Vmcg on high altitude airstrips
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Turbo-Props deliberately left out of the summary as you can play tricks with them (like injecting Water Methanol) to restore power loss with increasing Altitude and/or temperature
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Hi Jaguar Pilot,
Boy oh boy, we did go off at a few tangents here, all good clean fun!
For the normally aspirated piston engine, provided that full throttle is used for Takeoff, power, and therefore thrust output, decreases as Pressure Height increases. Thus, ACTUAL Vmcg and Vmca will decrease as Altitude increases.
As mentioned earlier the AFM value of Vmcg and Vmca is fixed at the 'worst case' value, unless, as Mad (Flt) Scientist has indicated, the manufacturer has sought some alleviation for decrease at higher altitudes.
Thus, if we're talking of examination questions, one must be very careful to ascertain whether the examiner is asking after the ACTUAL, or the Certified AFM value of Vmca/Vmcg.
The "provided that full throttle is used for Takeoff caveat was added earlier just in case some normally aspirated engine, somewhere, is limited to less than full throttle, e.g. 27" Manifold pressure instead of the near 30" available at Sea Level. That instance will be similar to a 'Flat Rating' up until the altitude where (for the example) 27" is no longer available.
Regards,
Old Smokey
Boy oh boy, we did go off at a few tangents here, all good clean fun!
For the normally aspirated piston engine, provided that full throttle is used for Takeoff, power, and therefore thrust output, decreases as Pressure Height increases. Thus, ACTUAL Vmcg and Vmca will decrease as Altitude increases.
As mentioned earlier the AFM value of Vmcg and Vmca is fixed at the 'worst case' value, unless, as Mad (Flt) Scientist has indicated, the manufacturer has sought some alleviation for decrease at higher altitudes.
Thus, if we're talking of examination questions, one must be very careful to ascertain whether the examiner is asking after the ACTUAL, or the Certified AFM value of Vmca/Vmcg.
The "provided that full throttle is used for Takeoff caveat was added earlier just in case some normally aspirated engine, somewhere, is limited to less than full throttle, e.g. 27" Manifold pressure instead of the near 30" available at Sea Level. That instance will be similar to a 'Flat Rating' up until the altitude where (for the example) 27" is no longer available.
Regards,
Old Smokey
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(I too have a pedant's license, right here in my wallet...)
Inasmuch as Vmc (a or g) is a function of assymetric thrust, and not necessarily engine shp, then propeller efficiency must be part of the equation. While I have no specific data at hand, I can readily conceive of a propeller optimized for higher altitude takeoffs, and thus sub-optimum at SL.
And gearbox limitations are common on turboprops, making them flat-rated up to an altitude where the gas generator no longer can overtorque the gears. Not sure if any recips have this limitation.
Inasmuch as Vmc (a or g) is a function of assymetric thrust, and not necessarily engine shp, then propeller efficiency must be part of the equation. While I have no specific data at hand, I can readily conceive of a propeller optimized for higher altitude takeoffs, and thus sub-optimum at SL.
And gearbox limitations are common on turboprops, making them flat-rated up to an altitude where the gas generator no longer can overtorque the gears. Not sure if any recips have this limitation.
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Not sure if any recips have this limitation.
Examples...R4360, R2800, both Pratt&Whitney.
CurtisWright, R3350 including the turbocompound series.
Speaking of the R4360, it is, without a doubt, the smoothest piston engine I have ever flown, almost turbine-like.
These were on the Stratocruiser, a typical example of a Flight Engineers airplane.
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If I could throw in a spot more pedantry to this debate and in particular OS's very wise move in his summary to ignore turboprops because:
......you can play tricks with them (like injecting Water Methanol) to restore power loss with increasing Altitude and/or temperature.
Not only can you restore lost power with water meth, some turboprop engines allow you to boost power (as opposed to restore) using water meth and thus it is possible to have 2 sets of performance figures and speeds etc for the same configuration and conditions ie with and without w/m.
Switches back to pragmatism,
Cheers,
mcdhu
......you can play tricks with them (like injecting Water Methanol) to restore power loss with increasing Altitude and/or temperature.
Not only can you restore lost power with water meth, some turboprop engines allow you to boost power (as opposed to restore) using water meth and thus it is possible to have 2 sets of performance figures and speeds etc for the same configuration and conditions ie with and without w/m.
Switches back to pragmatism,
Cheers,
mcdhu
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411A, I once had a boss who was ex-BOAC, served as F/E on several types including the B377. He wasn't very impressed with its performance, but would agree about the 4360.
