Question for Harrier pilots out there
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.
PDR
Last edited by PDR1; 4th Feb 2019 at 12:24.
PDR
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.
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.
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“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’.
’Plausible’ is closely aligned with ‘As explained to me by a trimmer and now repeated as good enough for a lay person’.
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.
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.
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.
I used to have a good memory, but these days I can't remember where I've put it.
Last edited by keith williams; 6th Feb 2019 at 13:17.