Ok, another aero topic ....
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Ok, another aero topic ....
Ok, have to set this up, have model heli buddies and want to answer a question, but I want my answer straight, so let's see the full scale response.
Hovering over a hard surface that is not level (a slope, duh
), no wind: will the heli follow the slope due to eneven ground effect circulation? (assuming no corrective cyclic inputs)
I can understand one following if there is a wind downslope ...
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Marc
Hovering over a hard surface that is not level (a slope, duh
), no wind: will the heli follow the slope due to eneven ground effect circulation? (assuming no corrective cyclic inputs)I can understand one following if there is a wind downslope ...
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Marc
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I'm not convinced that precession has much to do with the ground effect cushion. It would seem to reason that it remains where it is. Phase lag deals with changes in cyclical rotor pitch and the question states clearly that no cyclic inputs are to be made.
Experiments I read about where the effect of a 'split' ground cushion was tested by hovering half over a carrier deck and half over the ocean seem to support this interpretation. My vote goes to a move downslope.
Who's next?
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You are welcome to visit HELIDRVR here
Experiments I read about where the effect of a 'split' ground cushion was tested by hovering half over a carrier deck and half over the ocean seem to support this interpretation. My vote goes to a move downslope.
Who's next?
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You are welcome to visit HELIDRVR here
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Hi RW,
In still air it will not follow the slope.
Take in to account gyroscopic precession and apply it to a Heli pointing upslope (for instance). It will hover right skid lower than usual for anti-clock MR rotation. If no correcive input is made it will drift, but not down the slope. It will drift to the right.
Two factors make this happen. There is less induced flow at the blade at the front of the disc due to ground effect and that results in the blade being high to the left
as Alpha was increased at the front.
Conversely, more induced flow for the blade at the rear (less ground effect) means that it will be low on the right (Alpha decreased)resulting in right drift (But only in still air!).
In still air it will not follow the slope.
Take in to account gyroscopic precession and apply it to a Heli pointing upslope (for instance). It will hover right skid lower than usual for anti-clock MR rotation. If no correcive input is made it will drift, but not down the slope. It will drift to the right.
Two factors make this happen. There is less induced flow at the blade at the front of the disc due to ground effect and that results in the blade being high to the left
as Alpha was increased at the front.
Conversely, more induced flow for the blade at the rear (less ground effect) means that it will be low on the right (Alpha decreased)resulting in right drift (But only in still air!).
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Leading on from my last post, the interesting part (for me) is that if disc attitude is inclined this time, but the ground is level, we have identical variation in ground effect.
It suggests that tilting the disc forward to transition away from a still air hover results in identical effects on the disc. I beleive this is the root cause of Inflow Roll (to the right for anti -clock MR's)
That said, as soon as the Heli moves, all of the standard forces come into play and Inflow
Roll becomes greater.
I have left out flapback to simplify.
It suggests that tilting the disc forward to transition away from a still air hover results in identical effects on the disc. I beleive this is the root cause of Inflow Roll (to the right for anti -clock MR's)
That said, as soon as the Heli moves, all of the standard forces come into play and Inflow
Roll becomes greater.
I have left out flapback to simplify.
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Helidrvr,
Your experiment is a very good one but the conditions are not the same.
When over deck/sea half of the disc is experiencing dissimilar ground effect but on a slope it varies constantly.
Over the Deck there will be ground effect for 1/2 of the disc and over the sea there will be none for 1/2 of the disc (depending on height and the effect of the water). The blades will experience the resulting differences in ground effect earlier and indeed, if alongside the ship heading to the stern (for instance) it will drift to sea.
Not so with a slope though, different game altogether!
Your experiment is a very good one but the conditions are not the same.
When over deck/sea half of the disc is experiencing dissimilar ground effect but on a slope it varies constantly.
Over the Deck there will be ground effect for 1/2 of the disc and over the sea there will be none for 1/2 of the disc (depending on height and the effect of the water). The blades will experience the resulting differences in ground effect earlier and indeed, if alongside the ship heading to the stern (for instance) it will drift to sea.
Not so with a slope though, different game altogether!
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To: SPS
I quote,"Over the Deck there will be ground effect for 1/2 of the disc and over the sea there will be none for 1/2 of the disc (depending on height and the effect of the water). The blades will experience the resulting differences in ground effect earlier and indeed, if alongside the ship heading to the stern (for instance) it will drift to sea".
