AoA - What is it and how much?

[microtaylorcraft]

Hey everyone, I've had a book sitting around here given to me by my good friend and its about aircraft design. Now I've only just started it and have spent the better part of today establishing the points it's making in the first 5 pages of the introduction:D Anyway, in my readings there was a bit about angle of attack and how lift is produced on a symmetrical airfoil. It said that with very low AoA's it produces negative pressure on the upper wing surfaces, and negative pressure on the lower surfaces as well. Does this mean that it sucks the wing down and up at the same time? I'm assuming that if the AoA was neutral (0 degrees) it would produce no lift as there would be equal pressure on both wing surfaces. I am unsure though. It then proceeded to say that the higher angles of attack produce lift in both negative and positive pressures, with negative pressure on the top wing surfaces and positive on the bottom wing surfaces. Is this due to airflow pushing it up like a wedge? I'm essentially seeking clarification to make sure i haven't misunderstood what they've said, but any other info would be greatly appreciated! Thanks! :)

[MarkusN]

At zero AoA a symmetrical airfoil basically is just a body disturbing the flow. As the flow is forced around it, it is sped up slightly (Sort of the streamlines near the body being wedged between it and the more outward flow.) As per Bernoulli's law static pressure in higher speed flow drops, thus negative pressure on both sides of the airfoil.

As you increase AoA this effects gets stronger on the upper side, while it is more and more superseded on the lower side by braking effects (which increase static pressure). So pressure at the lower side at one point enters the positive range.

Lift is always the sum of all forces perpendicular to free air flow. So zero at zero AoA, then increasing (almost in a linear fashion for small AoA).

[microtaylorcraft]

Yes Bernoulli's law... thats familiar from today... :rolleyes: so i didn't misinterpret the book ha ha at least thats a start, only a few hundred more pages to go:p what sort of AoA would you recommend for a 3.5Kg 1.3metre wingspan plane with symmetrical airfoil? thanks for the quick reply:D

[ciurpita]

since this stuff is new to you, i hope you are looking for more thorough explanations at nasa's web site, "see how it flies", or even wikipedia.
http://www.grc.nasa.gov/WWW/K-12/airplane/bga.html
http://www.av8n.com/how/

the topic of lift is complicated. a very general answer is that according to bernoulli's theory, the faster a fluid (air) is moving across some surface (wing), the lower the pressure perpendicular to that surface. since the wing has both a top a bottom, there is an affect on both surfaces.

i hope you understand that the angle of attack is the angle of the wing relative to the fluid it moves through. as the AOA changes, the velocities of the fluid across the top and bottom surface change. when there is a difference in the total change in pressure between the top and bottom surfaces, perpendicular to the direction of the fluid, lift is generated. depending on the angle of attack, the lift can be positive or negative.

but you shouldn't assume that the velocity of the fluid is constant across the entire surface. the velocity across the nose may be much higher that across the trailing edge, which means there may be some sections (e.g. nose) that have a greater change in perpendicular pressure than other parts (trailing edge) of the same surface (top or bottom).

[Brandano]

...what sort of AoA would you recommend for a 3.5Kg 1.3metre wingspan plane with symmetrical airfoil...
Recommend ? You might be able to estimate one, if you had the actual wing surface (the span isn't enough), assuming a rectangular wing, fixed speed, fixed altitude, assuming also you are flying straight and level. I suppose you could recommend that the AOA to never exceed the stall angle for that airfoil (again assuming a rectangular wing etc) but the AOA isn't something you can really "set by project", it's something you can only measure. If you actually mean the decalage between the fuselage and the wing, then it's a matter of what will be the mean attitude of the plane while in flight. Making the fuselage as parallel as possible to the airflow reduces drag, but this sort of optimization works best only at a specific cruise setting

[MarkusN]

what sort of AoA would you recommend for a 3.5Kg 1.3metre wingspan plane with symmetrical airfoil? thanks for the quick reply:D
You don't "use" an AoA in a plane. The plane flies at an AoA for any given speed and weight / G-loading. Which means that it varies during flight.

However, you build the plane so that the angle at which the wing is set on the fuselage is in a sensible range, so that the fuselage is well oriented in the airstream for a certain design point. (Typically cruise speed for an airliner, more like maximum speed for a model airplane.) That angle is called angle of incidence.

To answer your question I would need more info, namely the wing area and the expected airspeed for which you want to design it.

[BMatthews]

If you wan't to get a feel for all this google for "foilsim" and go play. Input the wing size as the rough approximation of your models wing and then input the expected flying speed. Then adjust the angle until the lift produced is the same as the expected weight of your model. Then slow the speed to what you expect to be flying at for landings or low speed "hang" time. Then adjust the angle (this is the angle of attack we're talking about) until the lift value again matches the model's weight.

[kcaldwel]

Some pictures of the pressure distribution around a symmetrical airfoil at 0 and 5 degrees angle of attack. The arrows represent pressure: less (away from airfoil surface) than static atmospheric pressure, or above atmospheric (towards surface).

