December 18, 2021

Mail and Frequently Asked Questions Regarding Model Aircraft Aerodynamics

These are a variety of questions I have been asked about Aerodynamics and model aircraft engineering and design.

Other Mail and FAQ Pages

The information you kindly share at your website has helped me greatly in my first steps in kit building.

I believe it is a needed counterpart to much misleading information published everywhere, and stated by RC club experts.

I have stability and landing problems with a 52" span / 0.40 engine Goldberg Falcon I bought already built. The original plan shows the stabilizer with a positive 2.5 degree angle respect to the wing, so it will push up instead of down. It also locates CG at 35% of chord.Do you know of other models with this configuration and its reason to be?

You need to get a trim chart to figure out your stability problems. A lot of planes have positive incidence in the tail to prevent the plane from ballooning when power is applied.

What should happen is that if you’re flying in a level attitude the model should maintain a level attitude no matter what the throttle setting. Of course as the throttle gets too slow the plane will come down, but the plane shouldn’t climb or dive when power is applied. That means adjusting wing incidence, engine thrust and tail incidence.

There are a lot of tests to perform to figure out what needs adjusting.

There are several trim charts available. Print one out and take it to the field with you and get some help from one of the better aerobatic pilots in your club.

I was reading your articles on formulas and wing loading. I now understand the math and that a lower wing loading is better at any given airplane weight but at what wing load is a plane just too heavy?

Now that I know what the wing load of my model is what does it mean? I hope my question is not too confusing.

Your question isn’t confusing but the answer may be. “Too heavy” depends on how you want the model to fly and the power loading.

For aerobatics anything over 20 oz/ft² is getting marginal. The model just won’t be as nimble. Again, as the article states, wing loading can be higher as aircraft get larger.

For a decent size scale model up to 40 oz/ft² that may be acceptable as long as you understand what can happen and don’t get into trouble. If you try to yank the model around turns like a sport model you’ll probably snap roll it into a spin and then the ground in short order.

The question you’re asking is easier to answer when you have a specific application (model) in mind.

First a quick thank you for the intro to Klass Kote. I have almost finished spraying my Acro and the results are superb, although a very time consuming task the results are well worth while. I will send on a picture once it's complete so you can give me some feedback!!!!

On a linked note I have glassed and prepared the wing for painting but have decided to investigate some modifications first. I have recently purchased a second hand version of the same model from a hobby store and have used this as my 'hack' to learn the flying characteristics before trashing my pristine model.

We fly from a small grass patch and as there at least five of these models flying regularly. I have been asking the owners their comments on the model. All have commented that the plane is very easy to fly, responds well to most inputs, but is very slippery through the air and is hard to slow up for short landings. Getting the nose up is very important but with caution to prevent that wing tip dropping.

I have since been considering the possibilities of adding flaps or air brakes to the wing before painting, but have had mixed feedback from various people. More recently it was suggested to work the ailerons with separate servos (the original uses a single inboard servo) and apply something called butterfly mixing, which moves both ailerons up a few degrees and also dials in some down on the elevator to offset the effect of the upward moving ailerons. It still leaves the control over the ailerons for normal flying adjustments but is said to slow planes considerably.

I am not at the level of appreciating how mixes best work or indeed how to set them up for best affect, but this speed characteristic is one I would like to have control over. I have been told if the landing approach is gentle and flat and the nose is kept up then the speed will bleed off. The only problem with this is the approach to the patch is relatively short and fringed with tall trees!

Any help on this would be very interesting. The model is a 60 2c or 70-80 4c low wing with a 58" span, thickish wing fully-symmetrical profile and weighing around 6 lb in total weight. The manufacturer's web site is www.chrisfoss.co.uk and the plane is the Acro Wot.

The best thing you can do to help slow your plane down is to build it lighter. It will stay in the air at a slower speed.

The next thing you can do is make the leading edge more blunt but that will also restrict the top speed somewhat and make it more difficult to enter sharp stalls (required for some aerobatics).

What your friend has described is Flaperons, not Butterfly. Butterfly or Crow (the same thing) use a minimum of 4 servos and 4 control surfaces. Butterfly will slow your plane down, but that’s not all there is. If you airplane has to fly fast to remain flying then you don’t want to slow down below that.

The videos at the end of the Flaperons page show a butterfly mix. Butterfly is a good thing because it keeps the wing tips from stalling while using flaps to create more lift. Together they create a lot of drag and will slow the plane down in a hurry.

Does the shape of the wing tips have a significant effect on flight behavior? I'm fairly new to RC and have noticed several different wing tip shapes on planes. Some wings are cut off flat, some are fully rounded, others have flat angles on the lower half, etc. Does shape make a difference?