Actually, the helicopters as described above will not drift out to the sea. In fact they will fall into the sea. This was the subject of one of my posts on another thread. Many years ago a US Marine HRS (S-55) helicopter battalion was operating off of an aircraft carrier. They were positioned with their main landing gear at the edge of the flight deck. After loading of the troops the helicopters lifted off one by one, and one by one they tipped over backwards because of only one half of the disc being in ground effect. I don’t know how many fell into the sea before the rest of the air group stopped their take off.
I flew as a crewmember in an HTL-1 (early B-47) and we operated off of an Icebreaker. The pilots would normally pull collective and move sideways in relation to the ships path. One day the pilot decided to just lift off from the flight deck and let the ship move away from the helicopter. After the flight deck and fantail moved away from us we were about 40 feet in the air with no ground cushion. Needless to say, we almost bought the farm.
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The Cat
I quote,"Over the Deck there will be ground effect for 1/2 of the disc and over the sea there will be none for 1/2 of the disc (depending on height and the effect of the water). The blades will experience the resulting differences in ground effect earlier and indeed, if alongside the ship heading to the stern (for instance) it will drift to sea".
Actually, the helicopters as described above will not drift out to the sea. In fact they will fall into the sea. This was the subject of one of my posts on another thread. Many years ago a US Marine HRS (S-55) helicopter battalion was operating off of an aircraft carrier. They were positioned with their main landing gear at the edge of the flight deck. After loading of the troops the helicopters lifted off one by one, and one by one they tipped over backwards because of only one half of the disc being in ground effect. I don’t know how many fell into the sea before the rest of the air group stopped their take off.
I flew as a crewmember in an HTL-1 (early B-47) and we operated off of an Icebreaker. The pilots would normally pull collective and move sideways in relation to the ships path. One day the pilot decided to just lift off from the flight deck and let the ship move away from the helicopter. After the flight deck and fantail moved away from us we were about 40 feet in the air with no ground cushion. Needless to say, we almost bought the farm.
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The Cat
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Role reversal -
I bow to your greater experience as a crewmember!
My experience of flying off and onto a carrier is NIL! But skyscrapers are similar
(OK, they don't usually move!) and I've done that a few times!
OK, the point was that those conditions are quite different to a slope. I do remember reading something as applied to the V22 and have wondered about the effect of dissimilar amounts of ground effect (some and none)
on an aircraft with V22 rotor configuration
and its fuselage in line with the ship's hull.
Of course, the issue is totally different when a chinook is considered, unless it were to be flown sideways onto or off the deck.
Now, what about that Inflow Roll I brought up? Anyone want to explain why we have IR to the right when transitioning away but get almost none when transtioning TO the hover?
I bow to your greater experience as a crewmember!
My experience of flying off and onto a carrier is NIL! But skyscrapers are similar
(OK, they don't usually move!) and I've done that a few times!
OK, the point was that those conditions are quite different to a slope. I do remember reading something as applied to the V22 and have wondered about the effect of dissimilar amounts of ground effect (some and none)
on an aircraft with V22 rotor configuration
and its fuselage in line with the ship's hull.
Of course, the issue is totally different when a chinook is considered, unless it were to be flown sideways onto or off the deck.
Now, what about that Inflow Roll I brought up? Anyone want to explain why we have IR to the right when transitioning away but get almost none when transtioning TO the hover?
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To: SPS
On one of the V-22 threads I raised the question about the V-22 when it was leaving the deck and / or when it transitioned into a hover and landed on the deck. In either case the pilot would normally transition from flight to a hover or, vice versa and fly sideways relative to the centerline of the ship. I used the Marine HRS fiasco as an example and raised the question about one proprotor being in ground effect and the other not in ground effect and asked if the aircraft would roll towards the sea.
Here is another point to ponder in respect to RW-1s original question. In normal ground effect the wing masks a large part of the proprotor. Wouldn’t this in effect be the same as hovering in a no wind condition over a slope? A major portion of the downflow is being impacted on the wing so that the downflow is not equal around the disc and if what was posted previously the inflow would also be effected. Does the V-22 translate under these conditions? I don’t believe it does.
