Im looking for a few formulas to help me design my own RC airplanes such as wing to fuselage ratio and wing to tail area ratio. I understand it depends on the type airplane (slow or fast flyer, glider etc) but surely theres a master formula to work from to get a guy started right? Thanks for the help!


"Why don't they make the whole plane out of that black box stuff"

There's a lot of info from past discussions of this stuff over in the "Scratch Building" forum.

Scratcher, I am a professional aero engineer. There is no master formula, I do wish there were. We never started from scratch with a new design. If I were going to scratch build a nice pattern ship I would get a photo, or better, a drawing of Chip Hyde's Hydeaway airplane and copy it . You can cange the looks a little by canopy shapes, different nose cowls but it would be guaranteed to fly. It's the way most designers work if they would admit it. It is a form of evolution by cross breeding. If I were after a trainer I would copy a well known and regarded trainer. Same with gliders, racers, etc.

If you want formulas of your own. Measure the Hydeaway or the trainer and develope your own formulas. You will have horizontal and vertical tail areas as a percent of wing area, fuselage side area distribution, and the like. Wing and horizontal tail positions can be determined by careful observation of what works. There is nothing like competition success to prove the worth of aero formulas. It is much better than a years work with computers and formulas can produce. Given a good flying design, the knowledge of the aero formulas, etc. will help optimize your work but copying is the single best method of assuring success.

Now, there is a lot of fun in knowing why things work, certainly that is the reason I went into aero eng to start with. The last one even points out the interaction of experiment and test and design.

http://www.desktopaero.com/adw/welcome.html

http://142.26.194.131/aerodynamics1/

http://www.av8n.com/how/htm/intro.html

Congrats on the exciting career Ben, Im jealous. Thanks for the links, but I have another question. I noticed on the majority of all airplanes the elevators are above the wings in alignment. Why is this? Whats the advantage of having the elevator above the wings instead of level with the wings? Thanks

Also, Ive noticed on most RC airplanes the tail has no airfoil, just a flat surface - Even on some larger scale models, why is this? How big can you go and still have a flat tail? thanks!

Scratcher - A flat plate is easy to build and it will work for any scale. It's not as efficient, but it works. But, an airfoiled tail is always better and usually lighter. I just built and fully-sheeted a tail for a .40 size model and it weighs 2.1 oz. A flat plat of the same size would have weighed at least an ounce more, been more flexible and less efficient.

I think the reason the tail is above the wing is because of down-wash coming from the wing. The classic Ugly Stik has the stabilizer on the bottom of the fuselage and it works fine.

However, if you're building a precision aerobat, the relationship is important. You start your design using the best info you have and then build the second prototype using what you learn from the first.

There are no magic formulas. All designs are a compromise to get it to do what you want it to do as closely as possible, but the intent of the aircraft is the most important thing. Too many people DO NOT clearly define what they want before they start building. They say something generic and meaningless, like, "I want a plane that is aerobatic and flies well." OK, tell me one plane that doesn't fit that description. Actually - don't. I know there are a lot of dogs out there, but any decent design will fit that description.

My point being, before you ever start you need clearly defined design goals.

- Paul

Big enough?

Scratcher, I like Ben’s answer. I am also a career aero engineer (retired) and with the possible exception of Burt Rutan few full scale designers start from scratch. Aircraft design is highly empirical and is a process of evolution. A good bit of my experience is in engineering flight test, and I can confirm that surprises frequently arise there, even with the best of engineering design. I will try to address your last two questions.

The main advantage of locating the horizontal stabilizer a little above the wing, in an aircraft of conventional configuration is to remove it somewhat from the influence of the downwash from the wing. This generally provides a more linear stability curve than is the case where it is in line with the wing. The “T� tail has several other effects. By acting as a tip plate it increases the effect of the fin/rudder allowing a smaller area to be used. Some of this benefit is negated by the weight of additional structure required to support the stabilizer loads. For model aircraft, these effects are not particularly significant. In fact since many models are designed for aerobatics, the preferred position is in line with the wing which itself usually has a symmetrical airfoil. The reason for this is so that the aircraft will perform upside down the same as right side up.

All of the “rag wing� pipers (cubs, tripacers, colts, etc.) as well as many other of the fabric covered aircraft of that era, all have tails with flat surface airfoils. As far as R/C models there is no size that requires an airfoil shape. It’s really a matter of drag. An airfoil shape has a little less drag than a flat surface moving edgeways through the air. However I doubt if you could ever tell the difference in a model since they are so overpowered compared to full scale machines. In addition to the slightly lower drag, there could be a little difference in the control effectiveness at large deflection angles, but for model design it shouldn’t be significant. A flat surface is just simpler to build.

