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# Question for Harrier pilots out there

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# Question for Harrier pilots out there

4th Feb 2019, 12:08

Join Date: Nov 2015
Location: Farnham, Surrey
Posts: 1,190
Originally Posted by H Peacock
PDR. Surely Intake Momentum Drag is the result of the air coming through the intake (not sure it's sucked-in but I know what you mean) and then being 'stopped' at the engine. The engine is producing thrust vertically so the net momentum is rearward, i.e. it would push the Harrier backwards? You therefore need a tad of rearward nozzle to countwract this and remain stationary.
I didn't mention drag, just momentum. Consider an aeroplane sitting stationary in still air with the engine off. Now fire up the engine and some of the air in front of the aeroplane is induced ("sucked" if you will) into the engine. It also creates a big zone of low pressure in front of the aeroplane where all that air used to be (I exaggerate for effect) - lower pressure than the air behind the aeroplane. There are lots of ways you can model or look at this as a static pressure effect, a momentum effect etc, but the net result is a forward force on the airframe which is opposed by a slight net forward moment in the nozzle thrust.

PDR

Last edited by PDR1; 4th Feb 2019 at 12:24.
4th Feb 2019, 12:13

Join Date: Nov 2015
Location: Farnham, Surrey
Posts: 1,190
Originally Posted by orca
Most folk could mount a plausible explanation of Intake Momentum Drag and its effect if you didn’t play by the rules...everyone’s story on the effect was aligned but the explanation always seemed to be a bit woolly!
Intake inertia *drag* is a different thing altogether - that's the angular moment produced by same effect as the "P-factor" people talk about loosely with propellers - the angular reaction experienced when bending the sairflow into the intake. It's one of the reasons why the Sailor-Inhaler X-32's forward intake was such a fundamentally bad idea - intake distance from the CG being one of the main governing factors for the magnitude of IID effects.

PDR
4th Feb 2019, 12:19

Join Date: Nov 2015
Location: Farnham, Surrey
Posts: 1,190
[Duplicate post due to forum software suboptimality]
4th Feb 2019, 18:08

Join Date: Jan 2011
Location: England
Posts: 642
PDR1

Your explanation sounds convincing, but is it actually true?

The air initially has zero momentum in the fore-aft direction.

The fan draws in air thereby giving it some fore-aft momentum.

It the nozzles air pointing directly downwards, the air leaving the engine has zero fore-aft momentum.

The overall change in fore-aft momentum is zero, so there will be no resultant fore-aft thrust force.

Or we could say that the fan exerts a rearwards force on the air giving it a rearward aceleration. But the rear engine casing between the rear nozzles and the nozzles themselves, exert an equal and opposite forward force, reducing the rearward air velocity to zero.

The overall fore-aft forces and acelerations sum to zero, so once again we have no fore-aft resultant force.

5th Feb 2019, 07:13

Join Date: Oct 2001
Location: UK
Posts: 1,105
“Is it actually true?” is a very unhelpful question to ask when discussing IMD. The standard that answers are trying to achieve is ‘plausible explanation for phenomena witnessed’. 😉
’Plausible’ is closely aligned with ‘As explained to me by a trimmer and now repeated as good enough for a lay person’.
5th Feb 2019, 12:48

Join Date: Jan 2011
Location: England
Posts: 642
IIRC

The Hover Stop is fixed at 82 degrees relative to the engine centreline.

The engine centreline is set a 1.5 degrees nose up relative to the aircraft longitudinal axis.

The pitch attitude in a still air hover is 6.5 degrees nose up.

Adding the above angles together gives us 90 degrees.

Lifting the nozzle lever and moving it further aft can take the nozzles to the Nozzle Braking Stop, which is at about 100 degrees relative to the engine centre line. This can be used to slow the aircraft when approaching the hover. This may be the observed phenomena which leads to the idea that the nozzles are slightly forward in the hover.

Last edited by keith williams; 6th Feb 2019 at 13:16. Reason: Brain fade.
5th Feb 2019, 22:44

Join Date: Sep 2004
Location: Yorkshire
Posts: 487
I'm sure the Nozzle Lever comes fully backwards for the braking stop! Both levers forward = forwards & fast!
6th Feb 2019, 07:39

Join Date: Jan 2011
Location: England
Posts: 642
Quite right HP, my mistake. I've now corrected it.

I used to have a good memory, but these days I can't remember where I've put it.

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Last edited by keith williams; 6th Feb 2019 at 13:17.

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