Angle of Attack on a Wing

[Jack Olsen]

Engineer-types, please help. I want to confirm that the angle of attack for a wing is the measurement of the difference between the incoming airflow and a line drawn from the nose to the tail of an airfoil, even if that airfoil has been modified for additional camber, and even if it has a gurney flap.

To wit:



That's correct?

I want to do some quick wool tuft testing with the airflow coming toward a wing I just built, but want some numbers that mean something when I'm done.

Here's the wing:

[lateapex911]

Jack Jack, Jack, LOL. Your posts always make me ask questions ....like, you built a wing?? Is there a thread on it?? If not, spill! I'd love to see what you did.

I'm not an aero engineer, so I'll keep out of any technical answers... but, one thing I've always wanted to know is when you use your term "incoming airflow", do people assume thats horizontal to the ground, or is it based on the actual airflow in the region of that particular car?

Also, I imagine you will be using your wool tufts to look at airflow. I hope you'll show your methods and techniques.

[Dave Colangelo]

Sounds like an excellent excuse to build a wind tunnel and test different setups out...

Along with wool you could mount some pressure sensors like these,
Digi-Key - 223-1101-2-ND (Manufacturer - MS5535-CM)
and wire them up to an arduino or some thing like that, then record the pressure in multiple locations as you drive. I dont know much about aerodynamics, electrical engineering is my field but it could be an interesting project.

Regards
Dave

[jrolstin]

Hey Jack,

Yes AOA is exactly that, the key part you mentioned is that it is with respect to incident air flow angle.

Are you using any simulation software? If you have a CAD model of this setup I can run some simulation for you if you want to try and optimize your gurney flap?

I would like to attach a pdf but can't figure out how... here is a pic of some simulation I have done regarding 2D analysis via velocity cut plots.



You can see as you go higher in angle of attack you begin to get into seperation of the higher velocity airflow from the back side of the wing.

Regards,

[jrolstin]

BTW this is a front wing set up for a Formula SAE car that I designed. Rear wings are prone to large variations in incident airflow which changes across the span of the wing. That is why you see the latest lemans rear wings for GT cars having a unique jog in the center of the wing verus older single-dimensional style.

[304065]

Jack, strictly speaking AOA is defined as the angle between the chord line of the wing and the relative wind, which acts opposite to the direction of travel. With or without the Gurney flap, you can define it as you wish.

It seems to me that it's less about comparability of your setup to other setups via an AOA measurement than it is repeatability of measuring AOA for testing purposes. So you can define it however you want. I would use the router to fab a piece that conforms to the airfoil, mark a line on the airfoil at a certain spot, then glue an el-cheapo Harbor Freight electronic angle finder on the aluminum. Ordinarily I would say just glue it to the end plate but it might fly off and the end plates might get changed as you evolve the design.

Will be cool to see the range between zero lift and aerodynamic stall.

[McLaren-TAG]

a picture is worth... you know. you may want to fiddle around with flow vis paint, it makes it very easy to see the big picture of flow coming towards the wing and at the wing.

I came up empty on a quick google search but you may be able to make some yourself inexpensively with some watered down neon acrylic based paint from a hobby shop.

[aston@ultrasw.c]

Originally Posted by McLaren-TAG

you may want to fiddle around with flow vis paint

Jack - flow vis paint - you can do it!!!!

...and we will all learn something

[Jack Olsen]

I've done wool tuft and ride height testing on a wing with the same airfoil (but different chord length). I have theoretical data on the best angle (although it did not include the Gurney flap).



But I want to make sure I'm close to that actual angle now that I've re-done the wings and the supports. If I get any time today, I'll try to do a quick test just to confirm that my incoming air is close to 11°.



The previous version seemed to do its best at 20°, based on my wool tuft testing.

[jrolstin]

In my opinion, your end plates should be much bigger. Do you have a picture of your test setup through a long sweeper? You can play with end plates to see the difference in flow for yourself, you will be surprised.

[Jack Olsen]

I agree about the end plates.

I've run slightly bigger ones (and stranger ones). The current set was made that way for two reasons: 1) I was running out of aluminum in that thickness and 2) looks. People get used to the looks of ones on ALMS-type cars and the bigger ones look kind of funny.

[jrolstin]

I will try and dig up a pic for you later today, but when we ran world challenge in a GT3 cup car we would 'sneak' on larger end plates that weren't all that ugly for better cross-wind performance.

[jrolstin]

You've got the right idea with your slightly bigger ones. As for the stranger ones... No comment. Lol

[jrolstin]

Do you have a vacuum bag kit to do the carbon on your wing? We used an aluminum block as a large tool piece, put release agent on it, threw some pre-preg down and vacuumed it to the surface. You can cure at room temp, just takes longer (maybe 8-24 hrs I forget right now) and then you have some nice material for end plates, lighter and stiffer than your aluminum. Ideally you could water jet them out if you have some designs on the computer you can nest it and get a few styles at once. For our Formula SAE car we had to run 8mm thick end plates as per the rules. We laminated nomex core between a few sheets of CF to get our thickness, then ran strips of pre-preg around the perimeter were it would get cut in-between the panels so we could radius then after the fact. Again I will try and grab pictures when I can get on my computer - iPhone for now...

[jrolstin]

Here are some examples of what I was talking about...

Our endplate construction



What they looked like



Water-jetting...





Some ideas for wing construction








Final product

[jrolstin]

FWIW, I didn't design the nose cone... the wrap did set the whole thing off though, that's for sure. That chassis is quite the conversation piece too. Cold beer goes to anyone who can guess its construction

[Flieger]

This reiterates what some have already said here, but:

As I understand it, the angle of attack is measured between the incoming airflow (relative wind) and the chord line, and the chord line is the imaginary line which the net lift vector can be drawn perpendicular to. So the chord line will change with camber changes, and devices which modify the effective camber such as Gurneys, fowler flaps, etc.

But for testing purposes you should be able to use whatever reference line you find convenient. The absolute optimum angle of attack for your wing will vary from the true angle of attack, but that doesn't matter so long as you know how to set the wing up optimally on your car.

[Flieger]

Originally Posted by jrolstin

FWIW, I didn't design the nose cone... the wrap did set the whole thing off though, that's for sure. That chassis is quite the conversation piece too. Cold beer goes to anyone who can guess its construction

Guess:

Tube and fabric like and old airplane.

[jrolstin]

There's some aluminum honeycomb in there too :P - modern take on old F1 alu tubs. + hybrid with rear steel tube spaceframe. (front is put in place around an aluminum extrusion space-frame).

[Flieger]

Dang you guys sure have some money. You must have a whole lot of people too to be able to make all that stuff in time.