Re Transverse Flow Effect (Inflow Roll)
It would seem to me that in transitioning to forward flight there is a variation in the air inflow between the forward part of the disc and the rear part of the disc. This causes a variation in lift and gyroscopic precession causes the disc to tilt resulting in the right roll. Conversely, in transitioning into a hover the inflow is already established and when you transition from forward flight to a hover you pass through the transition with the inflow fully established and there fore, the lift across the disc remains the same and no right roll manifests itself. Again I add the caveat “I think”.
[This message has been edited by Lu Zuckerman (edited 08 March 2001).]
On one of the V-22 threads I raised the question about the V-22 when it was leaving the deck and / or when it transitioned into a hover and landed on the deck. In either case the pilot would normally transition from flight to a hover or, vice versa and fly sideways relative to the centerline of the ship. I used the Marine HRS fiasco as an example and raised the question about one proprotor being in ground effect and the other not in ground effect and asked if the aircraft would roll towards the sea.
Here is another point to ponder in respect to RW-1s original question. In normal ground effect the wing masks a large part of the proprotor. Wouldn’t this in effect be the same as hovering in a no wind condition over a slope? A major portion of the downflow is being impacted on the wing so that the downflow is not equal around the disc and if what was posted previously the inflow would also be effected. Does the V-22 translate under these conditions? I don’t believe it does.
Re Transverse Flow Effect (Inflow Roll)
It would seem to me that in transitioning to forward flight there is a variation in the air inflow between the forward part of the disc and the rear part of the disc. This causes a variation in lift and gyroscopic precession causes the disc to tilt resulting in the right roll. Conversely, in transitioning into a hover the inflow is already established and when you transition from forward flight to a hover you pass through the transition with the inflow fully established and there fore, the lift across the disc remains the same and no right roll manifests itself. Again I add the caveat “I think”.
[This message has been edited by Lu Zuckerman (edited 08 March 2001).]
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My response to LU on his last with the V-22, I would say it doesn't either; based on that you would have it happening on each side of said wing, in opposite directions, right?
(theoretically saying the vector sum was equal). Of course having one prop rotor out over the side would indeed have it's consequences though.
Anyway, lots of answers, what i want to do is copy the two answers that started me thinking about this. The original pilot was taking about haveing to correct position constantly (hover) and I just think he's overcontrolling as a beginner, though he could be out of trim. It was the two answers that I question.
"Something else to think about, if you are flying off of a hard surface that is out of level the helicopter will follow the slope. This is most noticeable when you are very close to the ground because the helicopter is in ground effect."
And a follow up:
"Tony is correct, I forgot to mention the "drain effect" of even a gentle slope."
I can go with SPS for still air. Unfortunately convincing my model bretheren is harder. Kinda like trying to kill the "Downwind turn" myth that they also still subscribe to (in FW flying)
I thought you guys would enjoy this topic!
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Marc
(theoretically saying the vector sum was equal). Of course having one prop rotor out over the side would indeed have it's consequences though.
Anyway, lots of answers, what i want to do is copy the two answers that started me thinking about this. The original pilot was taking about haveing to correct position constantly (hover) and I just think he's overcontrolling as a beginner, though he could be out of trim. It was the two answers that I question.
"Something else to think about, if you are flying off of a hard surface that is out of level the helicopter will follow the slope. This is most noticeable when you are very close to the ground because the helicopter is in ground effect."
And a follow up:
"Tony is correct, I forgot to mention the "drain effect" of even a gentle slope."
I can go with SPS for still air. Unfortunately convincing my model bretheren is harder. Kinda like trying to kill the "Downwind turn" myth that they also still subscribe to (in FW flying)
I thought you guys would enjoy this topic!
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Marc
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Initially I went for Helidvr because I can't remember GE having any effect on phase lag and consequently llift shedding over one quadrant of the disc. Then SPS came back with what seems to make sense also!!! Confused, as a result.
What I do know is:
When I lift now immediately in front of my hangar door and when lifting off a frigate in front of their doors, the recirc caused lift loss at the front of the disc and we drifted towards the hangar doors! This might be similar to sloping ground effects...no?
Lu...I cannot possibly believe a number of helos fell backwards into the sea because of reduced GE over the deck edge...what absolute b*****cks. I've flown several thousand deck manouevres on carriers and without question there is no discernable feedback thru the controls that persuades the pilot to respond rapidly to loss of 1/2 his GE!!!! Admittedly you make fine adjustments as you feel it off the deck but there is nothing waiting to bite you!!
