Aerodynamics Discuss the concepts of aerodynamics here

I really don't understand wing shape.

Old 05-27-2013, 12:17 AM
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mclarkson
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Default I really don't understand wing shape.

I've been noodling around for some ideas for a new scratch build (I've started to feel restless when I'm not building a plane.) I love the look of air racers of the thirties and was looking through pylon planes and their wing shapes. I found no real commonality at all: stubby wings, square wings, triangular wings, beautifully rounded wings...

It's hard to generalize what makes a good wing for a pylon racer.

And then, I was looking at photos of two of my favorite twin-engined planes, the de Havilland HD-88 Comet, and the Lockheed P-38 Lightning when I noticed something interesting. Their wings are very nearly the same shape but the Comet (or possibly the Lightning) has them on backwards. Here are the two planes:



If I lay one atop the other, you can see that the wings are similar, but that’s about it. The angles at the leading and trailing edges don’t match up very well.



However, if I flip one of the planes, nose-for-tail, the wings match up very closely indeed.

(The Lightning does have a slightly deeper chord.)

They're practically the same shape, but one is backwards. Why should that be? I have no idea.
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Old 05-27-2013, 12:38 AM
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Turner
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I think those slight differences in planform are far less significant than the aspect ratio and airfoil. More to do with artistic license I would guess.

http://www.dauntless-soft.com/PRODUC...ngPlanform.htm
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Old 05-27-2013, 12:44 AM
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hayofstacks
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read on elliptical wings. most write ups on those go pretty in depth.
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Old 05-27-2013, 02:12 AM
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I have the Durafly DH-88 Comet and it was fun reading up on the history of that bird. That baby was built for speed. I'm sure the full scale has a slightly different cross-section than the RC version. The elliptical wing was the trend of the day really spearheaded by the British designers like DeHavilland. Out on the tips the foil is folded down. Whereas today, winglets for better fuel economy are used.
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Old 05-27-2013, 04:21 AM
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Turner
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I really wouldn't call either of those wings elliptical. They are really in the category of tapered wings. The leading and trailing edges are perfectly straight for the most part.

Interesting discussion on wings:

http://www.ww2aircraft.net/forum/air...why-27645.html
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Old 08-09-2013, 08:02 AM
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Default Wing Shapes

I had a foam wing cutter from the 1970's through about 2002 or so. I cut many wings for our club and for many designers. Since I had all of these templates around I started to experiment. What really matters is your weight and area, or simply wing loading. You also have to consider aspect ratio when it comes to the shape of the wing. In the 1930's tapered and elipical wings was how they were reducing the loss of of lift near the tip of wing due to the air flowing outward instead of over the wings airfoil producing lift. Today this is acomplished by winglets or anti vortex generators as winglets were originally called. If you look at a sailplane you will see long narrow wings (Very high aspect ratio) great for lift, or short stubby wings (low aspect ratio) like on a fighter better for speed.

I played with swept wings, tapered wings, elipitacal wings, and constant cord wings, they all have there advantages, but it is mostly construction and flight envelope that drives the shape of the wing. For instance if you want to impove stall? Go with a blunt leading edge, good for a trainer, not good for a racer. If you want a fast roll rate go with shorter wings like on a patern airplane not like on a sailplane.



I bet if you overlay a model of a patern airplane it would be very close to the comet and the lightning factoring out the twin engines. But if you looked at the airfoils the patern plane would have a thicker airfoil for a more constant speed through maneuvers.
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Old 08-09-2013, 12:19 PM
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JetPlaneFlyer
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Actually taper is nothing directly to do with adjusting span wise flow. The main reason that wings use taper is to adjust span-wise lift distribution and so to produce less induced drag. The aim is to keep all parts of the wing operating at the same coefficient of lift and to make downwash constant across the wingspan and tip vortices minimised. using taper leads to a higher 'Oswald Efficiency Number'

The optimum wing shape as far as induced drag is concerned is elliptical, but that is hard to build and can have difficult stall characteristics, so in general taper attempts to approximate an elliptical wing shape whole retaining a simple build and safe stall.

For most power planes a single taper is good enough but for many sailplanes they attempt to get closer to a true ellipse by having two or three different taper angles.

The wing actually doesn't have to be a true ellipse (like the Spitfire) to get these benefits. What's important is that the chord of the wing reduces toward the tip at the same rate an an ellipse would do, so for instance you can have a straight trailing edge and a curved leading edge and still retain a perfect elliptical chord distribution.

