Preventing tip stall with aileron reflex?

Preventing tip stall with aileron reflex?
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All Forums >> RC Airplanes >> Aerodynamics >> Preventing tip stall with aileron reflex?

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Preventing tip stall with aileron reflex? - 3/8/2003 2:44:08 AM

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From: San Diego, CA, USA
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Can reflex on the ailerons be used to prevent tip stalling?

I thought of this when considering that changing of flap angle changes the AOA of the wing where the flaps are. If this were done to the ailerons it should change the AOA of the outboard section of each wing and thus make them stall before the inboard section of the wing.

Will this work? Do the ailerons need to be slightly UP or slightly DOWN to prevent tip stalls? What about flying wings that have ailerons?

-Q.

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-Q

Aileron reflex???? - 3/8/2003 2:54:40 AM

HalH

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Most times tip stall is prevented by twisting the entire wing. The tip section is at a lesser angle of incidence than the root section. So that being said I suppose that it would help sto some degree if both ailerons were tipped up slightly.
Please don't confuse angle of attack with angle of incidence. AOA is the angle between the mean aerodynamic chord and the relative wind.

(in reply to a088008)

Preventing tip stall with aileron reflex? - 3/8/2003 3:12:10 AM

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Most tipstalls can be prevented by flying the plane properly. This means having the c.g. in the right place, and the proper amount of control deflection for the condition.
Unless the plane is supposed to be super stable, crutching the design solely due to concerns about tipstalls doesn't accomplish much.
Reflexing the ailerons up a bit will do the same as twisting the entire wing, but it's more productive to adjust the system to stay away from the area where tipstalls occur most frequently..
Highly banked turns at low airspeeds, and too much elevator motion.

(in reply to a088008)

Re: Aileron reflex???? - 3/8/2003 5:26:19 AM

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I don't quite understand why you mention incidence, as I was referring to the effect of changing AOA indirectly by changing the MAC. Let me explain a little further, and I could very well be wrong here. The MAC at the wing tips is a function of the airfoil and the deflection of the aileron (my assumption). UP aileron will shift the MAC upwards and therefore decrease the AOA of the airfoil at the tip. Am I on the right train of thought here?

-Q.

[QUOTE]Originally posted by HalH
Most times tip stall is prevented by twisting the entire wing. The tip section is at a lesser angle of incidence than the root section. So that being said I suppose that it would help sto some degree if both ailerons were tipped up slightly.
Please don't confuse angle of attack with angle of incidence. AOA is the angle between the mean aerodynamic chord and the relative wind. [/QUOTE]

< Message edited by a088008 -- Mar 8 2003 12:50AM >

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-Q

(in reply to a088008)

I intend using reflex only for maiden flights - 3/8/2003 5:35:45 AM

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The reason I mention adding reflex to the ailerons is so that I can setup a super stable plane when doing a maiden flight. I have a new design ready for it's maiden flight and it's flight characteristics are a BIG unknown. The design in unconvetional (flying wing with forward lifting body) and the CG cannot be exactly calculated. The pitch control is reduced with increasing AOA. Also, there is no rudder, so tip stall is more likely. Also, due to the CG uncertainty, there might be lost of elevator corrections as I attempt to stabalize the plane after takeoff and during it's first turn after takeoff. This all makes for a plane that can easily enter a tip stall shortly after takeoff, hence my attempts to prevent it. Once a proper CG has been found through experiment the reflex on the ailerons will be removed.

-Q.

[QUOTE]Originally posted by Tall Paul
Most tipstalls can be prevented by flying the plane properly. This means having the c.g. in the right place, and the proper amount of control deflection for the condition.
Unless the plane is supposed to be super stable, crutching the design solely due to concerns about tipstalls doesn't accomplish much.
Reflexing the ailerons up a bit will do the same as twisting the entire wing, but it's more productive to adjust the system to stay away from the area where tipstalls occur most frequently..
Highly banked turns at low airspeeds, and too much elevator motion. [/QUOTE]

_____________________________

-Q

(in reply to a088008)

Preventing tip stall with aileron reflex? - 3/8/2003 5:59:45 AM

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MAC is the mean aerodynamic chord.. nothing more.
I expect you're confusing Cm, the pitching moment, with MAC.
Reflex (or flaps) change Cm.
It is generally a constant value for most reasonable values of alpha.
.
Lack of a rudder (or vertical surface) will create an unstable vehicle unless you've made other plans for keeping the pointy end at the front. Such as a lot of washout at the tips.

