Originally Posted by
Turbine D
megan,
I assume the "at cruise" to be above Mach 3 in which case most of the compression is coming from the inlet. I assume that was what Graham was conveying.
Axial loading on rotors do reverse with thrust settings, acceleration (pouring on the coals) and deceleration (hitting the brakes). These changes have to be accounted for in the design of the rotor components. The rotors even move slightly in the axial direction forward and aft as a result of acceleration or deceleration, so spacing between the stationary vanes and rotors has to be calculated plus a safety margin to prevent clashing between rotors and stators in either direction.
Thrust bearings are of a ball bearing design of which there are several types. They are designed to be contact as uniformly as possible and never free floating. Roller bearings are not used in the axial direction, but are used in some situations in a radial direction.
1. The rotors (sic) move slightly (axially) fore and aft. There is spacing between Stators and blades.
2. This movement is precipitated by acceleration and deceleration.
3. If there is a constant positive flow against the turbine, there will be no float.
4. The turbine is parasitic, removing only enough power to drive the compressor, by design.
5. If at any time the inlet “overpowers” the compressor, by eliminating the “suck”,
6. The drive is overpowered by the inlet, the parasitic turbine is “removed” from the flow now either in free stream,
7. Or in “compression’, now actually providing thrust, rather than removing it.
8. The net thrust (rotor) is forward.
9. The thrust (on the rotor) has been reversed, and the bearings must have unloaded, and are now loaded in the opposite direction.
OR,
Colonel Graham is mistaken, thinking a reduction in positive thrust amounts to a reversal into the (net) negative.
No?
Do you have an analysis of unstart? Seems to me this system is extremely sensitive to variations in thrust, and homogeneity of inlet compression?
So, in my simple mind, I want to know if Graham is talking about a “reduction” in thrust that causes “negative” only in “net”? Or, an actual reversal of vector of net rotor thrust, a
reversal.
If the rotor is pushed into the aft face of the forward bearing, instead of the usual forward face of the aft bearing, as the result of a reversal, then by definition there is “float” between faces, however undesirable that may be?
Bear in mind, the rotor is expected to be independent of the gas path as a whole, by definition as to variation of gas path flow. Within the limits of the thrust bearings.
It seems to me this reversal is not and should not be “catastrophic”. It merely means that for brief periods, the function of the rotor “flips”; it still remains in the gas path, its relative thrust doesn’t effect any substantial changes in the engine’s performance. It is the inlet that is the critical player.
You claim there is movement between stators (attached to the case) and blades of the rotor.
Stopping the rotor short of clash is the job of
both thrust bearings. You know there have to be at least two? Unless the design requires one bearing to protect both directions of thrust?
imo