Blackbird
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Torsional-Agility, High-Alpha, And EM Formula

Thu Aug 28, 2008 3:23 am

I'm wondering how having torsional-agility/super-maneuverability allows an aircraft with a light wing-loading, for a given thrust/weight-ratio, to sustain a given number of G's at a higher airspeed than would normally be allowed had the airplane not possessed torsional-agility/super-maneuverability?

Doesn't flying at high-alphas actually increase drag a lot especially as the alphas you're achieving exceed 45-60 degrees and approach 90-degrees? Also, wouldn't the lift vector be, at least partially at such high alphas in opposition to the direction of flight (ie. the force of lift is pulling up somewhat but also backwards as the plane's moving forward, which would provide a pulling force partially against the direction of flight, producing drag)


Blackbird
 
tdscanuck
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RE: Torsional-Agility, High-Alpha, And EM Formula

Thu Aug 28, 2008 6:06 am



Quoting Blackbird (Thread starter):
I'm wondering how having torsional-agility/super-maneuverability allows an aircraft with a light wing-loading, for a given thrust/weight-ratio, to sustain a given number of G's at a higher airspeed than would normally be allowed had the airplane not possessed torsional-agility/super-maneuverability?

As I understand torsional-agility/super-maneuverability, you tailor the aeroelastic response of the wing so that it unload as the G's increase. Basically, the wing protects itself by aerodyanmically unloading, so you don't need to carry as much strength. Sort of a souped up version of gust-load alleviation on commercial airliners.

Quoting Blackbird (Thread starter):
Doesn't flying at high-alphas actually increase drag a lot especially as the alphas you're achieving exceed 45-60 degrees and approach 90-degrees? Also, wouldn't the lift vector be, at least partially at such high alphas in opposition to the direction of flight (ie. the force of lift is pulling up somewhat but also backwards as the plane's moving forward, which would provide a pulling force partially against the direction of flight, producing drag)

Lift is, by definition, the force perpendicular to the freestream. Drag is, by defintion, in the direction of the free stream. So lift can never cause drag; they're orthogonal. That said, the wing actually experiences only one composite force, we just resolve it into lift and drag for convenience. As your alpha goes up the force on the wing goes up...both drag and lift increase, until you stall, at which point lift drops off but drag keeps going up.

Tom.
 
Blackbird
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RE: Torsional-Agility, High-Alpha, And EM Formula

Thu Aug 28, 2008 6:18 pm

Tom,

Wouldn't that mean that supermaneuverability and torsional agility are not exactly the same? I've seen old planes that were totally unstable doing crazy high-alpha maneuvers...

What allows the wings to unload?


Blackbird
 
tdscanuck
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RE: Torsional-Agility, High-Alpha, And EM Formula

Fri Aug 29, 2008 12:59 am



Quoting Blackbird (Reply 2):
Wouldn't that mean that supermaneuverability and torsional agility are not exactly the same?

I suspect they're not...what definitions are you using for each term?

Quoting Blackbird (Reply 2):
I've seen old planes that were totally unstable doing crazy high-alpha maneuvers...

That sounds right to me.

Quoting Blackbird (Reply 2):
What allows the wings to unload?

In a nutshell, having the right aeroelastic derivatives be negative. Specifically, you don't want to get into a positive feedback situation where loading of the wing distorts the wing in a way that increases the loading. If deflection tends to decrease wing loading (or lower the rate of load increase) the wing has some innate protection.

Tom.
 
Blackbird
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RE: Torsional-Agility, High-Alpha, And EM Formula

Fri Aug 29, 2008 2:37 am

Tom,

What does negative aeroelastic characteristics look like?

What kind of distortions increase loading and what kind of distortion forms decrease loading?


Blackbird
 
tdscanuck
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RE: Torsional-Agility, High-Alpha, And EM Formula

Fri Aug 29, 2008 5:38 am



Quoting Blackbird (Reply 4):

What does negative aeroelastic characteristics look like?

I'm not really sure what you mean "look like"...they're numbers, basically. You can't tell by looking at the structural form what it's going to do, you'd have to analyze it.

Quoting Blackbird (Reply 4):
What kind of distortions increase loading and what kind of distortion forms decrease loading?

The classic example is a swept wing...the center of pressure and the shear center aren't usually in line, so as the wing bends under load it also twists. In a conventionally swept wing, unless you do weird things to the internal structure, the wing tends to "twist down", towards lower angle of attack, so as the wing load increases (for example, tightening a turn) the angle of attack decreases and everything stays nice and stable.

If you are in the opposite situation, like a forward swept wing or some of the early straight-wing fighters when they got going too fast, the wing "twists up" as it loads up. The angle of attack increases, which increases the lift, which increases the load, which increases the twist, which increases the angle of attack. You end up in a positive feedback loop that can, literally, rip the wing off.

Tom.
 
Blackbird
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RE: Torsional-Agility, High-Alpha, And EM Formula

Fri Aug 29, 2008 4:07 pm

I thought all wings twisted leading edge down as lift increased...

Blackbird
 
tdscanuck
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RE: Torsional-Agility, High-Alpha, And EM Formula

Sat Aug 30, 2008 2:18 am



Quoting Blackbird (Reply 6):
I thought all wings twisted leading edge down as lift increased...

It depends on the internal structure (shear center) and the airfoil. For obvious reasons, you want to design this way, but you can design them so that they don't.

Tom.
 
Blackbird
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RE: Torsional-Agility, High-Alpha, And EM Formula

Sat Aug 30, 2008 2:20 am

Tdscanuck,

And the fact that the airfoils twist-down in response to load would provide good torsional agility?


Backbird
 
Blackbird
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RE: Torsional-Agility, High-Alpha, And EM Formula

Mon Sep 15, 2008 1:18 am

Tom,

Please tell me if I'm wrong here -- pertaining to maneuverability, it would seem that torsional agility's purpose primarily to reduce lift at higher airspeeds during maneuvers to avoid excessive lift (which on a lightly-loaded wing equates to more drag at high-speed) effectively reducing the drag that goes with the extra lift (L/D ratio -- basically, when you add more L, more D ends up going with it) allowing a lower loss of airspeed than would typically occur (for a typical wing of that area, at these higher speeds for a given number of G's pulled.

Whether torsional agility would reduce structural weight, I'm not sure, but it's primary function seems to be to allow airplanes with larger wings to be able to sustain more G's at higher airspeeds, while still functioning well at lower airspeeds.


Blackbird

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