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Old May 18, 2010, 05:21 PM
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rocketmagnet's Avatar
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Airfoil for maximum lift

Hi all

I'm looking for an airfoil which will provide as much lift as possible, especially at low speeds. I don't care about efficiency, drag, or L/D. I just want lift.

As I understand it, this would be a heavily cambered, thick wing flying at an AoA of 45. Am I right?

If so, where might I find such an airfoil?

Many thanks

Hugo
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Old May 18, 2010, 06:18 PM
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Well first off if you have enough power to pull a wing through the air at 45 degrees you don't really NEED a wing since the engine will have enough power to hold the plane up on it's own.

If you're looking for a wing to carry a payload in a larger model then the Selig 1233 designed for the SAE payload competitions would be the way to go. If it's for a smaller and slower model then some of the higher camber airfoils used in free flight would be better.

To point you in the right direction we'd need more information about what the model size and weight would be.

Also you may THINK you don't need to worry about all those other things but you do. If you create too much drag your motor or engine won't produce the airspeed required to remain aloft. And while lots of camber is good if you go much past 6 to 7% camber then the airfoil tends to suffer from separation bubble issues and again you suddenly need a lot more power to hold it up in the air. And the best solution to getting lots of lift to allow for slow flight is to give the model more wing area and lighten the wing loading. Airfoil ability can help but when the chips are down the biggest wing flys the slowest.

Oh, one more factor. The Selig 1233 is a good heavy lift airfoil, or a good slow speed airfoil, but it has to be fabricated to a high degree of accuracy. It's not an airfoil you can use with open structure and cover it. The section is critical as to shape far more than many other airfoils. So this means you need to make the wing using more complex methods that will ensure the high degree of accuracy that the airfoil requires. And it's not just THAT airfoil. Lots of the better performers require the same level of accuracy to achieve the promised performance. So again without knowing what you're trying to do and given the odd focus of the request it's hard to say what you should be using and why.
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Old May 19, 2010, 03:18 AM
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Also maximum lift occurs nearer 15 Deg than 45 Deg.. As noted previously planes that fly at very high alpha are using thrust from their props to supplement lift from the wings.. You dont see gliders flying with 45 Deg AoA

Steve
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Old May 19, 2010, 04:04 AM
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Originally Posted by JetPlaneFlyer View Post
Also maximum lift occurs nearer 15 Deg than 45 Deg.. As noted previously planes that fly at very high alpha are using thrust from their props to supplement lift from the wings.. You dont see gliders flying with 45 Deg AoA
But surely that's because normal wings, designed for normal flight will stall at about 15-25. Wouldn't an airfoil specifically designed to work at 45 give the most lift?
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Old May 19, 2010, 04:19 AM
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Originally Posted by BMatthews View Post
Well first off if you have enough power to pull a wing through the air at 45 degrees you don't really NEED a wing since the engine will have enough power to hold the plane up on it's own.

To point you in the right direction we'd need more information about what the model size and weight would be.

Also you may THINK you don't need to worry about all those other things but you do. If you create too much drag your motor or engine won't produce the airspeed required to remain aloft. ... Airfoil ability can help but when the chips are down the biggest wing flys the slowest.

So again without knowing what you're trying to do and given the odd focus of the request it's hard to say what you should be using and why.
OK, I guess I should be more specific. I'm just curious about making a plane to investigate the slowest possible landing. I don't care about flying, or acrobatics or anything, just launching it from my hand, and seeing it land slowly a few seconds later.

I'm thinking about 1m span, 500g weight (including payload).

I think that a large problem will be maintaining control at this slow speed.

Hugo
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Old May 19, 2010, 04:22 AM
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This floats like a feather
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Old May 19, 2010, 04:31 AM
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This floats like a feather
Haha. A helicopter wasn't quite what I had in mind.
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Old May 19, 2010, 06:03 AM
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Originally Posted by rocketmagnet View Post
But surely that's because normal wings, designed for normal flight will stall at about 15-25. Wouldn't an airfoil specifically designed to work at 45 give the most lift?
At model airplane Re numbers maximum lift usually occurs at 12-15 Deg AoA.. 25 Deg would be stalled unless some very fancy tricks were used such as slats, boundary layer control etc.

As for designing a model airfoil to stall in excess of 45 Deg... First you would need to design new air

Steve
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Old May 19, 2010, 06:10 AM
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Originally Posted by rocketmagnet View Post
OK, I guess I should be more specific. I'm just curious about making a plane to investigate the slowest possible landing. I don't care about flying, or acrobatics or anything, just launching it from my hand, and seeing it land slowly a few seconds later.