Unless a shape is really bad it doesn’t make a lot of difference unless the aircraft needs every bit of performance/efficiency it can get. Most of the shapes you see will work fine. If you gain an interest in high performance sailplanes then greater care in tip shape will make a difference, but you’d still have to be a superb pilot to notice it.

I’d like to try my luck in building a model Sailplane. Your site gave me lots of information on doing it (actually everything). There’s only one problem: Maybe I misunderstood it but as far as I’ve read about the different airfoils used for Model Aircraft, you say that Symmetrical Airfoils and Undercambered Airfoils are used for Sailplanes. But which one is best? Or doesn’t it matter? I own a small Sailplane from Robbe and that has Flat-Bottom Airfoils. But since you've said never to use them, I won't.

I don’t have much experience with sailplanes. I don’t think a symmetrical airfoil is good for a sailplane, but a semi-symmetrical foil is. There are many to choose from but you need to decide what you want the plane to do and what’s it’s flight envelope should be.

Flat bottom airfoils are used mostly because they are easy to build. That’s about all they have going for them.

Take a look at Selig airfoils to find one that suits your needs. Also ask at RC Universe about specific airfoils for your project.

I built a delta win model plane and was wondering how to find a centre of gravity.

15% Mean Aerodynamic Chord (MAC) is a good place to start. I’ve gone up to about 20% but don’t start there.

One thing you can always do is build a small, all sheet model of your model. Balance it until it glides properly. Use your findings to balance the larger model.

My friend and I are attempting to build a pair of Ugly Stik biplanes. Can we make a top wing and have the bottom wing be a thick plank of balsa? Would it be best to build complete top and bottom wings?

It’s best if the wings have the same flight characteristics. I would use the same airfoil on both wings even if the wings are different sizes. I would not attempt what you suggest but I don’t know that it won’t work.

I'm thinking about building another stick but this one's going to be a biplane. What do you think? I'm not sure what size the bottom wing should be. Is there a formula for this? I'm going to have to work out some things like the size of the fuse (little longer) and a few other mods. This project is still on scratch paper, so any of you input would be helpful.

I guess my biggest question would be.....will this fly?

I've never built an R/C plane this small so take my comments for what they're worth. I think this model would be a neat bipe conversion and don't see any particular difficulties with it.

Traditionally the lower wing is never larger than the upper wing and often times it's smaller. In this case I would probably make two wings the same size and perhaps clip one rib bay on the lower wing. I would not make the chord shorter on the lower wing because it may not be able to carry its own weight.

You will need to increase the tail surface areas accordingly.

Another thing to keep in mind is that there is more added drag than just the drag of the lower wing. There will be drag created at the wing saddle as well as drag from the airflow between the two wings. Space the wings about one chord apart. On larger models the wings can be spaced a little closer, but I think at this scale the effects of the wings being too close will be worse than a larger model.

I believe the Littlest Stik is designed for a .010 engine. You may need to go to as large as a Babe Bee .049 due to the added drag and weight. What I would probably do is first try a .020 and not go to a larger engine unless necessary.

This should be a fun project and inexpensive. Although there are no guarantees that all this will work, I think it will but it may take a couple prototypes to get it flying the way you like.

Hi I have been told that sweepback on wings actually creates dihedral.

Is this true for symmetrical section wings? If so is there any correlation between the two i.e. 1° deg sweepback is equivalent to 1° dihedral?

I have heard from a variety of reliable sources that wing sweep has a dihedral effect.

Unfortunately I don’t know the answer to your question as to how much sweep equates to how much dihedral. All I can do is suggest that you ask your question in the R/C Universe Aerodynamics forum.

My friend has just finished his first trainer, and after spending the last year in Egypt on business has returned home to find his manual with all the technical info well on its way back to where nature intended it, in a land fill site!

The plane is a Hi Boy II, made by Precedent I believe, running an OS 40. Do you have any idea as to the position of the C of G, or do we run with the 30% of cord guestimate?

30% should be fine. If you can move the battery around to shift the CG then that would be a good thing to do at the field after test flying the plane at 30%. If you can’t move the battery then you may want to take some stick-on weights and adjust that way. Once you find where the CG is best, do whatever to make it permanent. Hopefully you can move things to lock it in but if not then glue weight in as necessary.

Don’t stress about it. Airplanes are tolerant of CG changes within reason. As long as you’re not behind 35% or forward of 25% it will be controllable.

Dear Sir I am a bit at loss, having just completed a Powerhouse and the plan does not show the position of the Centre of Gravity.

Normally I would start at about 1/3 Chord, however this aircraft has a lifting tail plane hence the apprehension.