Thanks Ben, Lou, Paul, and Cafeen. You have all helped tremendously! Im in the middle of working on my design/formula and will post a picture as soon as I finish and hopefully you guys can critique it. My intent is an aerobatic flyer of course. Thanks again!

Many years ago, Ken Williard published a set of general ratios for designing RC aircraft. I think I still have a copy of that data sheet.
Send me an email ([email protected]) and a fax number, or address and I'll get a copy to you. I still refer to this from time to time.

Used to live in Tucson - moved there in 1957 and left in 1973 to go to Flagstaff to NAU.

Hope this helps. By the way, here are some pictures of my latest scratch built - a big ugly stick. Wing is 78" Chord is 22" Airfoil = semi-symmetrical Selig 8036, Fuselage = 68 1/2". I have a Homelite 25cc weedeater engine on it now, but am trying to buy a RCS 44 twin cylinder engine for it. As you can see - still have some work left, but hopefully will fly in 3-4 weeks????

Regards

ramrod25 - beautiful work. I love nice looking construction. The wing is excellent! [sm=thumbup.gif]

I can't tell from the photos, but it looks like you're using four main spars and then two more towards the aft of the wing. Is that correct? If so, what was your reason for the four main spars?

Thanks Ramrod, an e-mail is heading your way. I would love to take a look at Ken Williards general ratios for designing RC aircraft. I did manage to find a very useful website with a ton of airplane plans you can download for free. http://members.fortunecity.co.uk/slmohr/ ------ I had to find a program to open these zipped/CAD files. Its called DWG Viewer 3.0 and you can get it here http://download.com.com/3000-6677-85...ml?tag=lst-0-1 ------ What does "Ugly Stick" mean? I see those words used often on rc airplane sites. Thanks!

Scratcher - The Das Ugly Stik is a classic design by Phil Kraft. Everyone has built one or eleven. Basically a plane that looks pretty much like the one in this thread. Shoulder wing, conventional layout and no cowl. A basic requirement is to have Maltese Crosses on the wings. Just a really good, dependable sport plane.

Do a web search for "Das Ugly Stik." Also, check out google images with the same search.

Thanks mate, this forum is awesome. Ive never had my questions answered so fast or so nicely! You guys are very helpful! I found another link I would like to share that explains in detail several aspects of constructing RC airplanes from scratch. http://www.airfieldmodels.com/index.htm -------- On a different note, I have decided to test my designs using foam instead of balsa wood. My reasoning for this is speed of construction. Are there other factors Im unaware of that makes this a bad decision? Tips on shaping foam would be helpful also, lol. Thanks!

Scratcher - Thanks for the link. Looks like a great site. I'll have to check it out.

Foam is generally a little heavier, but not enough so to cause serious problems. I don't use foam so I can't tell you much about it. Search through RCU though. Cutting foam has been discussed many times.

Looking at your last reply, I noticed the site in your signature! LMAO, joke is on me! Thanks for not rubbing it in my face.

Small electric models made of foam are a great way of checking out a design. There are a lot of designs made by the GWS company that are great fliers. They are all molded foam bodies and molded wings. I have built quite a few of them and they fly nicely. They are also inexpensive. Buy one of the full bodies models, build and fly it. It will give a great handle on what is possible in foam. You can check out wing areas, fuselage shapes, tail sizes, etc. in a smaller scale model and if it works well then scale it up and do a balsa build.

CafeenMan - My wing really has two load carrying spars - the main one where the sheeting ends - at about 35% of the chord, and then one further back - I'll take some close up pictures of the wing and rib tonite and post them tomorrow.

Ok - Sent the file on general design criteria - but I'll post it here for everyone.

Regards - Ramrod25

I’m going to take a contrary view and say that in my opinion, if you are designing a model of conventional proportions, building a foam model is a waste of time, and here are the reasons.

If you stay reasonably close to the areas, moments, and airfoils used in other successful aircraft, your design can’t help but fly successfully. The real challenge to designing from scratch isn’t as much the aerodynamics, as the structure. Ramrod25’s stick is a good example. As long as the weight and balance is within the normal range for that type of aircraft, it will fly just like every other “stick� style model. What sets it apart as an original is the beautifully engineered structure. He has gone to great lengths to save weight with contoured cutouts in the ribs, judicious use of stub ribs, built up tail surfaces, flaps and ailerons, etc. Of course some of this is necessary due to the size which also makes it a little easier to do. Sometimes your goal may be to simplify the construction without adding too much additional weight. In which case you would avoid extensive cutouts, and use different density balsa and lite ply to control the weight. If you were designing a combat model, all that beautiful structure would seem kinda foolish. One of the benefits of building a lot of kits before launching out with original design, is to see different approaches to structure.

