question about flight model - sideway G force

[+malibu43 142]

This is exactly correct. Landings are done in cross control at touchdown...most heavy aircraft cannot handle large sideways loads unless they have steerable landing gear. It is easier to simply do a wing low landing and let one main gear touch first. Aircraft may fly on final in a crab (ie sideways, wings level) to maintain ground track, but then transition to the wing low method in the flare, or accept a small sideways load for light crosswinds.

 

Now, lets talk about the 'knife edge pass'. What makes you assume that a stock aircraft, even a fighter type aircraft can maintain altitude on a knife edge pass? You'll note in some aircraft, that pass is actually started in a climb, then rolls to 90 degrees bank, and even with full rudder deflection, the nose slowly tracks downward. What the crowd sees is an illusion caused by looking slightly upward as the aircraft traverses the showline.

 

For, other aircraft, they are slightly modified to exceed their normal rudder authority at high speed. High yaw angles, especially in aircraft with long, snaking intakes, are not something jet engines enjoy, and so at higher speeds rudder authority is actually reduced. This even occurs in jetliners for the same reason (and to avoid high sideloads). For instance, in the T-38A, which when it used to be in the Thunderbirds, would do knife edge passes. However, the stock T-38A could not hold altitude long enough to complete such a pass at the rudder authority of 6 degrees. The rudder actually had the ability to go a full 30 degrees, but that was only with the landing gear down. However, for the Thunderbirds, that limiter was removed in order to be able to do things such as a knife edge pass...while still being able to say the T-birds used a 'stock' aircraft. There isn't anything that says they haven't done the same thing for other aircraft.

 

Now, finally, lets talk about rudders in high speed, swept wing aircraft.

 

When any aircraft goes into a standard turn, more lift is created on the upper wing than the lower wing. This in turn creates more induced drag, which pulls the upper wing away from the turn (called adverse yaw). This can be countered by pro turn rudder, either manually, or automatically using flight control augmentation. Another way (which a lot of aircraft have done since the 1930s) is by deflecting the ailerons asymmetrically (sometimes called frise ailerons), increasing the deflection of the downward wing's aileron to increase the drag to match the upward wing's induced drag to coordinate the turn.

 

Next, when swept wing aircraft are put into a yaw, the forward moving wing presents more leading edge toward the relative wind, increasing lift, which increases induced drag, and increases frontal drag, both of which contribute to the lateral stability of the aircraft by tending to take out the yaw.

 

So, after that explanation, lets talk about the situation where you use full rudder to try to turn a swept wing fighter, while using cross aileron to keep the wings level.

 

First, rudder authority may have already been limited by the hardware or software at higher speeds. Secondly, the inherent drag properties of the forward wing in a yaw situation contribute to additional drag that wing is exerting against the turning force of the rudder. Third, the differential aileron you are using to keep the forward wing from rising is also adding more drag on that wing then the rearward wing. Fourth, the form drag of the air over the fuselage sideways will try to force the nose against the direction of yaw. All of these forces could easily add up and counter the rudder deflection and reduce the ground track change to trival levels, if any.

 

So, the answer is that there may not be enough rudder authority to change the ground track in the situation you stated in real life. It depends on the aircraft and specific situation.

 

FC

 

BEST. RESPONSE. EVER.

[Guest mlracing]

how about engine thrust -- yaw'ed.

something pushing the aircraft the opposite way to counteract the engine thrust?

[Guest mlracing]

and don't tell me that engine thrust can't NOT be yaw'ed just because there are only trim up/down, no trim right/ left available in SF series.

[Fubar512 1,350]

X-axis vectored thrust is not an option at this time in either TW series. There are, however, a few workarounds.....

Edited November 11, 2010 by Fubar512

[Fubar512 1,350]

F-15 Active, video from February of last year, TV in vertical-axis, "simulated" (workaround) TV in yaw-axis: http://combatace.com/topic/52317-day-of-the-cheetah/page__view__findpost__p__383752

[Stick 773]

This whole thread is hilarious!

[Derk 265]

This is exactly correct. Landings are done in cross control at touchdown...most heavy aircraft cannot handle large sideways loads unless they have steerable landing gear. It is easier to simply do a wing low landing and let one main gear touch first. Aircraft may fly on final in a crab (ie sideways, wings level) to maintain ground track, but then transition to the wing low method in the flare, or accept a small sideways load for light crosswinds.

 

Now, lets talk about the 'knife edge pass'. What makes you assume that a stock aircraft, even a fighter type aircraft can maintain altitude on a knife edge pass? You'll note in some aircraft, that pass is actually started in a climb, then rolls to 90 degrees bank, and even with full rudder deflection, the nose slowly tracks downward. What the crowd sees is an illusion caused by looking slightly upward as the aircraft traverses the showline.