First, I would do what I could to find where the CG is actually supposed to be. I’m guessing you’ve already tried searching the internet and came up empty handed. You might also ask at one of the online forums:

RC Groups or RC Universe

If no answer is found then I would do as you have mentioned – start with the CG at 33% MAC. I don’t think you can go wrong unless the plane has a very unconventional layout or the tail surfaces are unusually small, such as on an exact scale replica.

As I understand it (and I could be wrong), a lifting tail allows a more rearward CG. If it were my model then I would trust the 33% point to be fine for test flights and then I would adjust it as necessary.

I am scratch-building for the first time and would like to know how to use engine offset, finding Center of Gravity, the size of rudder or horizontal stab, etc.

Can you guide me to some freeware? Do you have some rules of thumb in building my first scratch build (.45 engine, 78" wing span, flat bottomed airfoil with 10 degrees of dihedral)?

The “rules” of design depend on what you’re trying to do. There is no set of “one size fits all” design parameters. Since this is your first scratch build, I suggest you stay on the conservative side of things. Make the tail larger than you think it might need to be, for example.

Engine offset is normally 2-3 degrees. I usually build my firewalls to have 2 degrees of right thrust and then make thrust wedges from plywood to fine tune the adjustment while trimming the plane.

10 degrees is a lot of dihedral. Will this plane have ailerons? If so I would make the dihedral at least half of what you suggest. If it is rudder only, then I would still knock it down to no more than 5-7 degrees. Dihedral is measured per panel, by the way. In other words, if a wing has 10 degrees of dihedral, then there will be a 160 degree angle between the two wing panels.

I suggest you look at these sites for more information. My site doesn’t contain much in the way of actual numbers for designing your own - just some general philosophy.

Another thing I suggest is that you decide now that you want to develop this model. By that I mean plan to build 2 or 3 prototypes to really get the design dialed in.

The first model will be rough, but will fly. After extensive flight testing and adjustments, build another model and make improvements you could not make to the original prototype. For example, change the size of the control surfaces if needed, adjust the dihedral, etc.

I recently found your web and think it is of great help. I read your comments on wing loading, but to have it clear: Is there a wing loading RATIO that indicates if the model will be able to fly or if it would be something like a sledge hammer?

The Wing Loading tells you how much Weight a given amount of Wing Area is responsible for lifting. In our realm, Wing Loading gives us enough information to have an idea of what to expect, but experience with a variety of types of models will make the Wing Loading figures more meaningful simply because some wings are more efficient than others and different types of wings fly differently.

Lift is distributed elliptically across the wing. Therefore, an elliptical wing is most efficient whereas a rectangular platform wing is not efficient towards the tips. The area near the tips doesn't do as much work as the rest of the wing.

A high-lift airfoil such as flat bottom wing can get away with higher wing loadings and still fly well. A tapered wing or one having a sharp leading edge, CAP-type aircraft, for example, can be vicious at higher wing loadings. You can expect nasty tip stalls, snapping out of loops, failure to recover from spins, or other unwanted behaviors if you let the weight get too high.

Another factor is the size of the model. A larger model with a higher wing loading will fly like a smaller model with a lower wing loading.

For example, a model having 2000 inches of area having a 35 oz per square foot wing loading will fly similarly to a model having 500 inches of area and a 20 ounce wing loading. Those are numbers off the top of my head, so don't take them literally. They are just for illustrative purposes.

Models .15 and below should keep the wing loading around 12 oz per square foot or lower. Models up to .60 size should strive for 22 ounces and less. Lighter is always better. If you build a .40 size model and really want it to perform well, try to keep the wing loading under 16 ounces per square foot.

Again, it depends on the model and how you want to fly it.

I just read your article on flaperons and this afternoon had my first go at using flaperons on my Midwest Aerobat. I was using 30% flaps and did not detect any adverse effects. It just plain landed easier.

The Aerobat ailerons are extremely long, starting at the tip and extend to within a few inches of the fuse. Since the effects of the flaps extend for almost the entire wing, does that make tip stalls less likely? Or is the tip stall based how aggressive the angle of attack is?

I am not an expert on this kind of stuff. When it comes to aerodynamics I work in "rules of thumb" and personal experience.

To be honest I'm not sure if the having full span flaps makes tip stalls more likely. It could be that the likelihood of tip stalls remains the same whether the flaps are deployed or not because the camber of the airfoil changes over the entire span at the same time.

I think it would largely depend on whether or not the airfoil is the same from root to tip. If so, then the wing probably behaves the same no matter what position the flaps are in. However, if the wing has tips having a different airfoil than the root, then lowering the flaps would probably make these two portions of the wing behave differently - for better or for worse.

What I do know is that not all planes are prone to tip stalling. It really depends on a variety of factors: overall weight, whether the wing balances laterally, leading edge shape over the entire wing (is the tip sharper than the root?), airfoil(s), wash-out, etc.