Even with the best effort, the proof is still in the testing. The little aircraft in my avitar is an original design of mine. It flew right off the board with no adjustments, and performs even better than I had hoped. However on the first hard landing the wing attach point broke. After it happened a second time, I designed a new support that didn’t add much weight but considerable strength. After the modification it has been a pretty rugged flying machine.

There is really no magic formula. An airplane design is a series of compromises depending on the mission and based on experience, appearance, past designs and a little luck. When your creation that you conceived, designed, and built takes to the air you will be a proud flier indeed.

CafeenMan - First of all - let me apologize - I mis-interpreted your previous question about the spars - so let me start over and try this again. The first image is a picture of the rib I started with. The main spar was built using an upper 3/8 square hardwood stick, and a lower 3/8 hardwood stick, and then they were joined with a 3/32 web on the front and back side, in effect making a box beam structure.
See picture #2. Then, I used two more 3/8 hardwood sticks for alignment spars - I use these mainly to hold the ribs in place and align them properly - and they also add strength without too much weight. OK - now - if you look at picture #3 you will see a thick 1/4" spar and it looks like it is the fourth rib - well - I have to admit that I changed my design late in the game. I orginally was going to use large "barn door" style ailerons on this wing - so - this 1/4" thick piece was going to be my original trailing edge. Instead, I decided I wanted to add flaps, so I went with the design you see now. This is the first "large" wing I ever built, so I was concerned about it's structural integrety. Perhaps I overbuilt it. Every time I build a wing, I learn something, and I think the design process continually evolves. In other words, I don't think I really ever build a wing the same way twice - I like to see how how other people build their wings, and learn from folks like yourself - which is what makes this a fun hobby.

Hope this does a better job of answering your question. I put the wingtips on this weekend and got the bottom of the wing covered. It was absolutely beautiful weather yesterday, could have flown it if it was ready. Will post some more pictures as it gets closer to flying condition.

Regards
Ramrod25

ramrod25 - Again, the construction looks really good. I do think the wing is over-built, but I suppose that would depend a lot on what you plan to do with this plane. Two 3/8" square basswood spars with front and back webs is sure not going to break in the air.

You are correct about designers evolving. My first designs were along the lines of acceptable building practices when I first came into the hobby. Those designs were way over-built. A .40 size model does not need 1/16" plywood doublers from the nose to the rear of the wing saddle. I would use 1/64" doublers on 1/8" sides and no doublers on 3/16" sides.

Anyway, I've found that my designs are getting lighter and stronger due to better techniques as well as being a better craftsman. New guys are always asking about what glue to use. My opinion is that the glue doesn't matter - the fit of the joint is much more important. Although I'm not going to test this theory, I believe Elmer's school glue is strong enough for any balsa to balsa joint and as flexible as it is, it would probably hold up very well.

BTW - that servo looks completely lost inside your wing. What kind of servo are you using and how big is the surface that it's moving? I'm sure it's fine, but I'm just curious.

CafeenMan - The servo is a GWS SOT3 (I think that's right - going from memory) - it's a standard size servo - puts out 111 oz-in at 6 volts - my aileron is about 2 1/2" x 18" - so roughly 45 sq in. I just used standard Tower Hobby 44 oz-in servos for the flaps (one per flap). Used the same GWS servos for the rudder and the elevator.

Ramrod25

Way back in the stone age (70's) when I first started flying pattern Jim Kirkland published an
article in Model Airplane News about designing and flight trimming a pattern plane. It was the
best article I ever read. I wish I could find a copy now. One of the things he was specific about was
the difference between flat and airfoiled stabs. He stated and I agree that an airfoiled stab has more
control around neutral. With a flat stab there is a dead spot around neutral where small inputs have
no effect. His experiments also showed the shape of the airfoil was not that important so he used a
diamond shape which could be built on a flat building board - no warps ! I lost my #1 plane and built
the replacement with the airfoil stab and the difference was very noticeable especially during landing.
I think most people use the flat stab just because it is easier to build.

tommy s

I also adopted the diamond airfoil tail groups.
These are used anywhere the scale model shows a rigged flat section.
The high point is the hingeline
These are extremely rigid- require no rigging and look flat to the observer.
They work very well -