Sikorsky X2 coaxial heli developments.
"push the lift" is not physically correct in any way, IFMU. I truly hope you didn't find such logic in any aero discussion!
Using 'ground cushion' as an example, I would say yes.
-- IFMU
A shakey youtube video of the mockup
Last edited by IFMU; 11th May 2009 at 00:56. Reason: Thanks SP I learned something from your edit
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IFMU,
Please view the first picture in your link on post #483 and envision that the slow turning rotors represent wings. The X2 pusher prop is totally below these 'wings'.
IMHO, the propeller is removing some of the lift from under a portion of these 'wings'. This removal is by; physical removal, and/or by causing the the velocity on the wing's lower surface to now exceed the velocity on it's upper surface.
Here is an example of a pusher propeller working with only a wing. These propellers are drawing most of their free-stream air from the upper surface of the wings.
Dave
__________________________________________
The following was to be the response to your post, until the above simple explanation came about.
For the very bored here it is.
_________
The use of 'ground cushion' may not have been the most appropriate example but since it was used, the following is an elaboration on it.
"Note that for operation IGE the power increases rapidly as the helicopter transitions from the hover state. This is because of the formation and the influence of flow recirculation at the leading edge of the rotor disk, which causes the rotor to experience a higher induced inflow than hover in OGE, and so power requirements will increase slightly." [Source ~ PHA Page 188]
[Source ~ RWP1 Fig. 3.11]
IMHO, the implication is that the flow recirculation causes a reduction in the pressure (1) differential between the higher pressure below the rotor and the lower pressure above the rotor. Therefor additional power is required to maintain lift.
Perhaps the better example of the same thing is Argument #2 in the link on above post #476.
No one can argue against the fact that two closely spaced coaxial rotors require significantly more power than two distantly spaced rotors, for the same amount of thrust. In other words, the upper coaxial rotor would be more efficient if the lower rotor was not 'consuming' the stream-tube from the upper rotor.
My position is that 'consuming' a PORTION of the X2 rotor's stream-tube is also of some detriment to the lift.
Graviman, should wish to present a position, please do. However, attack the subject and not the person.
Use the language of 'English' or the language of 'Math', your choice.
Use Momentum Theory or use String Theory; but provide the supporting algorithms and define the symbols if you wish to put forth an argument.
(1) The use of the word pressure could be argued until hell freeze over. Therefore please accept this easy to understand concept.
Please view the first picture in your link on post #483 and envision that the slow turning rotors represent wings. The X2 pusher prop is totally below these 'wings'.
IMHO, the propeller is removing some of the lift from under a portion of these 'wings'. This removal is by; physical removal, and/or by causing the the velocity on the wing's lower surface to now exceed the velocity on it's upper surface.
Here is an example of a pusher propeller working with only a wing. These propellers are drawing most of their free-stream air from the upper surface of the wings.
Dave
__________________________________________
The following was to be the response to your post, until the above simple explanation came about.
For the very bored here it is.
_________
The use of 'ground cushion' may not have been the most appropriate example but since it was used, the following is an elaboration on it.
"Note that for operation IGE the power increases rapidly as the helicopter transitions from the hover state. This is because of the formation and the influence of flow recirculation at the leading edge of the rotor disk, which causes the rotor to experience a higher induced inflow than hover in OGE, and so power requirements will increase slightly." [Source ~ PHA Page 188]
[Source ~ RWP1 Fig. 3.11]IMHO, the implication is that the flow recirculation causes a reduction in the pressure (1) differential between the higher pressure below the rotor and the lower pressure above the rotor. Therefor additional power is required to maintain lift.
Perhaps the better example of the same thing is Argument #2 in the link on above post #476.
No one can argue against the fact that two closely spaced coaxial rotors require significantly more power than two distantly spaced rotors, for the same amount of thrust. In other words, the upper coaxial rotor would be more efficient if the lower rotor was not 'consuming' the stream-tube from the upper rotor.
My position is that 'consuming' a PORTION of the X2 rotor's stream-tube is also of some detriment to the lift.
Graviman, should wish to present a position, please do. However, attack the subject and not the person.
Use the language of 'English' or the language of 'Math', your choice.
Use Momentum Theory or use String Theory; but provide the supporting algorithms and define the symbols if you wish to put forth an argument.
(1) The use of the word pressure could be argued until hell freeze over. Therefore please accept this easy to understand concept.
Dave,
I learned something that I didn't know- the slight increase in power when accellerating in ground effect. I never would have thought it true, either academically or from flight experience in little helos. So, thanks for the chart.
Regarding the stripping of lift, I still have a hard time with that one. When will the propeller be pulling lots of air? At high speed. Will the rotor downwash even be hitting the propeller? At Sikorsky's claimed 250 kts, that is a lot of air zipping under the rotor. If the prop did anything bad to the lift, I wonder if it would be measurable.
