Manufacturers angling engines to reduce VMCG"
For the Citation Sovereign.
"The engine tailpipes are angled out 4 deg. to reduce asymmetric thrust and minimum control speed. This results in lower contaminated-runway takeoff field lengths, especially a low takeoff weights." I thought angled inward would be do this but I guess outward is what works. Sovereign+ Builds On Cessna Marque?s Reputation As A Sensible Option | Business Aviation content from Aviation Week |
More tangential = more spinny = more bad.
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Originally Posted by JS
angled inward would be do this
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I get the theory, but assume more thrust per engine is required to get back to the original two engine performance? Seems a waste of energy/fuel for 95% of a flight.
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For the Citation Sovereign. "The engine tailpipes are angled out 4 deg. to reduce asymmetric thrust and minimum control speed. This results in lower contaminated-runway takeoff field lengths, especially a low takeoff weights." I thought angled inward would be do this but I guess outward is what works. |
I get the theory, but assume more thrust per engine is required to get back to the original two engine performance? Seems a waste of energy/fuel for 95% of a flight. IIRC Blackhawk helicopter has a similar arrangement with the tail rotor - it is angled by 6 degrees (or thereabouts), yielding a significant lift contribution for minimal loss of side force/tail rotor torque |
Angling the tailpipe out does not reduce the asymmetric thrust, it reduces the yawing moment due to the thrust asymmetry. F28 tailpipes were angled out for the same reason. C_Star is correct about sine and cosine. Actually the sine is beneficial with one engine out and the airplane yawed towards the live engine.
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VC10
On the VC10, the engines appear to be pointing in at rear. :confused:
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True - but we are talking a 50-year difference in understanding of nacelle-fuselage interference and a host of other little aerodynamic details, which may have changed in relative importance to designers over 5 decades.
All the little problems the Vickers guys were trying to address in '61 may be different from the mix Cessna is trying to juggle today. And the -10 had 4 engines, so a loss of one produced a smaller assymetric thrust problem. |
It's only worth the other (albeit often minor) penalties for toe-in to improve the thrust asymmetry for VMC if you are going to be VMC-limited for performance and if the fin size is driven by VMC.
If VMC isn't going to be a performance constraint, it's unlikely I'll care enough to drive the engine layout by it. Instead I'll be looking for the most aerodynamically efficient layout, considering both thrust and drag issues. If VMC is important, but something else is driving the fin size more - such as high speed directional stability, say - then I can take advantage of the large fin that the other requirement drives, and I get it "for free" for VMC. That means I probably won't need to worry about toe-in for that case either. As always, the best design is a compromise, and the weighting factors on that compromise can be different, which can subtly (or radically) affect the design decisions. Which is why there's rarely a single right answer to a design question. (Though there are also lots of bad answers!) |
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