Mind you they (the 55 helo drivers) probably saw their sqdn boss tip over, so followed orders........
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Thermal runaway.
What I do know is:
When I lift now immediately in front of my hangar door and when lifting off a frigate in front of their doors, the recirc caused lift loss at the front of the disc and we drifted towards the hangar doors! This might be similar to sloping ground effects...no?
Lu...I cannot possibly believe a number of helos fell backwards into the sea because of reduced GE over the deck edge...what absolute b*****cks. I've flown several thousand deck manouevres on carriers and without question there is no discernable feedback thru the controls that persuades the pilot to respond rapidly to loss of 1/2 his GE!!!! Admittedly you make fine adjustments as you feel it off the deck but there is nothing waiting to bite you!!
Mind you they (the 55 helo drivers) probably saw their sqdn boss tip over, so followed orders........
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Thermal runaway.
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LZ,
Very intersting stuff and I'll come back to you on it separately, want to try and keep this (my present)post focussed on slopes, ground effect and recirculation with obstruction on one disc only.
All-
We did touch on some of the recirculation effects on a thread a month or so ago but I think it is well worth going into with more depth.
Thomas - You are perfectly correct on the drift toward the obstruction, it most certainly does happen (as you have demonstrated) and hopefully I can explain.
When we consider recirc. we are generally speaking about the effect at the tip area
in the first instance, in that most
(through disc) recirculation happens there. The effect will be manifested 90 degrees on and a Heli (anti clock MR) with the obstruction on its right would drift forward if uncorrected.
But this is only good for recirculation affecting the tips only - If the Heli becomes closer to the obst. more of the disc will be affected (across its diameter). If the area affected by recirc. becomes half of the disc then the blade over the tail will be low by the time it gets to the (90 deg travel)right hand position.
The cumulative effects of recirc. on the blade at the right side and now at the tail will result in drift to both the right (toward the obst.) and front.
There is a small balancing effect at work as air recirculated at the disc's FRONT has the effect of tilting the disc low on the LEFT
at the same time, so resulting drift toward
the obstacle is diminished but it is not neutralised.
But the problem is also made WORSE as stable ground effect may be present on the left, out of the area of greatest recirculation, (Alpha is increased) tilting the disc to the right again as that blade climbs.
Drifting into an obstruction is of course avoidable by keeping your distance, if only the tips are affected by recirc. on the right then only drift to the front would result (still air, uncorrected).
I prefer to keep ground effect as it pertains to slopes and recirculation as it pertains to obstruction seperate as they are different causes (although they are related by their effect on tip vortices, induced flow and Alpha) and may have simliar end results. The common factor in both is gyroscopic precession and to predict the resuting drift you must find out how much of the disc is being affected.
If only the tips experience the max. effect (in considering only one blade position) as with a slope (constantly varying ground effect) then the drift will me manifested 90 degress on from the max. position.
If the maximum effect moves 'into' or across the disc diameter (toward the blade root)then the resulting effect (disc tilt) will be manifested earlier in the cycle as gyroscopic precession will also begin its work earlier on any blade approaching.
This is why sloping ground will give a wholly different drift direction to recirculation caused by an obstacle very close to the disc.
Sloping ground will always have its max. effect at the top of the slope which will constantly strengthen and diminish through the blade's 360 dgree cycle.
It would be possible for a very steep slope to produce its max. effect on disc tilt earlier to produce drift toward the highest part of it and the logical extension of that is to think of an 'adjustable' slope becoming steeper until you end up at vertical....Which brings us to obstructions like Hangars...
Recirculation due to vertical obstruction can have a much greater effect over more of the disc area (up to a half) as seen from above.
It really only depends on when you stop calling a slope a slope at start calling it
a vertical obstruction!
Very intersting stuff and I'll come back to you on it separately, want to try and keep this (my present)post focussed on slopes, ground effect and recirculation with obstruction on one disc only.
All-
We did touch on some of the recirculation effects on a thread a month or so ago but I think it is well worth going into with more depth.
Thomas - You are perfectly correct on the drift toward the obstruction, it most certainly does happen (as you have demonstrated) and hopefully I can explain.