 

For, other aircraft, they are slightly modified to exceed their normal rudder authority at high speed. High yaw angles, especially in aircraft with long, snaking intakes, are not something jet engines enjoy, and so at higher speeds rudder authority is actually reduced. This even occurs in jetliners for the same reason (and to avoid high sideloads). For instance, in the T-38A, which when it used to be in the Thunderbirds, would do knife edge passes. However, the stock T-38A could not hold altitude long enough to complete such a pass at the rudder authority of 6 degrees. The rudder actually had the ability to go a full 30 degrees, but that was only with the landing gear down. However, for the Thunderbirds, that limiter was removed in order to be able to do things such as a knife edge pass...while still being able to say the T-birds used a 'stock' aircraft. There isn't anything that says they haven't done the same thing for other aircraft.

 

Now, finally, lets talk about rudders in high speed, swept wing aircraft.

 

When any aircraft goes into a standard turn, more lift is created on the upper wing than the lower wing. This in turn creates more induced drag, which pulls the upper wing away from the turn (called adverse yaw). This can be countered by pro turn rudder, either manually, or automatically using flight control augmentation. Another way (which a lot of aircraft have done since the 1930s) is by deflecting the ailerons asymmetrically (sometimes called frise ailerons), increasing the deflection of the downward wing's aileron to increase the drag to match the upward wing's induced drag to coordinate the turn.

 

Next, when swept wing aircraft are put into a yaw, the forward moving wing presents more leading edge toward the relative wind, increasing lift, which increases induced drag, and increases frontal drag, both of which contribute to the lateral stability of the aircraft by tending to take out the yaw.

 

So, after that explanation, lets talk about the situation where you use full rudder to try to turn a swept wing fighter, while using cross aileron to keep the wings level.

 

First, rudder authority may have already been limited by the hardware or software at higher speeds. Secondly, the inherent drag properties of the forward wing in a yaw situation contribute to additional drag that wing is exerting against the turning force of the rudder. Third, the differential aileron you are using to keep the forward wing from rising is also adding more drag on that wing then the rearward wing. Fourth, the form drag of the air over the fuselage sideways will try to force the nose against the direction of yaw. All of these forces could easily add up and counter the rudder deflection and reduce the ground track change to trival levels, if any.

 

So, the answer is that there may not be enough rudder authority to change the ground track in the situation you stated in real life. It depends on the aircraft and specific situation.

 

FC

 

Great answer and very instructive !!!. Wonder about one thing: in the years we operated the NF 5 in the KLu, there were wonderful demo's flown all over Europe during the season (with special paint jobs and smokewinders) where the empty central pylon tank was always used generating lift during the long and spectacular knife edge passes that were part of the program. It must have added quite a bit to the available lift.and thus may have reduced the sideways AoA and probably the necessary amount of rudder. Have you got any idea if increased rudder authority was used ? In any case it was always stated that the NF5 was very well suited to knife edge flying. What I do know is that it posed considerable strains on the airframe and the demo plane was changed every year or so for that reason.......

 

Hou doe,

 

Derk

Edited November 11, 2010 by Derk

[FastCargo 412]

and don't tell me that engine thrust can't NOT be yaw'ed just because there are only trim up/down, no trim right/ left available in SF series.

 

How about losing the attitude and just tell us what you are trying to achieve and we'll tell you how it can be done.

 

FC

[Guest mlracing]

How about losing the attitude and just tell us what you are trying to achieve and we'll tell you how it can be done.

 

FC

 

never mind.

i just felt amazed.

[FastCargo 412]

Great answer and very instructive !!!. Wonder about one thing: in the years we operated the NF 5 in the KLu, there were wonderful demo's flown all over Europe during the season (with special paint jobs and smokewinders) where the empty central pylon tank was always used generating lift during the long and spectacular knife edge passes that were part of the program. It must have added quite a bit to the available lift.and thus may have reduced the sideways AoA and probably the necessary amount of rudder. Have you got any idea if increased rudder authority was used ?

 

Well, our resident F-5 driver Jug said that the F-5E/F model did not have reduced rudder authority, but had the full 30 degrees available, so I'd imagine the NF-5 had it too.

 

FC

[Derk 265]

Well, our resident F-5 driver Jug said that the F-5E/F model did not have reduced rudder authority, but had the full 30 degrees available, so I'd imagine the NF-5 had it too.

 

FC

 

 

THx FC & Jug !!

 

Hou doe,

 

Derk