(in reply to a088008)

Preventing tip stall with aileron reflex? - 3/8/2003 6:36:49 AM

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a088008,

Yes, this could be used to help prevent tip stall. This is because the reflex would decrease the AoA, as you say, and also because it would decrease camber. Both of these effects will delay stall. Similarly, if you have flaps inboard, they increase camber and AoA there, which tend to cause the inboard part of the wing to stall earlier. Since you talk about using the reflex during takeoff, you might be able to use flaps for the same effect, if flaps are feasible. You could even mix the flaps or reflex with elevator and/or throttle. I personally like these ideas better than fixed geometric twist ( washout ), since the twist that is appropriate for upright flight is exactly wrong for inverted flight. This probably only matters for a plane that is intended for aerobatics, but I still find geometric twist kind of unsatisfying.

Now that I've typed all this, I have gone back and read your most recent post. As I recall from pictures of 'adrenaline', the wing has no taper. More than likely, you will not have a tipstall problem. Also, my comments about flaps and elevator don't apply.

Good luck,

banktoturn

(in reply to a088008)

Preventing tip stall with aileron reflex? - 3/8/2003 8:23:49 AM

Johng

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Yes, aileron reflex intruduces washout to the wing - but so do flaps. Using either or both reduce effective angle of attack on the outer wing. Now, during takeoff, the wing alpha can be very dynamic and the reflex can help alot.

Gliders routinely use "crow" for landing, which includes flaps deployed downwards and ailerons reflexed upwards. The main purpose of this is to add drag. However, it also makes the long wing almost unstallable. I have sunk-in my glider at a 45 deg angle with the fuse level. The other consideration is that reflexing ailerons causes pitch-up force, noticable even on a conventional layout.

Same thing goes for funfly planes, but they usually call it "airbrake". You can get a plane going real slow with that setting engaged. I used it an a giant stinger. That thing could creep along. Drag the tailwheel on landing too.

I have used a "crow" setting specifically for takeoff one one of my planes. It's a 4m sailplane with a self-launch pylon motor that weighs 10 lbs. The tip chord is about 3 inches. I launch it from a dolly cart, using a bungee-assist launch hooked under the nose. So, there's all kind of wacky stuff going on at once. You pull back this plane on a slingshot, sitting on a cart, with an offset-high thrust line. It takes full up elevator to get the thing up off the cart clean. Then I let off the elevator off as it picks up speed and the bungee drops away. For takeoff I have the flaps set down about 20 degrees and the ailerons are up between 5 and 10 degrees. I have never had a wobble or hesitation in 15 or 20 flights. Would it work as well without the reflex? I have no idea, since I'm not going to bet this big plane on trying it without. It works, I ain't changing

I can't get any more flap travel for landing( strange - it came hinged like that) , so I use the same settings for landing, but I also have spoiler available for glide control.

Anyway, from what I've read, I understand that this is essentially a flying wing, which means pitch control is thru the trailing edge devices. Which means that setting the ailerons "up" will also increase the pitch up forces. That could be ugly on a frst flight all on it's own. This will have to be balanced with deflection of flaps downward. If there is any sweep in the wings, the ailerons will have more pitch authority than the flaps. If you want to use reflex, not more than 10 degrees should be fine.

_____________________________

John

(in reply to a088008)

Preventing tip stall with aileron reflex? - 3/8/2003 9:12:22 AM

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Thanks for the very interesting post. I'm intending using about 5 degrees or less reflex on the ailerons. Another nice thing about reflex is that I get less down-travel on the ailerons which further reduces the changes of tip stall.

-Q.

[QUOTE]Originally posted by Johng
Yes, aileron reflex intruduces washout to the wing - but so do flaps. Using either or both reduce effective angle of attack on the outer wing. Now, during takeoff, the wing alpha can be very dynamic and the reflex can help alot.

Gliders routinely use "crow" for landing, which includes flaps deployed downwards and ailerons reflexed upwards. The main purpose of this is to add drag. However, it also makes the long wing almost unstallable. I have sunk-in my glider at a 45 deg angle with the fuse level. The other consideration is that reflexing ailerons causes pitch-up force, noticable even on a conventional layout.

Same thing goes for funfly planes, but they usually call it "airbrake". You can get a plane going real slow with that setting engaged. I used it an a giant stinger. That thing could creep along. Drag the tailwheel on landing too.