When we consider recirc. we are generally speaking about the effect at the tip area
in the first instance, in that most
(through disc) recirculation happens there. The effect will be manifested 90 degrees on and a Heli (anti clock MR) with the obstruction on its right would drift forward if uncorrected.
But this is only good for recirculation affecting the tips only - If the Heli becomes closer to the obst. more of the disc will be affected (across its diameter). If the area affected by recirc. becomes half of the disc then the blade over the tail will be low by the time it gets to the (90 deg travel)right hand position.
The cumulative effects of recirc. on the blade at the right side and now at the tail will result in drift to both the right (toward the obst.) and front.
There is a small balancing effect at work as air recirculated at the disc's FRONT has the effect of tilting the disc low on the LEFT
at the same time, so resulting drift toward
the obstacle is diminished but it is not neutralised.
But the problem is also made WORSE as stable ground effect may be present on the left, out of the area of greatest recirculation, (Alpha is increased) tilting the disc to the right again as that blade climbs.
Drifting into an obstruction is of course avoidable by keeping your distance, if only the tips are affected by recirc. on the right then only drift to the front would result (still air, uncorrected).
I prefer to keep ground effect as it pertains to slopes and recirculation as it pertains to obstruction seperate as they are different causes (although they are related by their effect on tip vortices, induced flow and Alpha) and may have simliar end results. The common factor in both is gyroscopic precession and to predict the resuting drift you must find out how much of the disc is being affected.
If only the tips experience the max. effect (in considering only one blade position) as with a slope (constantly varying ground effect) then the drift will me manifested 90 degress on from the max. position.
If the maximum effect moves 'into' or across the disc diameter (toward the blade root)then the resulting effect (disc tilt) will be manifested earlier in the cycle as gyroscopic precession will also begin its work earlier on any blade approaching.
This is why sloping ground will give a wholly different drift direction to recirculation caused by an obstacle very close to the disc.
Sloping ground will always have its max. effect at the top of the slope which will constantly strengthen and diminish through the blade's 360 dgree cycle.
It would be possible for a very steep slope to produce its max. effect on disc tilt earlier to produce drift toward the highest part of it and the logical extension of that is to think of an 'adjustable' slope becoming steeper until you end up at vertical....Which brings us to obstructions like Hangars...
Recirculation due to vertical obstruction can have a much greater effect over more of the disc area (up to a half) as seen from above.
It really only depends on when you stop calling a slope a slope at start calling it
a vertical obstruction!
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Lu - some thoughts on V22.
NONE of this is from experience, I doubt they would let me near one and I have no burning desite to get into one. So, theory only and no other source than my portable hard drive!
I'll put the two most common configurations of twin but not co- axial main rotor helis (Chinook and V22) against each other. In both types the rotors are contra-rotating which has a benefit of cancelling out torque induced rotation of the fuselage(which it does, strictly speaking, but the the two opposing torque forces 'meet' at a point where they are balanced and sheer forces exerted on that area of the fuselage are the result).
The Chinook has the MR discs placed fore and aft whilst the V22 has them right and left,
and this can mean a lot of difference when considering dissimilar ground effect.
Only still air conditions are considered;
If the Chinook experiences loss of GE on the left of the fuselage whilst it is 1/2 over the deck and half over the sea (fuselage oriented in line with fore/aft with the ship's hull) then the max. effect of that will be at both left hand position blade tips, carried on by gyroscopic precession so both blades (discs) will be lowest over the centre of the fuselage. The lever will have to be raised to maintain height as a result but no drift will be induced.
If the Chinook's fuselage were oriented at 90 degrees to the ships hull and it were flown forward to place the front disc over the deck only that disc would benefit from the GE created. The result would be drift in an opposite direction to the blade high point 90 degrees on from the max effect (depending on its rotation).
So I assume that the best way to move on or off a deck in a Chinook is sideways because less drift would result.
The V22 would be a differnt game altogether and if we think of it as no more than a Chinook being flown sideways then it seems it might be wise to modify deck aproach/departure technique by 90 degrees to avoid unnecessary drift, ie. that it would be better to fly to or from the deck forwards at right angles to the ships hull.
