Forum: General F-22A Raptor forum

Look at the cool way the stabilators fit into the wing trailing edge flaps.

Steve Pace

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I'm guessing they did that so they could have large stabilators without having to lengthen the fuselage to fit them.

Actually this is for stealth. Tucking the unit up behind the wing flap provide protection to the stabilators. One of the Lockheed engineers pointed this out in an interview at the Paris airshow, it was in response to the Eurofighter's Canard and why the F-22 did not use them.

Both reasons seem like good ideas to me. The larger the control surfaces the less they have to deflect too, angle wise. It kind of makes you wonder about the canards on some proposed fifth generation fighter concepts.

It's a very large control surface, which helps in producing the torque needed to pitch or roll the airplane very fast. I'm not an aerospace engineer (I'm studying computer engineering) but from what I've heard and read... maybe another benefit to putting the stab 'nacelled' into the flap is that you get a little less drag. By definition you'd need a bigger stab to compensate for less drag in the straight and level flight regime.

Tinito_16 wrote:

By definition you'd need a bigger stab to compensate for less drag in the straight and level flight regime.

By whose definition?

I don't quite follow that one, Tinito. Care to explain it further?

wrend wrote:

The larger the control surfaces the less they have to deflect too, angle wise. It kind of makes you wonder about the canards on some proposed fifth generation fighter concepts.

Not really.
You need a certain amount of force to rotate aircraft around it's lift point/Cp. This depends on several factors. Total lift (area, cambering/lift coeff) and leverage (the distance from Cp) play important part.
Now, F22 has large elevators, but since they're so close (short lever) to the wing (and Cp), they need to be larger to be able to produce the required amount of force.
As for Eurofigter (for example) it can go with smaller foreplanes, since they're pretty far from the Cp.
Another thing is that the canard receives a steady boundary layer flow, unlike elevator situated just behind the wing. Don't have numbers on this one, but it surely knocks some efficiency off the elevator's envelope...

cola has it right. His explanation of tail area and moment arm is called tail volume (area x arm). And another reason for smaller canards on the Eurofighter is that it also has elevons to assist with pitch control. Tail volume is critical for low speed cases, like the takeoff rotation he mentions. That was one of the reasons for enlarging the Block 15 F-16 tail.

He is also almost right about the the wing affecting on tail effectiveness (not efficiency). Yes, the wing does affect the tail, but another big driver is the flap directly in front of the tail. A few years ago, I did an analysis of T-50 (Korean trainer) flight test data which showed that every degree of flap deflection reduced tail effectiveness about 1/3 as much as each degree of tail deflection. So if the tail is 5 degrees trailing edge down and the flap is 15 degrees down. the tail is totally ineffective. That is a subsonic case, and supersonic would probably be different.

johnwill wrote:

Tinito_16 wrote:

By definition you'd need a bigger stab to compensate for less drag in the straight and level flight regime.

By whose definition?

I don't quite follow that one, Tinito. Care to explain it further?

I'm not an aerospace engineer so bear with me Cool

. Part of what I meant, cola said:

Quote:

Another thing is that the canard receives a steady boundary layer flow, unlike elevator situated just behind the wing. Don't have numbers on this one, but it surely knocks some efficiency off the elevator's envelope...

Quote:

Now, F22 has large elevators, but since they're so close (short lever) to the wing (and Cp), they need to be larger to be able to produce the required amount of force.

From the little I know of statics and dynamics, this makes sense.

Quote:

He is also almost right about the the wing affecting on tail effectiveness (not efficiency). Yes, the wing does affect the tail, but another big driver is the flap directly in front of the tail. A few years ago, I did an analysis of T-50 (Korean trainer) flight test data which showed that every degree of flap deflection reduced tail effectiveness about 1/3 as much as each degree of tail deflection. So if the tail is 5 degrees trailing edge down and the flap is 15 degrees down. the tail is totally ineffective.

Interesting fact there. Are there any measures/design changes that fix this, or is it something you just 'live with'?

cola wrote:

wrend wrote:

The larger the control surfaces the less they have to deflect too, angle wise. It kind of makes you wonder about the canards on some proposed fifth generation fighter concepts.

Not really.
You need a certain amount of force to rotate aircraft around it's lift point/Cp. This depends on several factors. Total lift (area, cambering/lift coeff) and leverage (the distance from Cp) play important part.
Now, F22 has large elevators, but since they're so close (short lever) to the wing (and Cp), they need to be larger to be able to produce the required amount of force.
As for Eurofigter (for example) it can go with smaller foreplanes, since they're pretty far from the Cp.
Another thing is that the canard receives a steady boundary layer flow, unlike elevator situated just behind the wing. Don't have numbers on this one, but it surely knocks some efficiency off the elevator's envelope...

I was just generalizing. Larger control surfaces don't need to deflect as far as smaller ones would to get the same amount of force, in general. I wasn't comparing differing distances on multiple platforms in regards to their center of mass and lift.

My reference to canards had to do more with stealth, wondering if they were that good of an idea, or not.

Quote:

My reference to canards had to do more with stealth, wondering if they were that good of an idea, or not.

Well, apparently not because we've had a few experimental a/c use canards and it seems the cons outweigh the pros at least for that particular application (stealth).

Tinito16 said,

Interesting fact there. Are there any measures/design changes that fix this, or is it something you just 'live with'?

Placing the tail so close to the flap (as on F-22) is certainly a compromise, as all aircraft design is. Certainly, losing effectiveness is not desirable, but many other factors (LO, weight, cost, drag, etc) must have made that the best choice. To answer your question, you live with it. The tail may lose effectiveness, but if it still has enough, go with it. If not, make it bigger or otherwise modify it within all the compromises required.

From Code One magazine regarding the F-22's design:

"The structural engineers wanted a diamond wing because it provides a larger root chord, which carries bending moments better," Hardy notes. "The aerodynamicists wanted a trapezoidal wing because it provides more aspect ratio, which is good for aerodynamics. Dick Heppe, the president of Lockheed California Company, made the final decision, and he was right. The aerodynamics were not all that different, but the structure and weights were significantly better. So we went to a diamond shape. The big root chord, though, moved the tails back. Eventually we even had to notch the wing for the front of the tails. If the tails moved farther back, they would fall off the airplane."

Maybe the TVC more than compensates for any compromise.

Just an observation, not particularly revelatory, merely interesting: the F-22 has often been described as a tailed delta, not like the Mig-21, mind you, actually even more so, but as the LM engineer said, if the tails were moved farther back, they'd fall off the airplane. Try to picture the F-22 without its empennage (no stabs or tails). It would be a pure delta.

BTW, John, it would appear that part of the reason the stabs are set so far back is for greater leverage or torque, if you will, further from the CG. I am not at all an engineer but that is what I would imagine. Is that about right?

Forum: General F-22A Raptor forum

Stabilators