Designing an airplane

Joshua,

Sounds like a great project. I suggest starting with the book: Simplified Aircraft Design for Homebuilders
http://www.amazon.com/Simplified-Aircraft-Design-Homebuilders-Daniel/dp/0972239707/
It will help you through the mathematics of wing shape, lift, and drag.

There are some real tradeoffs in designing any airplane. The most efficient wings are long, horizontal, and skinny, like you see on high-performance gliders. Unfortunately, people fit best in tall, vertical spaces. An airplane's tail makes it more stable at the expense of drag. The flying wing concept attempts to blend the wings into the fuselage and eliminate the tail. This removes a lot of drag from the design, but it comes at the expense of lower stability and wing efficiency.

The Dyke Delta is a plans-built experimental flying wing. An interesting design, but only 50 have been built in the last 50 years!
http://en.wikipedia.org/wiki/Dyke_Delta
http://www.youtube.com/watch?v=5-qSl0yQ4pY

Be sure to keep posting as you work through your design. All of us would love to hear about your progress!

Hey, thanks for the reply. I looked at the Dyke Delta, and it looks pretty cool, even though it is not a textbook example of a flying wing since it has a tail. Anyway, that looks like a good choice if I don't get through designing this.

So, I went on Paint last night and drew up a sketch of what the plane will look like. There's a program my school uses called Geometer's Sketchpad that I really want and think would be good to use since you can accurately make arcs and any geometrical figure on it, but you have to buy it. I don't know how much it costs. I can find out later, but for now I'm doing paint.

And I looked at the book you recommended. It looks like just what I need, but I'm doubtful about spending $30 on something I might already have. Have you read the book? If so, can you take a look at the article from 1954 that I was using and tell me if it is seriously outdated? Thanks!

Hi, Joshua.

      Your crescent moon shaped wing is very interesting. I built a Mitchell Wing B-10 and learned that the placement and size of the control surfaces must be carefully calculated. I would suggest changing the shape of the crescent to allow mounting the elevators on the points as far to  the rear as possible- maximizes the moment arm so the elevators can be smaller and hopefully more docile. Another point I would emphasize is the relationship of the engine thrust line to the center of gravity. A high thrust line, especially with a pusher engine will cause the nose to rise or fall with each throttle change requiring the pilot to counter with elevator. Your low wing with a high thrust line will be very susceptible to this. A tractor engine would be easier to "get right" and would also make landing gear placement and size much less problematic. Good luck with your project. Many different configurations have successfully flown, but it takes a lot of calculation and testing to make a design safe and pleasurable to fly.

Ok. The only problem I see with a tractor configuration is that the pilot would have limited visibility being in the middle of the wing. On the new drawing I have made several modifications, including making the wing curve less sharp so the control surfaces will be straighter, and slightly curving the control surfaces to maintain the surface area where the wing curve would cut in. I have also made the engine a dashed line instead of a solid line, to show that it would have to be incorporated into the front of the wing for less drag. The famous Northrop flying wing was a pusher configuration, and I know that some of them took onboard computers to control them, but was it only the pusher configuration that made the Northrop wings unstable?

I have come into a problem with the crescent shape. The wings will come to a point, unlike normal wings, so the ailerons can't go to the wing edge. Will ailerons a little bit away from the wing edge work as well?

I'm having computer trouble right now, so I can't edit my first post, but here's the link to the article:

How to Design and Build Your Own Airplane 

Try and find a copy of the AIAA book "Tailless Aircraft in Theory and Practice" by Nickel.  It is 512 pages long, and includes over 100 pages on designing swept wing tailless aircraft. 

One of the best introductions to aircraft design in general is "The Design of the Aeroplane" by Stinton.  642 pages cover every aspect of aircraft design in simple terms.

Joshua:  Keep going!  You're doing great!

Why do you want the aileron to go all the way to the wing tip?  It is actually more effective if it doesn't go all the way out.  The extra differential pressure will just dump off the wing tip, adding to the wing tip vortex ... a bad thing.

We do a lot of (8-1/2") styrofoam plate gliders using a penny for ballast, and (although we have patterns that we know work) we allow participants (kids and adults) to cut any shape they want.  Your crescent moon shape actually works really well.  Try out your configurations using this method (or any cheap and easy way you like), and see how it works.  If you can get a styrofoam glider to fly ...

Getting a little technical, here's why the "crescent" moon works well in foam plate gliders.  Wing sweep (straight or curved) puts wing area further aft, which adds a little pitch stability.  The sweep also adds a little directional (yaw) stability.  We also keep part of the lip of the plate to act as winglets, which add aerodynamic lateral (roll) stability.