Kevin

[microtaylorcraft]

thanks for the replies everyone:) wasn't expecting so many so soon!

ciurpita:
At this time I'm avoiding the internet mostly for this sort of thing as I frequently get stuff that isn't entirely true, however, I will continue to use my book (Design for Air Combat by Ray Whitford), Presumably there will be more detail later on in the book, but I'm only in the introduction! I will get there eventually though! So far I've established that lift doesn't really have a unit, rather there is a coefficient of lift which depends on a lot of things such as free stream velocity, free stream density, compressibility, Viscosity, aerofoil shape and one or two other things (AoA included!) I shall have to find out if i can calculate the lift coefficient of my wing some day:)

Brandano:
I thought that might be the case, oh well! I was thinking today perhaps between 5 and 10 degrees positive? that should be enough I'm thinking. And there will be no general attitude for the plane in flight for the first few flights, after that its general attitude will be....psychotic? It's aerobatics after all:D

MarkusN:
Are you saying that the entire aircraft moves to vary the wing AoA? Rather than having the wing set at a fixed location? I also don't currently have any more data on the wing or aircraft at this point however I will do some more measurements soon:) The plane is an aerobatic so i was thinking a faster plane, perhaps 60-70Mph (96-112Kmph)? I'm not sure what sort of speeds these types travel at to be honest, and I'm aiming for less than 2.0Kg in weight, though it will be somewhat less by current progress:)

BMatthews:
Thanks:) I'm on dial up here so it will take me some time to download it, but I'm sure it will be worth it.

kcaldwel:
Thanks for the picture, I am aware of pressure (and hence velocity) variations around a wing section:) I also found in my book a cause of lift loss, being spanwise flow of air on the wing. its due to the wingtips, so my first thought was "remove the wingtips!" then "how do I do that?" I believe I have a solution, anyone else got one?

btw to all: I didn't think I was doing too bad for a fifteen year old:p

[Andrew McGregor]

Winglets reduce spanwise flow. However, it's not possible to remove it entirely.

Yes, the whole plane's attitude establishes the wing's AoA. You can feel it happen in a fullsize plane sometimes, as you change the airspeed you can feel the deck tilt.

Most airframes like to cruise at around 3° AoA (as a gross oversimplification), you'd only use larger angles for slow flight. However, there's a little complication… every airfoil has a 'zero lift AoA', which might not be zero if the airfoil isn't symmetrical. So, some very efficient glider airfoils have a -3° zero lift AoA, and cruise at 0° AoA, making quite a lot of lift and very little drag.

Those two links are pure gold. Have no fear of relying on that information, it's good stuff.

A 2kg aerobatic model is going to fly around the speed range you're estimating; my 50-size Lancair with 1300W of power on it does 150 kph, by actual measurement. It weighs just under 4kg.

[BMatthews]

thanks for the replies everyone:) ....BMatthews:
Thanks:) I'm on dial up here so it will take me some time to download it, but I'm sure it will be worth it.....


There's a downloadable java applet version as well as the online running version. But you'll need to download and install Java if you don't have it already. Once you download the applet version of it to your computer you can run it without going online.

It's a simple enough program but it can teach you a lot about how lift coefficients alter with speed and weight and how camber affects the angle of attack and lift coefficients and even let you get a good estimate on your model's stall speed.

It doesn't model the stall speed very well. It "stalls" at 10 degrees all the time which is OK for big full sized stuff but our models stall more typically around 6 degrees due to the Reynolds number effects and the greater ease to forming separation bubbles at our sizes and speeds.

[microtaylorcraft]

Andrew McGregor,
I don't know a lot about winglets, I only know what they are. My solution (I don't know whether it'd work) would be a circular wing, such as the prototypes the Germans had in WWII (I don't believe they had any in service) because my book said it is the tips of the wings that cause the spanwise flow of air. Would this actually work?

[Andrew McGregor]

A circular wing works to reduce tip drag, but like everything else in aerodynamics, it's a compromise. In that case, the extra drag from the sheer amount of wing and the extra weight more than cancels out the benefits. No free lunches. If you think about it, the steeper the angle of the wing, the less effective it is at producing lift (because the lit is pointed inward, at an angle to airframe-up). A circular wing has a lot of wing area that isn't producing much lift at all.

Go read the chapter on lift in 'See How it Flies', one of the links up above. That should let you see how that part of airplane design works (and it's a fairly easy read). In essence, some form of vortex generation is essential to lift production; the tip vortices are what produce tip drag, you can modify them, but you can't eliminate them because they're part of what made the lift you're using to fly.

[Ricardo RW]

...Those two links are pure gold. Have no fear of relying on that information, it's good stuff...

Could not agree more with you.

Thank you guys for sharing gold with us (ciurpita)

[macboffin]

Very good info source on all this "Theory of Wing Sections" by Abbot and Doenhoff if you can find a copy. Includes co-ordinates and characteristics for many many sections as well as more general information.

[biber]

Forget about circular wings.
They can't cancel induced drag, not even a bit. :p
That's the reason why you rarely see them in service (not one at the moment). :rolleyes:

Physics are not so easy to trap.
The air won't see a wing without tips,
it will see a wing with a hole in the middle and of course with tips. ;)

biber