A lot of model planes can get away with things that would make full-scale pilots cringe.

Another thing to keep in mind is that there is a stall angle for any given airfoil. So even with a wing having wash-in, the plane shouldn't tip stall as long as the tips remain below the stall angle.

I've had the same experience as you with My Stik 30. It also uses full-span flaperons and it has never shown any signs of tip-stalling - even when the flaps were lowered to 45 degrees.

The point I was trying to make in the flaperons article is that by having separate flaps and ailerons you can get the best of both worlds; wash-out at the tips to significantly reduce any tip stalling tendency while having flaps down at the root to lower flight velocity and gain lift.

I am about to start building a powered .46 engine glider for use in aerial photography. I want it to be stable with slow flight capabilities so I can take my eyes of it for a while and not look back to see it in a spiral dive. I want the engine as a pusher config/high wing with the engine behind the trailing edge of the wing.

Could you give me some idea of what wing loading to look for to achieve this slow flight speed and what wing section?

I have never attempted aerial photography, but some things seem like givens to me regarding performance attributes of the plane. That being said, I will tell you how I would go about the design, but I also suggest you check out the photography forum at RC Universe. There are people there who have practical knowledge that I don’t have.

I would build the fuselage from sticks to get the best strength to weight ratio. The camera compartment should be designed into the fuselage for best strength and least amount of weight.

I would truss the camera compartment to spread the load out as much as reasonably possible. The camera will be a massive item and will have a lot of inertia. By spreading the load with trusses the camera will be less likely to break loose in a hard landing.

What I would avoid is trying to gain a lot of strength using slabs of plywood. You will need some plywood for mounting purposes, but use as little as possible. Balsa has plenty of strength if you arrange the grain correctly. More plywood = more mass and inertia = greater damage and poorer flight characteristics.

The main point I’m trying to make is that you should avoid trying to beef up the plane in ways that have a poor strength to weight ratio. Take a careful look at my design, Great Gonzo, to see how you can build a very strong airplane with very little weight. It’s all about building, but not over-building, strong beams and trusses and avoiding slabs of wood. Also look at some of the flight shots of Bride of Gonzo to see more of how the wing was built. The wing is incredibly strong and weighs only 5-1/2 ounces.

With a .46 engine, you can reasonably build a motor glider having 1,500 or more inches of wing. I would shoot for a wing loading no more than 16 ounces per square foot and closer to 10 if you can do it. A lot depends on what type of flight video you want. If you want to soar at altitude, then light weight will allow the plane to fly gracefully in the air currents with some slight bobbles.

However, if you want very steady video at lower altitudes, then you may want a higher wing loading so that the plane is less affected by turbulence near the ground. My personal choice would be to make the plane as light as I could. I suspect a plane having a 1,500" wing and a .46 engine could come in around 6 lbs ready to fly without the camera.

Some other ways to save weight would be a carbon fiber landing gear, using a wood prop instead of a glass/nylon prop, lightweight wheels, pull-pull controls, etc.

I would build the wing exactly as for the planes I mentioned above. Use a good pair of spars with shear webs and turbulators at the leading edge. Avoid unnecessary sheeting. You may want a second pair of smaller spars toward the trailing edge to stiffen the wing a little. I would make this pair from significantly smaller wood than the main spars as they will not be carrying much of a load. Maybe 1/8" x 1/4" with shear webs.

As for an airfoil, I would look at a Selig foil in the neighborhood of 13%-14% thickness.

By the way, taking your eyes off the plane is a bad idea. If you think you’ll need to do this, you may want to have a spotter to watch the plane for you.

I am currently doing a project on building a electric model airplane. But sad to say that I am lack of knowledge in this field. Thus, I would like to seek you advise in this project.

Actually I have come out a preliminary design. Below are some of the values that I have come out with.

Wing Span: 1500 mm

Wing Area: 304 500 mm2

Chord Length: 203 mm

Total Weight: 5000 g (inc. battery, motor, servo, propeller etc.)

* It is a rectangular wing *

Could you tell me how to calculate the torsion, moment, wing loading, forces exerted on the wing? Because I need to select the graphite rod diameter to be fixed at the wing. I totally have no idea how to start calculating these values.

I do not use electrics and couldn't begin to give useful advice for electric power. I work strictly in glow and will eventually use gasoline engines when I have more room to store larger models. At this point I can not help you with questions about gas engines either.

I am not an Aerospace Engineer. Many of these questions are over my head. Useful formulas are here, but they do not delve into the type of aerodynamics you are asking about. Try the Aerodynamics forum at www.rcuniverse.com. That is where I ask questions that I never understand the answers to.

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