jet engine gyro effect
 

Hello everybody

Having flown jet aircraft for almost 10 years, it appeared to me that gyro spin effect was only on propeller driven engine.
Now I know that jet engine (high or low bypass jet engine) are subject to this phenomenum even if thrust vector is much higher than spin gyro.
Anybody has any clue on that please?

Yeah, engines twisted off due to that effect, aircraft lost (B720) and I personally remember one poor guy who accelerated the two hughe blowers of an A310 too much when he was about to enter the runway. Found himself in the grass.

Has never come up in my years of training.

With the weight if the big fans & comps. there must be some effect.

Look forward to the comments. ;)

AA 587 out of JFK in 03 demonstrated the forces that separated both engines of the A300 because of extreme yaw.

The question is
Is there a gyro effect on a jet engine such as the prop engine
If so what is this effect in terms of vector force compare to thrust force?

Need to be careful in comparing a vector with a force, they are not the same.
But the gyro direction is perpindicular to the long axis of the fan/prop shaft. It also follows the right hand rule which is 90 degree out of phase with any lateral motion of the fan shaft. It does not respond to fore or aft motion of the shaft (thrust), however it does generate force in response to RPM, but not torque, but only in combination with a lateral motion of its shaft.
OK, so if the fan/prop shaft is held on its bearings, and the bearings are held rigidly within the engine cases, which are held firmly on the pylon mounts and you don't horse the aircraft arround enough to break off its tail or wings, then the gyro forces are quite minimal and almost never show up in literature having to do with commercial aviation.
Comparatively speaking the gyro forces are very minor with the small diameter engines but very significant on the large diameter fans should you happen to break off an engine while its running at very high power and allow it to take its own pitch or yaw attitude free of the wing.

On the other hand failed engines run down (sometimes abruptly0, and while generating torque, they do not generate significant gyro loads.

Yes, it is a problem, though only (in general) if you spool up prematurely whilst entering the runway. It puts massive loads on the pylons and can cause an embarrasing moment in the flight deck, too.

Avoid it by making sure you are fully lined up before advancing the levers beyond, say 40%.

The Pegasus in the P1127 and later the Harrier was, and is, the result of true genius.

Just simply contra-rotating the low pressure and high pressure compressor spools (each with attached turbines) is not enough. It's the product of each assembly's moment of inertia with its angular rotation speed, its angular momentum, that has to be equal and opposite to the other - so that the net angular momentum of the engine is zero.

Because the two spools rotate at different speeds, they had to be designed so that their moments of inertia came out at the right values for this to be the case in the hover. Moreover, this principle had to be maintained throughout the engine's development and thrust growth.

A design triumph, thanks to Sir Stanley Hooker and team.

You beat me to it D120A! Yep, the pegasus is a fantastic bit of kit - when you watch the abruptness of some of the manoeuvres performed in the hover (when theres no air flow over the aircraft to damp it all out), you can guess at the results if both spools were spinning in the same direction.. Genius.

Gyro effect was a real problem in the design of the large-fan engines powering the B-777 (such as the GE-90, RR 8xx, and PW-4090/98).
With the size of the fan, a simple turn onto a runway puts a heavy strain on the fan shaft and bearings.
Problems are to be solved so we have these engines working quite well today.

Many Jet fighter planes having Rolling G limits and it is because of this effect.

The F100 had a MASSIVE rotating mass in the engine and many aircraft were lost early in its service life from Gyroscopic forces. Later models received a much larger Rudder and vertical stab to counter this. But early models had a lot of unexplained departures from controlled flight that werer later chalked up to gyropscopic forces.

F-4 phantom as I recall had a rather low rolling G limitation to keep from breaking the motor mounts from gyroscopic forces.

Cheers
Wino

Rolling g limits are actaully more to do with a phenominum known as 'yaw divergence'. A rapidly rolling aircraft, if pitched slightly will develope two c of gs, one at each end of the fuselage. The resultant moment's of inertia have an effect on the longitudianal axis and the airaraft will pitch further causing loss of control.

Many early jet fighters were lost to this before it was understood. IIRC, the Hawk has a rolling limit of one and a half turns because of yaw divergence.

For what it's worth the latest Trents (900 and 1000) have contra-rotating shafts but I believe that this has been done for aerodynamic efficiency rather than gyroscopic effect mitigation. Any engine designers out there, I'm just a controls person.

VnV.

The internal advantage with counterrotating rotors lies in the aerodynamic coupling between the HP and LP turbines. One stator (nozzle) stage can be eliminated, thus less pressure loss and lower weight.
Gyro loads, and worst-case rotor-seizure loads, are also reduced - permitting further weight reduction. :)

Thanks, Barit, sounds like a win/win for counter rotation then!


VnV

if you ever have the chance of watching a ground take-off power run from close, you will see that torque is there. The engine really dances on his pylon and the wings will go up and down. I think most vibrations and torque forces are taken up by the pylon-engine connection. (pylon-wing connection being more rigid but having shear bolts).

There's a world of difference between high power engine ground runs without aerodynamic damping effects and the like, and the way the engine runs on the wing in flight. :hmm:

I can't remeber the maths now - it was a long time ago but I do remember in Mechanical Engineering at Uni having to do two calculations about gyro effects.

One was how much a prop aircraft would pitch in a turn due to precession.

The other on was a frigate with a gas turbine with a shaft length of x, turning at y rpm while the ship was tuning at z deg/s and having to calcualate the forces in the shaft and on the bearings.

The forces are definately there.

haven't we left out the very critical parameter germain to fan engine discussions like diameter or polar moment of inertia :confused:
Just as an aside, the bearings and bearing supports in a gas turbine can take some pretty high loads for a short time, but the nut holding the bearings and shaft together has a real difficult time when the gyro load precesses and tries to unscrew it like a tyre iron releasing the bead from a tyre rim

read in article once that the F15 turbofan was designed with cr dual shaft engines to counteract engine torque tendencies. My .02$