Wing heaviness VS IAS

[Mr.Vortex]

Hi all,

I'm reading th F-86 Manual and it said that the wing heaviness or the roll
capability of the aircraft tend to decrease at high IAS or Mach no

Why that happened? If the airspeed increase the aileron should be able to
create more lift or downforce than the low airspeed. Or is it cause by the
shockwave on the aileron? But if the aircraft fly around sea level which
it would take a lot of airspeed to reach around the M1.0 it shouldn't be a
problem.

Also if i recall correctly, the reason why the B747 have the inboard aileron
instead having the only outboard aileron is to decrease the roll rate at cruise
speed. If that is the reason why the A330-300 didn't have the inboard aileron?
Or is it due to the shockwave on aileron that cause the structure problem?

Thanks for all reply.

[BarbiesBoyfriend]

Mr Vortex

Inboard ailerons are there because at high IAS, when you move the ordinary aileron, instead of rolling the a/c, it simply bends (flexes) the wing tip in the opposite direction. I think it's called 'aileron reversal'. The inboard ailerons are mounted on a very thick almost bendable inboard section of wing.

Most a/c have reduced roll rates and heavier aileron forces at high IAS.

Someone more knowledgable than me will soon explain exactly why.

[Mr.Vortex]

Thanks for your reply.

So as the airspeed increase the force that aileron produce to cause the roll was
losing to the bend the wing more than it roll the aircraft. Is that right?

Best regards

[ImbracableCrunk]

F-86 was one of if not the first jet to have

hydraulic power- operated irreversible controls with “artificial feel” for all movable horizontal tail and ailerons

The Sound Barrier and the Evolution of the F-86 Sabre

That could be the reason.

[Mr.Vortex]

Hi there,

Thanks for your reply. But why irreversible flight control have the effect on roll rate?

Thanks for reply

Best regards

[Pugilistic Animus]

But why irreversible flight control have the effect on roll rate?

Mr. Vortex I'm not sure exactly what you mean?

Generally the dynamic pressure feedback is used to create a natural feeling force increase with speed in order that a pilot not inadvertently overstress the airplane ...


let me guess---you want to design iPods?---

[ImbracableCrunk]

Generally the dynamic pressure feedback is used to create a natural feeling force increase with speed in order that a pilot not inadvertently overstress the airplane

That's my guess. Maybe the control system trades a reduced rate of roll to prevent control reversal. The wing/aileron isn't constructed to be unable to reversed, but the feedback system limits you so can't get into that region and thus you get a reduced roll rate.

That's just my guess, though.

[galaxy flyer]

"Aileron reversal" was a characteristic of the B-47, the first large-scale production plane with aeroelasticity designed into the wing structure. Being flexible, at high IAS, the action of the ailerons would twist the wing to the degree that would overcome the desired roll input. When descending into low level routes, the pilot would allow the airspeed to build to a point where aileron inputs became ineffective at producing roll in the desired direction due to wing flex. The route was then flown at a speed below that. Each plane was rather individual in it's flexing, within a narrow range around 410 KIAS. The B-52, I believe used spoilers to overcome this problem, having very flexible wings.

Most Boeing airliners (707, 727 and 747, 757, 767, I can't say about the 737 or 777) had inboard and outboard ailerons, the outboards locked in trail after the leading edge devices were retracted. I doubt very much if Boeing was anticipating high IAS at low levels, however. The planes did need the roll authority in normal operations provided by outboard ailerons.

GF

[Denti]

The 737 does not have inboard ailerons, unlike all the other boeings. However ailerons are of course spoiler assisted and usual roll rates in commercial air traffic are pretty slow.

[SMOC]

757 doesn't have I/B ailerons either, the wing is stiffened to prevent aileron reversal, same on the A330/340. Which is usually why the ride is a little rougher.

I think the A380 gets around the aileron reversal with some clever manipulation of the 3 O/B ailerons as they move independently to each other at times, which has been discussed in the tech forums.

[BOAC]

Mr V - the reasons for reduced roll effectiveness at high IAS/Mach numbers are many.

Two primaries being wing torsion as explained and loss of aileron effectiveness due to trans/supersonic airflow effects towards the tips - in some cases outboard ailerons can be almost ineffective. Plenty of reading around if you Google.

You can get deep into theory if you wish and look at modified roll damping in high subsonic flow as well but I suggest you don't

[Mr.Vortex]

Thanks for all of your clear reply.

Just a final question. When the aircraft with hydraulic flight control fly faster
, does the dynamic pressure press on aileron and reduce their deflection or
it has the same deflection as it was on the ground if the stick or conrtol column
was on the same position.

Best Regards

[BOAC]

The same unless a system of reduced/increased deflection with IAS is fitted. Hydraulically powered controls normally work at 3000psi.

[Mad (Flt) Scientist]

... or unless the hydraulic power control units don't have the force to overcome all of the airload. Even with 3000psi hydraulic pressure, that's a possibility since the PCU ram usually has a small piston area (so its a very high pressure but over a small area) and the PCU usually has only a small arm to apply the force (because you don't want a long lever that will protrude further into the airflow).

Since you don't want large control deflections at high speeds, being "hinge moment limited" isn't really a bad thing, but it does complicate other parts of the design if you are.