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Manufacturers angling engines to reduce VMCG"

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Manufacturers angling engines to reduce VMCG"

Old 10th Dec 2014, 02:16
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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.

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Old 10th Dec 2014, 03:36
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More tangential = more spinny = more bad.
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Old 10th Dec 2014, 09:20
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Originally Posted by JS
angled inward would be do this
- stbd engine out = yaw to starboard, agreed? Now cant the port engine tailpipe OUTWARDS and increase thrust = a touch of yaw to port? If in doubt, imagine the tailpipe at 90 degrees (yes, I know, not much forward thrust....)
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Old 10th Dec 2014, 10:55
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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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Old 10th Dec 2014, 13:29
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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.
You're thinking of wing mounted engines. With tail mounted engines, the thrust vector of the operating engine will be behind the CG. (I did the same thing)
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Old 10th Dec 2014, 13:49
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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.
I guess it is due to properties of sine and cosine - you'll get a significant yaw/vmc reduction for a minimal loss of thrust.


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

Last edited by C_Star; 10th Dec 2014 at 15:36. Reason: grammar
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Old 10th Dec 2014, 15:24
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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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Old 10th Dec 2014, 18:02
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VC10

On the VC10, the engines appear to be pointing in at rear.
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Old 10th Dec 2014, 18:42
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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.
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Old 11th Dec 2014, 01:32
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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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