View Full Version : Question for Harrier pilots out there
Andy_RR
2nd Feb 2019, 02:49
How much lift does the full flap deployment generate in hovering flight, especialy out of ground effect? Do you have to apply any aft thrust vector to compensate for wing lift?
Just curious about the aero brought about ffrom some design work and studying the Hopflyt eVTOL concept
Thanks in advance!
Flap62
2nd Feb 2019, 06:12
Hmmm. Not really sure what you mean. In the hover the wing produces no lift therefore no compensation is required. I seem to recall the 7/9 wing would produce about 1500lbs down to about 30 kts (doubtless some worthlessness QFI will be along soon to tell me it was 1569lbs at 31kts)
Easy Street
2nd Feb 2019, 06:24
I thought that too... an oddly-phrased question.
Hovering = no lift and no drag to compensate for.
Transitioning to the hover = some lift and some drag, but you want to stop so no compensation needed. Fine adjustment of deceleration uses coordinated pitch and throttle control; coarse adjustment uses coordinated nozzle and throttle control.
Transitioning out of the hover = some lift and some drag, but you want to accelerate so an aft thrust vector is used anyway.
Slow speed jet borne flight = some lift and some drag. A small aft thrust vector (about 8 degrees off the vertical IIRC) maintained a steady 50kts.
Andy_RR
2nd Feb 2019, 08:10
Something like this is what I had in mind. Note that thrust vector is not completely vertical to hover
https://www.youtube.com/watch?time_continue=6&v=8UPVfRmUMio
As above, the wing ( including flaps) will produce lift all the time there is a (sub-stall) angle of attack and an airflow from ahead. In a still-air hover, there is no lift BUT there is resistance to the downward movement of entrained air over the airframe. This means that flaps will increase this resistance unless they are "down", thus reducing the plan area of the wing. This vertical "drag" will off-set some of the vertical thrust, thereby reducing the weight at which a hover can be established.
Aerodynamics get a bit complex in the hover!
Mog
Andy_RR
2nd Feb 2019, 09:03
So, what is the purpose of the rather extreme flap angle deployed in the hover?
jimjim1
2nd Feb 2019, 09:19
Note that thrust vector is not completely vertical to hover
https://www.youtube.com/watch?time_continue=6&v=8UPVfRmUMio
Hmmm.
The very definition of hover is that the lift vector is vertical.
Any non vertical component to the thrust (net thrust from all sources - if you like) will produce an acceleration in some horizontal direction until the drag equals the horizontal component of the thrust.
I guess I am missing something.
SammySu
2nd Feb 2019, 09:45
The point of the very large 62 degree flap angle was to enable the wing to deliver meaningful lift at low IAS during the transition to from the hover. Numbers above are correct so you would only start to “fall off” the wing into purely jet borne flight below about 30kts. That meant that until that point you didn’t need so much jet thrust which in turn meant saving the time limited engine ratings at higher JPTs or limited volume of water injection available until you really needed it in the hover.
Once in the hover the 7/9 had a range of lift improvement devices like the strakes, front LIDS door and rear gear door to help entrain airflow close to the ground and cushion ROD.
When considering the Harrier nozzles also remember that true 90 to the ground was not 90 to the airframe deck angle so the hover was around 82 degrees nozzle for a vertical thrust vector.
Jimjim - I agree for still air. The other 90% (or more!) of days would require a small forward (assuming straight vane!) component from the thrust vector.
To answer another facet of the question: On really strong wind days or when the landing surface (let’s call it a CVS) was moving into wind at a rate of knots - and therefore the wing was producing more lift than ‘normal’ - one still had to trim forward - so any moment requiring aft pitch (if there was one!) was overcome by the thrust vector needing to be angled forward to keep position.
And no, I’m not a trimmer.
Andy_RR
2nd Feb 2019, 10:39
Hmmm.
The very definition of hover is that the lift vector is vertical.
Any non vertical component to the thrust (net thrust from all sources - if you like) will produce an acceleration in some horizontal direction until the drag equals the horizontal component of the thrust.
I guess I am missing something.
I probably should have said the fan lift vector is not vertical. I think in this case the fan inlet flow over the wing is producing wing lift perpendicular to the wing surface, the vertical component of which is additional lift and the horizontal component being cancelled by the horizontal component of the angled fan thrust vector.
WRT the Harrier, I was wondering if the jet plume induced any airflow over the wing to speak of that could generate a noticeable lift component (or more likely a noticeable horizontal thrust component that needed compensating for)
So, what is the purpose of the rather extreme flap angle deployed in the hover?
I refer my learned friend to #5!
As well as producing lift to very low air speeds during the decel, it enables a higher hover weight (or reduced RPM).