I'm thinking about 1m span, 500g weight (including payload).

I think that a large problem will be maintaining control at this slow speed.

Hugo
In that case you should reduce your weight? If all you are interested in is gliding slowly from a hand launch then why so heavy?.. What do you need a payload for if all you want is to hand launch and glide for a few seconds?

It would be very easy to build a 1m span model down to 50g or so.. You would then have a plane that would fly at walking pace, no fancy tricks required. As mentioned in the first reply; by far the most significant variable in minimum flying speed is wing loading, especially when you get down to very low Re numbers.

Providing the wing is not stalled control should be easy enough... But why if you just want to glide for a few seconds would you want 'control' just make the glider freeflight and save the added cost/weight of RC gear..

Steve
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Old May 19, 2010, 07:09 AM
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Originally Posted by JetPlaneFlyer View Post
It would be very easy to build a 1m span model down to 50g or so.. You would then have a plane that would fly at walking pace, no fancy tricks required. As mentioned in the first reply; by far the most significant variable in minimum flying speed is wing loading, especially when you get down to very low Re numbers.

Providing the wing is not stalled control should be easy enough... But why if you just want to glide for a few seconds would you want 'control' just make the glider freeflight and save the added cost/weight of RC gear..
OK, I know that reducing weight is more important. However, getting back to the original question ....

If one had to design an airfoil just to produce maximum lift, forgetting about everything else, what would it look like ?

I assume it would be quite thick with a lot of camber (After all, aren't slats and flaps there to basically increase the effective camber?). But what's the maximum camber? Cambering the wing into a tube is probably too much.
e
My guess would be about 45 of camber from the LE to the TE.

Hugo


Hugo
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Old May 19, 2010, 07:15 AM
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Originally Posted by rocketmagnet View Post
OK, I know that reducing weight is more important. However, getting back to the original question ....

If one had to design an airfoil just to produce maximum lift, forgetting about everything else, what would it look like ?

I assume it would be quite thick with a lot of camber (After all, aren't slats and flaps there to basically increase the effective camber?). But what's the maximum camber? Cambering the wing into a tube is probably too much.
e
My guess would be about 45 of camber from the LE to the TE.

Hugo


Hugo
Easiest way to get more lift out of a wing is to make it go faster

At lower speeds, you can trade lift for drag. fully flapped wings with slats are probably as good as it gets.

Don't fly at any higher speed worth a damn though.

You have to work out what you are really trying to achieve.
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Old May 19, 2010, 07:33 AM
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Originally Posted by vintage1 View Post
You have to work out what you are really trying to achieve.
I realise what I'm asking is unusual, and you're all trying to help me by suggesting things that are good advice for building normal practical aircraft. However, what I'd really like is some insight to the orignal question.

What I'm trying to achieve is this:

Lift: I want the maximum
Drag: Don't care
L/D: Don't care
Max flying speed: Don't care
Suitability for practical aircraft: Don't care
Efficiency: Don't care
Weight: Irrelevant to the question
Wing area: Irrelevant to the question

Hugo
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Old May 19, 2010, 07:50 AM
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Then the Selig 1233 as suggested in the first reply is possibly a good choice. But it very much depends on how slow you are going and the size of the model. At very low Re (e.g .walking speed flight on small/medium size models) then a simple curved plate airfoil of minimal thickness will work as good as anything. The thick airfoils you see used on large scale models and full size planes dont work at all well on very slow flying models.

You say weight and wing area are irrelevant but nothing could be further from the truth. Weight and wing area determine wing loading and wing loading largely dictates flying speed. Flying speed and size dictate Re number and Re number is critical in deciding what type of airfoil will work best.


Regarding camber (i.e. the degree of 'curve' of the airfoil centre line)....You cant just add more camber to give more lift because there very soon comes a point when the air wont follow the curve of the airfoil and stall occurs. This comes much sooner at model Re numbers than at full size... Also forget 45 Deg angle of attack, the wing will stall long before that regardless of airfoil.

Steve
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Old May 19, 2010, 08:23 AM
Herk
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Excessive camber works against your objectives - requiring increased downforce on the tail to offset the pitching moment of the airfoil. Your concept about high angle of attack misses the fact that your model would develop a very high rate of sink - would not land softly

The most reasonable approach to high lift at high angle of attack is a full span leading edge slat added to a good airfoil such as have been suggested.
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Old May 19, 2010, 09:46 AM
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