Aerodynamic Question re VNE
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Joined: Oct 2011
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From: New York
Aerodynamic Question re VNE
I fly an Enstrom 480B and have been mulling over why VNE decreases so much as a function of rearward center of gravity. Sea Level 0*C 2400lb gross weight VNE is 125k ias at full forward center of gravity, and 105 at full rearward. A couple of possibilities occur to me but none feel quite right:
1) Not VNE but actually estimate of how fast you will be going when you run out of forward cyclic. Experience has proven this not to be true.
2) Structural stress on mast given relatively forward tip plane path and relatively rearward mast orientation. Possible but attitude indicator shows decent nose down attitude at VNE speeds i.e. doesn't seem that severe an angle.
3) Flapping limitations given relatively forward tip plane path and relatively rearward mast orientation. Possible but as above, angle doesn't seem that severe.
4) Drag created by non-aerodynamic positioning of fuselage means you are putting more stress on the transmission at same speed. Certainly true, but I am only pulling about 55 torque when I hit VNE w/ rear center of gravity, and there are configurations that have you pulling in excess of 65 torque with more forward center of gravity while still below VNE. Only takes certain amount of energy to maintain fixed altitude at fixed weight, so everything else has to be forward pull I reckon. Transmission is capable of handling it.
Thoughts?
1) Not VNE but actually estimate of how fast you will be going when you run out of forward cyclic. Experience has proven this not to be true.
2) Structural stress on mast given relatively forward tip plane path and relatively rearward mast orientation. Possible but attitude indicator shows decent nose down attitude at VNE speeds i.e. doesn't seem that severe an angle.
3) Flapping limitations given relatively forward tip plane path and relatively rearward mast orientation. Possible but as above, angle doesn't seem that severe.
4) Drag created by non-aerodynamic positioning of fuselage means you are putting more stress on the transmission at same speed. Certainly true, but I am only pulling about 55 torque when I hit VNE w/ rear center of gravity, and there are configurations that have you pulling in excess of 65 torque with more forward center of gravity while still below VNE. Only takes certain amount of energy to maintain fixed altitude at fixed weight, so everything else has to be forward pull I reckon. Transmission is capable of handling it.
Thoughts?
Joined: Jul 2004
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I have no experience with Enstroms
But I'd go with number 2 and additionally I'd say it's about retaining rotor RPM in case of engine failure. With a disc that far forward and the mast relatively far back the rpm decay might be unrecoverable.
But I'd go with number 2 and additionally I'd say it's about retaining rotor RPM in case of engine failure. With a disc that far forward and the mast relatively far back the rpm decay might be unrecoverable.
Last edited by GoodGrief; 26th September 2013 at 15:26.
Thread Starter
Joined: Oct 2011
Posts: 7
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From: New York
Huh
Heard back indirectly from factory. Evidently the answer is... a variant of number 1. No issue with retreating blade stall (at lower altitudes) or with structural stress. Just that they don't want you to run out of forward cyclic in case you hit a gust and need some nose down to stabilize.