Mog
WRT the Harrier, I was wondering if the jet plume induced any airflow over the wing to speak of that could generate a noticeable lift component (or more likely a noticeable horizontal thrust component that needed compensating for)
The harrier's nozzles would always have a net forward angle at the hover (still air) to offset the intake momentum. Essentially the engine sucks in a large massflow of air, so it effectively "sucks itself forwards". When high enough to be out of the jet-sheet the wing and fuselage play very little (if any) part in hover lift. But closer to the ground the let efflux hits the ground and spreads sideways into a jet-sheet. the jet sheet heading outboard is lost, but the inboard-heading part collides with the inboard-heading jet-sheet from the other side, deflecting the flow upwards. There is still quite a lot of energy in this flow, and where it hits the airframe it generates additional hover lift. Right from the get-go the Hawker chappies at Kingston recognised the potential benefits, so they designed features into the fuselage to trap this deflected jet-sheet. These are mainly comprised of the two strakes or cannon pods (Harriers are always fitted with strakes or pods - they never fly with neither) and the main-leg door which (with the gear down) closes off the gap between the pods/strakes at the back to trap air. Then at the front there is a small air dam called a "LID" (Lift Improvement Device) which extends when the wheels go down - the LID blocks the gap between the pods/strakes at the front so that the Pod/strake-LID-Mainleg-door combination forms an enclosed box to trap the deflected jet sheet. I no longer have the data to hand, but if I remember correctly the LIDs on the Harrier II provide something like 2,000lbs of extra hover lift.
There is something very strange going on in that video - I'm struggling to see what force the forward rake of the thrust vector is opposing. I'd need to look at it in a lot of detail to understand what's going on.
PDR
Fortissimo
2nd Feb 2019, 13:54
It is threads like this that remind me how much John Farley used to contribute to such discussions. We would have had a definitive answer to the original question. 😔
Jhieminga
2nd Feb 2019, 14:13
There is something very strange going on in that video - I'm struggling to see what force the forward rake of the thrust vector is opposing. I'd need to look at it in a lot of detail to understand what's going on.
I would hazard a guess at the lift from the 'wings' below the four props. The video states that 20% of hover lift is generated by these wings. If you look at them as a wing surface with the flow from the props providing the high speed flow over the top of the wing, you get a lift vector that's angled back and up a bit. The upwards bit is useful for hover lift, the rearwards bit needs to be offset by the canted thrust from the props.
Sun Who
2nd Feb 2019, 14:45
Note that thrust vector is not completely vertical to hover
Because it's not in the hover.
Sun.
Actually the dams/pods/lids etc did not affect the HOVER performance at all. If anything they REDUCED it because of the extra weight. What they did affect was the vertical LANDING and TAKEOFF performance. As stated above, interaction between the airframe and the vertical jet eflux and recirculation of hot air into the intake was what was managed by all the bits and bobs.
The hover was always carried out in "free air" outside the effects of recirculation, airframe impingement etc. It was possible to hover at a weight that would "spike" the engine if you attempted a VL.
Swing the Lamp!
Mog
H Peacock
3rd Feb 2019, 23:24
The harrier's nozzles would always have a net forward angle at the hover (still air) to offset the intake momentum. Essentially the engine sucks in a large massflow of air, so it effectively "sucks itself forwards".
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.
With the intakes forward of the CofG it is this Intake Momentum Drag that makes the Harrier directionally unstable in the hover and caused all sorts of nasty problems if any appreciable yaw is allowed to develop. Hence the yaw vane on the nose and I believe vibrating rudder pedals!
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!
In my experience it was because the vane being straight made perfect sense, the speed band of impending disappointment seemed logical - and at some point alpha came into the conversation as a ‘it just does’ ingredient.
Hence most conversations about IMD seemed to end with someone saying ‘Look, let’s not get bogged down in this - just take one element of ‘the triangle’ out and you’ll be fine...’
Still, I suppose it allowed the QFIs to talk knowledgably about something that didn’t include a single weapon system or tactic...😉
Jhieminga
4th Feb 2019, 08:49
It is threads like this that remind me how much John Farley used to contribute to such discussions. We would have had a definitive answer to the original question. 😔
I don't think he covered the original question, but this post does explain the IMD issue: https://www.pprune.org/military-aviation/414038-harrier-transition-hover.html?highlight=intake+momentum+drag#post5675384
weemonkey
4th Feb 2019, 10:33
Hmmm.
The very definition of hover is that the lift vector is vertical.
Any non vertical component to the thrust (net thrust from all sources - if you like) will produce an acceleration in some horizontal direction until the drag equals the horizontal component of the thrust.
I guess I am missing something.
In the hover as the aircraft descends there is a "pocket" of high pressure air "retained" fore and aft by the airbrake & dam and left and right by gun pods or strakes. It may be that there if some residual that can be retained likewise by flaps?
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
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
[Duplicate post due to forum software suboptimality]
keith williams
4th Feb 2019, 18:08
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.
“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’.
keith williams
5th Feb 2019, 12:48
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.
H Peacock
5th Feb 2019, 22:44
I'm sure the Nozzle Lever comes fully backwards for the braking stop! Both levers forward = forwards & fast!
keith williams
6th Feb 2019, 07:39
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.