Taper can also have some structural advantages. If you taper a wing sharply then the tip makes less lift so the wing bending moment is reduced and the wing can be made thinner and/or lighter. This was most likely why planes like the Comet had such sharply tapered wings. The downside is a really tricky stall.
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Old 08-09-2013, 11:00 PM
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mclarkson
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This is all proving my point: I really don't understand wing shape! Thanks for all the cool information.
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Old 08-10-2013, 09:11 AM
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JetPlaneFlyer
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Originally Posted by mclarkson View Post
This is all proving my point: I really don't understand wing shape! Thanks for all the cool information.
If you remember one thing remember this:

Highly tapered wings = tip stall

If you want an easy flying plane with no nasty habits it's best to be wary of strongly tapered wings (anything with a tip chord less than about 70% of root chord is likely to be tricky).
A good designer can use some tricks like washout to compensate but unless you know the model to be a good flyer be suspicious of highly tapered wings. The DH-88 Comet for instance would be one I'd be very concerned about, and the videos of the Durafly version spinning in only go to confirm.
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Old 08-10-2013, 10:19 AM
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mclarkson
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Originally Posted by JetPlaneFlyer View Post
The DH-88 Comet for instance would be one I'd be very concerned about, and the videos of the Durafly version spinning in only go to confirm.
Yeah, but it's so pretty, who cares?
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Old 08-10-2013, 01:16 PM
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And to be fair that sort of behaviour is what might be called "accurate scale behaviour". All the reports suggest that the original DH88, while very fast and very pretty, was also a bit of a pig to fly when it wasn't up to speed i.e. on take-off or landing .

Even the revered DH Mosquito with a roughly similar wing shape was reputed not to be the friendliest of things at slow speeds.

Though on an aerodynamics note, I find it fascinating that no documentation on full-size aircraft aerodynamics or piloting ever seems to mention the existence of anything called a "tip stall" though just about every aeromodeller knows and fears it .

Steve
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Old 08-10-2013, 02:32 PM
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JetPlaneFlyer
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You dont look hard enough.

Here's an extract from NASA paper # 1589 on Leading edge modifications to prevent stall/spin on light aircraft:


And what about the old NACA paper tipled "A comparison of several tapered wings designed to avoid tip stalling"

http://naca.central.cranfield.ac.uk/...aca-tn-713.pdf

It's true that in full size aviation a 'tip-stall' would often be called a 'stall/spin'. But call it what you want, it's bad.
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Old 08-10-2013, 05:45 PM
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slipstick
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You're right it's time I gave up trying to talk about anything on here. It's now way too full of people who are determined to disprove any statement one makes if it's not perfectly qualified.

Steve
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Old 08-10-2013, 06:05 PM
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I don't think he is motivated to disprove, but rather to inform. Knowledge moves us all forward. I value those here that bring us accurate information and dispel myths. I've learned much from JetPlaneFlyer and hope he will continue to participate.

Don't take it personally.
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Old 08-10-2013, 06:36 PM
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JetPlaneFlyer
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Sorry

Guess we both feel the same way because I know whenever I mention 'tip-stall' someone will come along and say it doesn't exist because full size pilots don't use the term.
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Old 08-17-2013, 12:27 AM
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quorneng
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Surely the whole object in full size design is to ensure the root stalls before the tip.
In model sizes this is harder to do as even a perfect scale wing with all the scale aerodynamic features is unlikely to behave the same way so is much more likely to have rather dramatic stall characteristics.
We should also take into account that in RC there is no feedback to the controls so you cannot easily detect any pre stall buffet and changes to their 'feel'. Both of these are likely warnings in full size.
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Old 08-18-2013, 01:35 AM
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Just for fun I would like to fit a stall horn to one of my planes.

Placing it properly and tuning it to work properly would be most of the fun.
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Old 08-18-2013, 04:00 AM
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dahawk
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Dave,

Why not go all out and design a stick shaker ? LOL
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Old 08-18-2013, 05:07 PM
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Only because i hadn't thought of it!
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Old 08-18-2013, 05:48 PM
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JetPlaneFlyer
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Originally Posted by dahawk View Post
Dave,

Why not go all out and design a stick shaker ? LOL
Maybe do-able with telemetry and vibrate alert already available on some Tx's
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Old 03-05-2015, 08:14 PM
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Yah. If you look at a modern airplane you will see that the root of the wing usually has a higher pitch than the tip(slightly twisted wing!) so that the tip is always at a smaller angle of attack compared to the root so that it doesn't stall first, allowing you control of the plane's roll during stall of the root.

just don't install a stall horn on a 3D plane
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Old 03-05-2015, 08:33 PM
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JetPlaneFlyer
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Originally Posted by theapplepi3.14 View Post
Yah. If you look at a modern airplane you will see that the root of the wing usually has a higher pitch than the tip(slightly twisted wing!)
It's called 'washout' and it's the oldest aerodynamic trick in the book. Not so good for aerobatic planes that have to fly inverted though!
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Old 03-05-2015, 08:48 PM
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Yup. Do you know if it pre-dates NACA? I am starting to learn that 3D flyers break every rule for normal aircraft design.
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Old 03-05-2015, 09:07 PM
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Washout can be seen on many WWI and earlier aircraft.

To be honest 3D planes don't really brake any 'rules'. They are just lighter, have better thrust/weight ratio and have much larger control surfaces than the typical aerobatic model. You wouldn't find washout used on 3D models just because they need to fly well inverted. There again you wouldn't find washout on a traditional aerobatic pattern model either (for the same reason).
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Old 03-05-2015, 09:14 PM
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Spektrum has gps and pitot tubes available for telemetry, among others. And I think you can have the nice lady in the tx warn you if you go below a certain speed and even shake the tx. Not certain if that's available with the GPS, but it's already possible.
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