The main goal with all of this is that you continue to dream, experiment and learn.  If you (or anyone else for that matter) have questions, e-mail me at fly-in-home@att.net.  I'd be glad to help.  -Ron

Do a Google search for TWITT (The Wing Is The Thing). It's an active group of flying wing enthusiasts and they may be able to point you towards some helpful resources.

 The reason I tried to put the ailerons on the tips in the second design was because I forgot "elevators" and thought "ailerons".

And thanks for the idea about making a styrofoam glider. I was thinking of building an RC model airplane, but I should probably get the design flyable before investing in the servos and everything needed for that.

Thanks for the encouragement!

Why do you need separate surfaces?  Can't the elevator and aileron be the same surface?  It makes the controls a little harder, but you wouldn't be the first one to do this.  For example, on "V"-tail Bonanzas, the aft surfaces (ruddervators) act as both the elevators and the rudders.  Delta wings (typically fighters) also use elevons (combined elevators and ailerons).  You're on the right path.  Plus, with today's fly-by-wire technology, controls can do amazing things.

 A pilot's license DOES NOT qualify you for building an airplane. Don't get me wrong - it's a great start, and I'm sure that pilots that have been flying for years can tell you a lot about aircraft design.

You don't design an aircraft to be cool, but they often are. You design them for function and safety.

If you wish to succeed:

1) Study calculus based physics (and calculus).This may require local community college for some.

2) Get yourself a book on Aircraft Design. Cambridge has a good series of books. "Ajoy Kundu" wrote the book that I read. Get yourself lots of Aircraft related books, electrical engineering books, structural engineering books, etc.

3) Learn to build models that are controlled via radio connection.

There's no such thing as an _article_ that teaches you how to design airplanes!! You need lots of knowledge. There's a damn good reason why universities offer AEROSPACE SCIENCE as a DEGREE.

Please be careful.

Ok, I was serious about this being a hypothetical airplane and that I will not attempt to build something like this for a while, but I was kidding about doing it as soon as I got a job and a pilot's license. I realize that there are inherent risks with an airplane of your own design, and this whole thing is to see if I can design an airplane that flies. A glider and RC model are enough for that.

In response to the question about why not make the elevators and ailerons a single control surface, I do not see how one control surface could effectively manipulate pitch, yaw, and bank, mainly at the same time. For example, if a pilot is flying the aircraft and wants to bank right and pitch up and yaw right for a turn, the aileron/elevator/rudder on the right side is opening for right yaw, moving to bank the wing down, and moving down for upward pitch. At the same time, the left aileron/elevator/rudder is remaining closed for no left yaw, moving to bank the wing up, and moving down for upward pitch. Since the ailerons have to move opposite directions to bank the wing but the elevators have to move the same direction for pitch, the controls become jammed or just don't work until the maneuver is finished being attempted.

Another thing, about TWITT, it looks like it could have been good but the site hasn't been updated since 2006 apparently. From the looks of the features, membership and subscription of the newsletter cost too much to be worth it, mainly because I can't even tell if the group exists anymore or if they just left their site up. The flash "updated" signs are on 2005-2006 updates... For now I'll stick to EAA, but thanks anyway.

Yesterday I was thinking about the plane and wondered what it would be like if I switched everything around. This means ailerons/rudders and elevators on the broader side of the curve, and the engine in the original spot of the pusher configuration but in a tractor configuration. Does anyone know if putting everything like this (reversed) might have any good or bad effects on possible flight characteristics?

Hi, Joshua.

    Wings are swept for aerodynamic and structural reasons to improve the performance of the aircraft compared to the same aircraft with straight wings. Early straight winged jet fighters had problems in the high subsonic-supersonic speed range that were solved by sweeping the wing back. Several aircraft have flown with forward swept wings which while aerodynamically equal to or superior to rear swept wings also require that the wing be made heavier/stiffer to withstand the twisting loads. In my Mitchell Wing, the wing was swept, tapered and had the outer sections mounted with 6 degrees of dihedral. The designer felt that the aerodynamic and handling advantages of this configuration were worth the extra time and effort required to build such a complicated wing. Other flying wings have had different shaped wings- even straight- because the designer used a shape that delivered the characteristics that he wanted from his aircraft. Almost any shape can be made to fly as long as the center of pressure of the wing, the center of gravity of the airframe, and the thrust line of the engine are in balance. Then, control surfaces must be designed to allow the pilot to direct the aircraft. Again, my Mitchell Wing used elevons and tip-rudders.The purest flying wing has to be the trike- controlled only by weight shift and power. I prefer stick-and-rudder aircraft, but that's just personal preference. A successful designer understands the forces he is trying to control and uses a configuration that best performs the desired mission. And it helps if the aircraft looks pretty cool.

