a 1/4 scale RC model before I got into the real thing. Will the data that I get from the model be of any use when designing the real plane?

Dan Raymer Says: December 12th, 2007 at 6:24 pm
Scale RC models will teach you a lot, but don’t trust them completely. There are “scaling laws” that tell you how to change the weight of a subscale model so that it flies just like its full-size brother, and when you work out the numbers it turns out that the smaller scale version has to be proportionally very heavy - probably too heavy to fly at model speeds. Also, there are “Reynold’s Number” effects that cause the air to flow differently over a small wing than over a big wing. That’s why bumblebees fly just fine but if you scaled one up to the size of a 747, it wouldn’t work so good (but it would look really cool and scary!). So, subscale models are good confidence builders but you still need to do some serious calculations at full scale before strapping yourself into the real thing.

Cameron Says: December 13th, 2007 at 6:39 am
I bet wasn’t 15 minutes after I wrote my initial question before I realized that a 1/4 scale model would have to weigh 500 lbs, and have a 40hp engine. My initial thought was that a scale model would be a good way to experiment with the aerodynamics stuff, since it would be alot easier and cheaper to change, but it didn’t take me long to figure out that it wouldn’t be practical in that respect. Your reply just reinforces that.

Sahaj Panchal Says: March 25th, 2008 at 8:33 am
I am a 3rd Yr UnderGrad student, studying Aeronautical Eng. in India. I wanna do my masters from the US. Now I wanna be an Aircraft Design Engineer. So anyone there, please please tell me what courses I should to be the one. Also tell me in which University to apply for in the US?

Dan Raymer Says: March 27th, 2008 at 5:30 am
Great! I’ve been meaning to write a short piece on my advice to people like you. I’ll do it - but it may take a while. When it is ready I’ll post it....

Update - I did! See it here - Raymer's Rules for Would-Be Aircraft Designers.

Sreenivas Rao Says: March 26th, 2008 at 8:20 am
How to prevent rain water entry into military aircrafts even after taking proper care while designing the aircraft. Are there any guidelines or reference or industry practices to follow.

Dan Raymer Says: March 27th, 2008 at 5:30 am
This would be a detail design, production, and maintenance issue, involving things like rubber seals and water diverters. I’m the wrong guy to ask! Website Praise & Haze

Jaspreet Singh Says: February 23, 2008 at 06:37
Praise for Aircraft Design Website: I am a recent student of Aircraft Design from Bangalore, India. I am all praise for your unique website! Your best-selling work through AIAA is not only motivating but enticing. And the "Raymer's Rules" are admittedly spot-on.

Resumes & Job Postings

(Those looking for jobs, and those looking for people to fill jobs, are welcome to post on the blog. Kindly keep it to the field of aerospace, please. There is no charge, but there are no promises either!)

Unrelated But Interesting Topics

(so far everything is either aerospace-related or uninteresting!)

Go ahead - post something on Dan Raymer's Aircraft Design Blog! Updated 1-09

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