Of course, this only considers drift resulting from dissimilar GE. To begin thinking of the possible effects of recirculation on only one side of the helicopter or worse still, VRS on one side (disc) only (mentioned on another thread I think) and my organic processor just says 'ERROR'!
The V22 must roll to the right or left due to recirc or VRS affecting only one disc or the other. For a Chinook it appears slightly less serious as it would result in pitch and not roll, easier to handle. Stabilisers could help to balance the effect for a short time.
Moving on the the masking of the proprotor by the wing, it isn't quite the same as a slope as the two (engine nacelle and wing) have a physical and fixed relationship whils a heli and the slope do not, but I do see where you are going on that. I would agree that recirculation is the inevitable result. This must make both proprotors (discs) hugely inefficient and I would estimate that the discs would need to induce 30% more air to flow than similar ones with a 360 degree unimpeded airflow in order to make the same amount of thrust.
None of the above is to say the V22 cannot work, but it does point to a very inefficient HELICOPTER design in my opinion.
Of course, in fixed wing terms it may be very good and competetive, but I wouldn't have a clue!
I have thoughts on the inflow roll question but it'll have to wait 'til tomorrow!
NONE of this is from experience, I doubt they would let me near one and I have no burning desite to get into one. So, theory only and no other source than my portable hard drive!
I'll put the two most common configurations of twin but not co- axial main rotor helis (Chinook and V22) against each other. In both types the rotors are contra-rotating which has a benefit of cancelling out torque induced rotation of the fuselage(which it does, strictly speaking, but the the two opposing torque forces 'meet' at a point where they are balanced and sheer forces exerted on that area of the fuselage are the result).
The Chinook has the MR discs placed fore and aft whilst the V22 has them right and left,
and this can mean a lot of difference when considering dissimilar ground effect.
Only still air conditions are considered;
If the Chinook experiences loss of GE on the left of the fuselage whilst it is 1/2 over the deck and half over the sea (fuselage oriented in line with fore/aft with the ship's hull) then the max. effect of that will be at both left hand position blade tips, carried on by gyroscopic precession so both blades (discs) will be lowest over the centre of the fuselage. The lever will have to be raised to maintain height as a result but no drift will be induced.
If the Chinook's fuselage were oriented at 90 degrees to the ships hull and it were flown forward to place the front disc over the deck only that disc would benefit from the GE created. The result would be drift in an opposite direction to the blade high point 90 degrees on from the max effect (depending on its rotation).
So I assume that the best way to move on or off a deck in a Chinook is sideways because less drift would result.
The V22 would be a differnt game altogether and if we think of it as no more than a Chinook being flown sideways then it seems it might be wise to modify deck aproach/departure technique by 90 degrees to avoid unnecessary drift, ie. that it would be better to fly to or from the deck forwards at right angles to the ships hull.
Of course, this only considers drift resulting from dissimilar GE. To begin thinking of the possible effects of recirculation on only one side of the helicopter or worse still, VRS on one side (disc) only (mentioned on another thread I think) and my organic processor just says 'ERROR'!
The V22 must roll to the right or left due to recirc or VRS affecting only one disc or the other. For a Chinook it appears slightly less serious as it would result in pitch and not roll, easier to handle. Stabilisers could help to balance the effect for a short time.
Moving on the the masking of the proprotor by the wing, it isn't quite the same as a slope as the two (engine nacelle and wing) have a physical and fixed relationship whils a heli and the slope do not, but I do see where you are going on that. I would agree that recirculation is the inevitable result. This must make both proprotors (discs) hugely inefficient and I would estimate that the discs would need to induce 30% more air to flow than similar ones with a 360 degree unimpeded airflow in order to make the same amount of thrust.
None of the above is to say the V22 cannot work, but it does point to a very inefficient HELICOPTER design in my opinion.
Of course, in fixed wing terms it may be very good and competetive, but I wouldn't have a clue!
I have thoughts on the inflow roll question but it'll have to wait 'til tomorrow!
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Folks,
An interesting discussion.
I think you should be very cautious about considering ship-borne antics as any reliable indicator of the theoretical aerodynamics of our rotor systems. I had a particular interest in this topic many moons ago (1977!) and sought advice from ETPS and Westlands as well as Pax River: the considered view was that deck edge effects invariably masked the classic aerodynamic responses and, given the dreadful recirculation environment that surrounds vessels under way, no one was prepared to commit to give me the numbers that I sought.