No matter what, keep going.  You have started a really good discussion.

Multi-function control surfaces take a little more thinking.  Your example had a slight error in it, but here is how a climbing right turn could work.  YAW - as you stated and if you have no vertical surfaces, you must open the right wing drag device (you said this well by splitting the right wing control surface).  PITCH - Both surfaces have to go UP a little.  ROLL - the right surface must go up a little (to lower that wing) and the left one must go down a bit (to raise that wing).  Remember that control surface movement in flight (unless you're at extremely low speeds) are very small.

So let's use some numbers.  YAW - We split the right control a little, say +/- 5 degrees (probably way too much).  ROLL - The right control goes up 5 degrees, and the left one goes down 3 degrees.  PITCH - Both go up 3 degrees.  Sum the numbers to come up with the right control is UP 8 degrees (5+3) and split, and the left control is neutral (3-3).  This will roll, yaw and pitch the airplane the way you want.  Also remember control input is only used when the airplane is being maneuvered.  (Dang, this is further than I wanted to go).  In other words, when you roll an airplane into a turn, you don't continue to hold the ailerons in throughout the turn (or the airplane would continue to roll around).  Note: you will need a small amount of aileron, though, to offset (fight) the natural stability of the airplane to want to fly straight and level).

As for control surfaces on the leading edge of a wing ... they are unstable (in this case a very bad thing).  As a passenger in a car, hold a small piece of cardboard (like a large McDonalds drink cup) out the window (a cool, cheap wind tunnel).  While holding the front of the cup (on the upwind side), you should be able to hold it and rotate it around a little.  Now, hold it by the back side (down wind), and try to do the same thing.  It is either much harder, or it was ripped out of your hands.  Go back and pick it up ;ob........

Thanks for listening, Ron

Ok, that explains it, thanks. I was confused at first about how a neutral input on the left wing could affect it, but your explanation makes sense. As for the control surfaces on the leading edge, by reversing everything, I meant everything, including the direction the plane moves. Think of a pod racer from Star Wars. Not the best comparison, but the pod racers have two engine things out in front of them. On the airplane, instead of an n shape, with the top of the letter here representing the leading edge, think of it as a u shape, so the pilot sits behind the curving projections on the outer edges. That way, the control surfaces are not on the leading edge of the plane.

Cool idea.  Forward sweep has structural problems ... but those can be overcome.  Forward sweep also blocks the pilot's view ... harder to sell.  All good ideas nonetheless.  I think that you will have a lot of fun with styrofoam plates (and/ or temporary signs ... like for elections, garage sales, ...).  I know that I do.  You will also have fun playing with the CG for various planforms (sweeps, curves, tapers, ...).  The sky is the limit!  -Ron

A characteristic called yaw/roll coupling has allowed several flying wing ultralights to be controlled with only tip rudders. The Pterodactyl, Kasperwing and Easy Riser are very maneuverable without ailerons of any kind. Your split ailerons should act similarly. For example, deploying the right tip rudder creates drag slowing the right wing and reducing its lift. It moves to the rear and down compared to the left wing which because of its faster relative speed has more lift (rises). The left tip rudder/ split aileron is not deployed at all, so has less drag (continues forward at the same speed). This results in an automatic controlled turn to the right. My Mitchell Wing had elevons. I seldom used them in their "aileron" function, because it was so easy to just push on the rudder pedal, deploy the tip rudder and make a nice turn. Doing this also eliminated a bad characteristic of ailerons- adverse yaw. What happens is the down aileron creates more drag than the opposite up aileron. Even though the wing is banking to the right, the drag of the down aileron pulls the nose to the left. Then the tip rudder had to be deployed to make a coordinated turn. So, to make a climbing turn to the right, you would simply deploy the right split aileron and add power. To return to level flight, deploy the left split aileron and reduce power to your cruise setting. To make a steep descent, reduce power and deploy both split ailerons an equal amount. This eliminates the need for separate spoiler controls. Your elevators would be used to control pitch attitude during take-off and landing with engine power controlling the rate of climb or descent. One problem with split ailerons is the difficulty in mass balancing them to eliminate "flutter". Several Mitchell Wings (with their pilots) were lost as bigger engines were mounted pushing speeds to over 80 mph. The elevons could develop vibrations- flutter- strong enough to rip them off. Adding weight ahead of the balance point solved the problem. Hope all this makes sense.