I have used a "crow" setting specifically for takeoff one one of my planes. It's a 4m sailplane with a self-launch pylon motor that weighs 10 lbs. The tip chord is about 3 inches. I launch it from a dolly cart, using a bungee-assist launch hooked under the nose. So, there's all kind of wacky stuff going on at once. You pull back this plane on a slingshot, sitting on a cart, with an offset-high thrust line. It takes full up elevator to get the thing up off the cart clean. Then I let off the elevator off as it picks up speed and the bungee drops away. For takeoff I have the flaps set down about 20 degrees and the ailerons are up between 5 and 10 degrees. I have never had a wobble or hesitation in 15 or 20 flights. Would it work as well without the reflex? I have no idea, since I'm not going to bet this big plane on trying it without. It works, I ain't changing I can't get any more flap travel for landing( strange - it came hinged like that) , so I use the same settings for landing, but I also have spoiler available for glide control.

Anyway, from what I've read, I understand that this is essentially a flying wing, which means pitch control is thru the trailing edge devices. Which means that setting the ailerons "up" will also increase the pitch up forces. That could be ugly on a frst flight all on it's own. This will have to be balanced with deflection of flaps downward. If there is any sweep in the wings, the ailerons will have more pitch authority than the flaps. If you want to use reflex, not more than 10 degrees should be fine. [/QUOTE]

_____________________________

-Q

(in reply to a088008)

Preventing tip stall with aileron reflex? - 3/8/2003 9:14:02 AM

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From: San Diego, CA, USA
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I do have tail fins on the plane, just no rudders installed. You can see what it looks like in the "Adrenaline" post in this forum.

-Q.

[QUOTE]Originally posted by Tall Paul
MAC is the mean aerodynamic chord.. nothing more.
I expect you're confusing Cm, the pitching moment, with MAC.
Reflex (or flaps) change Cm.
It is generally a constant value for most reasonable values of alpha.
.
Lack of a rudder (or vertical surface) will create an unstable vehicle unless you've made other plans for keeping the pointy end at the front. Such as a lot of washout at the tips. [/QUOTE]

_____________________________

-Q

(in reply to a088008)

Preventing tip stall with aileron reflex? - 3/8/2003 1:10:43 PM

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From: Haslemere, UNITED KINGDOM
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To reduce the likelihood of tip stalling you need to have a wing in which th etips are effectively working at a lower AoA than the root, either by physical means (wash-out or reflex on inset ailerons which are not full span) ailerons or by having a less cambered section at the tips than at the roots.

If you have a parallel chord wing with constant width strip ailerons I do not see how reflexing them can be seen as a measure against tip stalling - you have surely simply changed the aerofoil of the whole wing to one with less camber.

Mike

(in reply to a088008)

more thoughts on reflex - 3/8/2003 9:37:41 PM

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The problem is that when you reflex the ailerons, you do cut the lift at the tip, but the "negative flap" also cuts the lift the airfoil CAN make.
Stall happens when you ask the airfoil to make more lift than it can,
so....

What happens in most cases is that you get a premature stall near the inboard end of the reflexed airfoil. Now, if it was going to stall right at the tip, this is an improvement, but if it was going to stall at the root, its worse.

Jack

(in reply to a088008)

In most cases? - 3/8/2003 9:57:03 PM

Johng

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The possibility that Vortex00 put forward is possible I guess, but it is remote and would depend on the specific setup . It will not happen "in most cases" The root problem is this statement:

[QUOTE]Stall happens when you ask the airfoil to make more lift than it can[/QUOTE]

The only thing that governs an airfoil stall is angle of attack. By reflexing the aileron, you lower the local angle of attack relative to the inboard wing, shifting the load to the inboard section. Also, reflexed airfoils tend to stall at higher angles of attack, while creating lower lift. This effect is even greater if inboard flaps are deployed. The flapped section will almost certainly stall first. In most cases, this is how it works.

_____________________________

John

(in reply to a088008)

Preventing tip stall with aileron reflex? - 3/8/2003 11:16:00 PM

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By reflexing the aileron, you lower the local angle of attack relative to the inboard wing, shifting the load to the inboard section<<

Johng

In the case of inset ailerons yes. In the case of tapered strip ailerons I think so - but not in the caase of parallel chord wings with constant chord strip ailerons. All you are doing is changing the wing section.

Mike

(in reply to a088008)

Preventing tip stall with aileron reflex? - 3/9/2003 1:12:00 AM

Johng

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OK, I haven't been able to download any pictures of this thing. Does it have strip ailerons, or outboard ailerons or what?

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John

(in reply to a088008)

Link to pictures - 3/9/2003 10:19:40 AM