[Jan F]

Jan F  asked the Naked Scientists:
   
Hi Chris & Team,

You recently had a very interesting feature on jet engines. I have a related question:

How does a plane cope with the gyroscopic forces generated by its jet engines? My little toy gyroscope is surprisingly hard to topple. How does an airframe handle these forces when it turns? Do they cancel each other out?

Love the podcast!

Cheers,
Jan F

What do you think?

[Johann Mahne]

Jan,
You are referring to the angular momentum of the jet engine. This depends on the diameter of the jet engine and it's mass and how fast it's spinning.
 L=RXMV for a particle and L=∑RXMV for the whole engine where ∑R is the sum of all the particles.
  Jet engines are normally long and narrow, (well they have to be for aerodynamics), and their mass has to be relatively low so that the jet can get lots of lift. Their construction is also designed to be light and strong.
  So even if they have do have lots of angular momentum, my guess is, that it's kept to a mimimum by the aeronautical engineers.

[wolfekeeper]

So far as I am aware aeroengines nearly always all spin in the same direction, so that the parts are interchangeable. It will affect turning to some degree, the aircraft will turn more easily one way than the other, but it's not a big deal- the angular momentum isn't so very large, and aircraft don't turn so very quickly, and the wings have a long lever arm, and the bearings and mountings of the engine can handle the loads generated.

On Concorde though they had a minor problem with (just) the right hand engine because of the direction of spin; the vortex from the wing entered the engine in the opposite direction from the rotation- this caused issues at takeoff, it ran rough, so they throttled it slightly differently, but it wasn't a gyroscopic issue.

[syhprum]

I wonder if it is a problem on high perfomance fighter aircraft that are designed to be very agile making 9g turns.

[martys2002]

I stumbled upon this interesting video. It shows small a one man jet propelled vertical take off flying vehicle. I guess. the WASP.

[martys2002]

I just found this other video.

It is hard to hear but they explain how it works, and it doesn't say any thing about any gyroscopic forces.

[CliffordK]

  Jet engines are normally long and narrow, (well they have to be for aerodynamics), and their mass has to be relatively low so that the jet can get lots of lift. Their construction is also designed to be light and strong.
  So even if they have do have lots of angular momentum, my guess is, that it's kept to a mimimum by the aeronautical engineers.

The Trent 900 series engines used on the A380 Airbus.
Compressor Fan Diameter: 116" (nearly 10 feet)
Length: 179"
The engine weighs about 14,000 lbs, or 7 tons.
The 116" fan operates at 3,000 RPM, with the outer edge turning at about 1.5 x the speed of sound.
The high pressure compressor runs at 12,500 RPM, with the outer edge running at about 1,200 MPH.



They are massive engines.
Nothing light or small about them.

They do mention a contra-rotating HP spool internal to the engine, perhaps it helps somewhat with stability.  I don't see any mention on whether engines on opposite sides of the aircraft have the same or opposite rotation. 

Anyway, I can't see any way they could avoid some gyroscope effects, but the whole A380 jet is HUGE too.