At that time, I was working at very high altitude to small pads on extremely steep slopes and I thought that any research on boat and oil-rig operations might have been relevant. Practically, I learnt to be extremely careful because the helicopter's behaviour was fairly inconsistent - even 2-3 knots of wind created huge controllability effects.
Lu, I love your thoughts...
Like Thomas C, I have come along-side, stabilised OGE and transitioned sideways across the deck to park on quite small civil and military vessels. I always departed much the same way and never crashed or sensed the "great suck" that you suggest. Find another theory.
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Stay Alive,
[email protected]
An interesting discussion.
I think you should be very cautious about considering ship-borne antics as any reliable indicator of the theoretical aerodynamics of our rotor systems. I had a particular interest in this topic many moons ago (1977!) and sought advice from ETPS and Westlands as well as Pax River: the considered view was that deck edge effects invariably masked the classic aerodynamic responses and, given the dreadful recirculation environment that surrounds vessels under way, no one was prepared to commit to give me the numbers that I sought.
At that time, I was working at very high altitude to small pads on extremely steep slopes and I thought that any research on boat and oil-rig operations might have been relevant. Practically, I learnt to be extremely careful because the helicopter's behaviour was fairly inconsistent - even 2-3 knots of wind created huge controllability effects.
Lu, I love your thoughts...
Like Thomas C, I have come along-side, stabilised OGE and transitioned sideways across the deck to park on quite small civil and military vessels. I always departed much the same way and never crashed or sensed the "great suck" that you suggest. Find another theory.
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Stay Alive,
[email protected]
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SPS :
Minor query - the Chinok is contra-rotating. In the scenario where it's ship fore/aft but only 1/2 on and 1/2 off, the the forward rotor will precess to effect flight rearwards, but the rear rotor will precess to try to fly forwards. Net effect zero ?
This is simplistic because it's so bloody early on a Sunday, and it's also academic to me as I have not seen this effect for real. Maybe I'll have to ask someone nicely if I can have another go in the sim to demonstrate this !!
Any real Wokka drivers care to comment ?
Minor query - the Chinok is contra-rotating. In the scenario where it's ship fore/aft but only 1/2 on and 1/2 off, the the forward rotor will precess to effect flight rearwards, but the rear rotor will precess to try to fly forwards. Net effect zero ?
This is simplistic because it's so bloody early on a Sunday, and it's also academic to me as I have not seen this effect for real. Maybe I'll have to ask someone nicely if I can have another go in the sim to demonstrate this !!
Any real Wokka drivers care to comment ?
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Smack on as they say.
It does not matter which way the front or rear rotors rotate, a long as they contra- rotate, then the net effect IS zero.
They will either precess jointly to a high point over the centre of the fuselage or to the front and rear of the discs (depending on rotation and on which side of the fuselage the deck lies). Neither of these will affect pitch or roll attitude.
That was one of my points although I may not have put it over very well - If the Chinook is moved to or from the deck sideways, fuselage oriented in line with the ships hull, the net effect is nil (the benefit of the ground effect on one side but on the other side the benefit is lost) and no drift will result (in perfectly still air, impossible practically but you must start somewhere).
Now think on the V22 (effectively a chinook flown sideways) and that is why it seems it might be better to approach or leave the deck at 90 degrees to the hull.
All achedemic of course, as has been said - wind of even a light amount will change things, and if the ship is moving, likewise.
SPS late on Sunday night!
It does not matter which way the front or rear rotors rotate, a long as they contra- rotate, then the net effect IS zero.
They will either precess jointly to a high point over the centre of the fuselage or to the front and rear of the discs (depending on rotation and on which side of the fuselage the deck lies). Neither of these will affect pitch or roll attitude.
That was one of my points although I may not have put it over very well - If the Chinook is moved to or from the deck sideways, fuselage oriented in line with the ships hull, the net effect is nil (the benefit of the ground effect on one side but on the other side the benefit is lost) and no drift will result (in perfectly still air, impossible practically but you must start somewhere).
Now think on the V22 (effectively a chinook flown sideways) and that is why it seems it might be better to approach or leave the deck at 90 degrees to the hull.
All achedemic of course, as has been said - wind of even a light amount will change things, and if the ship is moving, likewise.
SPS late on Sunday night!
