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
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 youíre 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 shouldnít 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
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
Now that I know what the wing
load of my model is what does it mean? I hope my question is not too
Your question isnít confusing
but the answer may be. ďToo heavyĒ 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 wonít 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 donít get into trouble. If you try to yank the model around
turns like a sport model youíll probably snap roll it into a spin and then the
ground in short order.
The question youíre 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 thatís not all there is. If you airplane has to fly fast to remain
flying then you donít 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
Unless a shape is really bad it doesnít 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 youíd still have to be a superb pilot to notice it.
Iíd like to try my luck in building a model Sailplane.
Your site gave me lots of information on doing it (actually everything).
Thereís only one problem: Maybe I misunderstood it but as far as Iíve 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 doesnít 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 donít have much experience with sailplanes. I donít
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
whatís itís flight envelope should be.
Flat bottom airfoils are used mostly because they are easy to
build. Thatís 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.
Mean Aerodynamic Chord (MAC) is a good place to start. Iíve gone up to
about 20% but donít 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?
Itís 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 donít know that it wonít
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
Is this true for symmetrical section wings? If so is
there any correlation between the two i.e. 1į deg
sweepback is equivalent to 1į dihedral?
I have heard from a variety of reliable sources that wing
sweep has a dihedral effect.
Unfortunately I donít 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 canít 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
Donít stress about it.
Airplanes are tolerant of CG changes within reason. As long as youíre 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
First, I would do what I could
to find where the CG is actually supposed to be. Iím guessing youíve already
tried searching the internet and came up empty handed. You might also ask at one
of the online forums:
RC Groups or
If no answer is found then I
would do as you have mentioned Ė start with the CG at 33% MAC. I donít 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 ďrulesĒ of design depend on what youíre trying to do.
There is no set of ďone size fits allĒ 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 doesnít 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
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
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 donít 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
The main point Iím 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,
to see how you can build a very strong airplane with very little weight. Itís
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 youíll 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
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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