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airfoil max thickness position

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airfoil max thickness position

Old 07-17-2005, 12:22 AM
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Default airfoil max thickness position

I'm wondering whether anyone has some insights on relation between the % of chord point of maximum airfoil thickness and CG range. The question comes up because of a no-name Chinese ARF, sort of a sport aerobat mongrel that looks like a hash of Extra, CAP, Edge, etc. One of the characteristics that caught my eye when I was putting it together was that the maximum airfoil thickness looked to be a lot closer to the leading edge than I am accustomed to seeing in my scale aerobat and pattern models. Anyway, the thing is snappy as heck in the air, and I've come close to losing it a few times.

I set it up with CG at 35% of MAC, which is my habit with pattern and SA models. It flies a little tailheavy, but not awfully much. I need to check the wingloading, but it feels heavy in my hands and flies heavy in the air. It flies nicely at good speed, but snaps at the slightest provocation as soon as it gets below cruise speed.

I remember playing with some interactive website on airfoils that seemed to indicate that a more rearward placement of maximum airfoil thickness would allow a greater range of CG, while one more forward would make CG more critical. This seems to make sense intuitively as well.

My snappy mongrel has moderate throws, about 12˚ on elevators, 15˚ on ailerons, expo on both. I know it's on the high side for wingloading, and I'm pretty sure the aerodynamic reality is my CG is too far back. This is a 65" wingspan with MVVS 77 piped, so it has gobs of power. I will see if I can tame it by moving CG forward, but just looking for any theoretical observations that might be educational.
Old 07-17-2005, 12:30 AM
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Default RE: airfoil max thickness position

It is angle of attack that stalls, not airspeed. You have too much elevator throw for the CG. The further back the CG is, the less movement you will need. If it's flyable as is, I would suggest just reducing elevator travel until it no longer snaps (although for an aerobatic airplane, snapping is somewhat desirable). Perhaps just a dual rate?
Old 07-17-2005, 03:17 AM
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Default RE: airfoil max thickness position

Just a couple of thoughts for you here. If the thickest part of the wing is closer to the leading edge, It generally makes the leading edge more rounded. If this is true for your plane, the blunter leading edge has caused a larger high pressure area on the leading edge. This in turn causes the tail to be much more responsive to elevator input. Couple that with a more rearward CG and you will begin to experience the problem you describe. I agree that it can be cured by reducing elevator input or moving cg forward. I have been exparmenting with wing thickness and leading edge shapes for about 5 years to explore the relationship between wing thickness / leading edge shape and low speed 3D characteristics. What I have come to believe is this. Thickest part of the wing should be between 30% and 50% of the wing chord back from the leading edge for best stall characteristics. Sharp leading edge lines cause a wing to be a bit more erratic in flight until very high speed is achieved. The rounder the leading edge the more stable in slow flight and the more manueverable at high angle of attack.

To bear this out, I built a 60 sized profile plane loosely based of the Morris Hobbies "Knife" I built a 48" wing 4" thick. thickest point of the wing at 35% behind the leading edge. Wing chord including ailerons is 22" with 5" ailerons. I built this to be extreme on purpose to push the outside of the envelope. What I got was outstanding. Stall Characteristics are beauiful it will fly st stall very slowly and without a bit of wing rocking. It Harriers like a dream. Slow speed flight is incredible. Bring up the flight speed and manuverability in pitch and yaw is great. Roll rate is slow even with large aileron throws. Although with a 22" chord I sort of expected this. Landing is somewhat different than other 3D profiles I have flown because the plane slows down very quickly, but will float to a point where you have less than a 4 foot roll out. I can land this thing on a table top and not roll off.

I hope this provides some insight to the conditions you have experienced too
Old 07-17-2005, 05:58 AM
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Default RE: airfoil max thickness position

I've found that a far forward chord thickness makes for a wider range in centre of lift. With around a 35% CG, as you slowly pull up elevator, it will reach a point where the centre of lift moves ahead of the CG and the nose will tilt up on it's own and you can let the elevator go to neutral and the nose will stay up, with enough power on of course. Still stays very stable. So moving the CG forward fixes this characteristic.
Old 07-17-2005, 09:14 AM
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Default RE: airfoil max thickness position

Airfoils with their maximum thickness well forward of the normal 30% chord point are used on the full-scale Extra, Edge 540, Sukoi, and others intended for all-out competition aerobatics. They produce a higher maximum lift coefficient at the expense of higher drag. They also stall more abruptly, which helps to get clean entry to snaps and spins. I believe that John Roncz designed the Edge 540 airfoil, but I have been unable to find coordinates for it on the web.

I have used such airfoils on models with good results. The center of pressure appears to be at about the same location as that of normal airfoils, so the CG should be at about the same location, and not more than about 1% mean chord further forward.

If the more abrupt stall is a problem, a bit of washout will help. I would be inclined to avoid such an airfoil on a model with a high wing loading, where the stall problem could be more troublesome. Except for extreme 3-D type aerobatics, I like to carefully fine-tune elevator travel to just enough to stall the wing - this greatly reduces the chance of inadvertently getting into a stall/snap roll.
Old 07-17-2005, 01:05 PM
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Default RE: airfoil max thickness position

Well, guys- I recall from designing airfoils many years ago that between 30 and 40% from the leading edge was about right. Note however, the full scale Extras have their high point closer to the leading edge and there is almost no airfoil shape from there back to the trailing edge.
This is, of course, a highly aerobatic airfoil.
You mentioned a Chinese built ARF. I had one that was built, a Patty Wagstaff Extra, in Viet Nam sold by Hobby Lobby and it had a habit of snapping at most unexpected times. One of the things that I did was peel back the covering and rounded off the almost square leading edges. After trying several different wings I finally got the aircraft to perform.
I recently gave it to one of our 13 year old Ace pilots and he is doing well with it
But, it was not built in China but instead was built in Viet Nam where many of our planes are coming from now.

3dbob in the mountains

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