-- IFMU
I learned something that I didn't know- the slight increase in power when accellerating in ground effect. I never would have thought it true, either academically or from flight experience in little helos. So, thanks for the chart.
Regarding the stripping of lift, I still have a hard time with that one. When will the propeller be pulling lots of air? At high speed. Will the rotor downwash even be hitting the propeller? At Sikorsky's claimed 250 kts, that is a lot of air zipping under the rotor. If the prop did anything bad to the lift, I wonder if it would be measurable.
-- IFMU
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Originally Posted by Dave Jackson
Graviman, should wish to present a position, please do. However, attack the subject and not the person.
After the PPRuNe discusion on pressure from ground effect, i went away and relearned stuff that at uni i thought i had understood. The fact that you are mentioning "pressure", but not qualifying it as "total pressure", "static pressure" or "dynamic pressure" means the discussion gets completely stuck on fundamentals. Total P = Static P + Dynamic P , and must be constant along one streamline, but can be different between two streamlines .That is why pitot-static ports are best placed on a probe in the same streamline for experimental work. When considering a streamline through a disk the assumption is that the same streamline passes through the disk with an increase in total pressure (which can be induced dynamically or statically). What actually happens is that the assumption that the same streamline flows normally through the disk plane is just a mathematical convenience. From the moving aerofoil reference frame, the actual streamlines are helixing into the disk then helixing out at a steeper angle. Downstream the flow contracts or expands which determines whether the assumed total pressure increase is actually static or dynamic.
This was all stated concisely in my last post. How you came to feel that this was a personal attack is quite beyond me!
This is getting away from X2.
----
Interesting walkaround the LTH mockup, IFMU.,
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IFMU,
The propeller must be pulling air at a higher velocity (plus from a larger area) then the 'remote velocity'' if it is to provide thrust.
There must be some way. Perhaps, in a wind-tunnel.
CEFOSKEY,
Prouty's column 'Fly-By-Wire' in the Spring 2009 issue of Vertiflite gives an overview of this subject.
Graviman,
Thanks for the elaboration. As you know, there are probably dozens of 'theories' on aerodynamics, with each one attempting to get closer to the ever-elusive reality.
Your position, which proposes a consideration of the individual rotor-blade and the individual propeller-blade helix interactions has probably never been computed; at least not in a non-coaxial arraignment. The basic question relates to the interaction of a lower tractor propeller and a rotor, and this should not necessitate the consideration of helix-helix interactions.
Dave
When will the propeller be pulling lots of air?
If the prop did anything bad to the lift, I wonder if it would be measurable.
CEFOSKEY,
I am still curious with regards to the mixing between pilot inputs and the resulting FBW controlled servo movements.
Graviman,
Thanks for the elaboration. As you know, there are probably dozens of 'theories' on aerodynamics, with each one attempting to get closer to the ever-elusive reality.
Your position, which proposes a consideration of the individual rotor-blade and the individual propeller-blade helix interactions has probably never been computed; at least not in a non-coaxial arraignment. The basic question relates to the interaction of a lower tractor propeller and a rotor, and this should not necessitate the consideration of helix-helix interactions.
Dave
Last edited by Dave_Jackson; 11th May 2009 at 22:53.
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Dave,
This is all textbook stuff. What i have described is the basic principle behind combined blade element and momentum theory. My vantage point is that i have also created a vortex panel method, which can be as accurate as CFD but without the computational overhead (mine should eventually cope with drag, trailing edge stall, and transonic flow). Integrating VPM and BEMT in one rotor/fan model has left me with the understanding about the helices described above. I think it is this lack of perspective which leads to a lot of misunderstanding (including my own a while back) about how flow alters the power required for a given thrust/lift.
Regarding X2, it does not matter where the prop is located relative to the rotor. If you think of the helicopter as being in a black box, it will require thrust power to cruise and to lift power to hover - This will not change significantly with configuration as long as rotor and prop area are constant. In some flight conditions puller prop will allow a slightly greater total area of induced flow, ie prop & rotor streamlines are kept apart, reducing power requirement. In power hungry conditions of hover and cruise-dash i doubt power requirements are any different with puller or pusher prop. Transition is one example of a single flight condition, but it a transient event requiring perhaps a brief blip in power increase for a given envelope.
This is all textbook stuff. What i have described is the basic principle behind combined blade element and momentum theory. My vantage point is that i have also created a vortex panel method, which can be as accurate as CFD but without the computational overhead (mine should eventually cope with drag, trailing edge stall, and transonic flow). Integrating VPM and BEMT in one rotor/fan model has left me with the understanding about the helices described above. I think it is this lack of perspective which leads to a lot of misunderstanding (including my own a while back) about how flow alters the power required for a given thrust/lift.
Regarding X2, it does not matter where the prop is located relative to the rotor. If you think of the helicopter as being in a black box, it will require thrust power to cruise and to lift power to hover - This will not change significantly with configuration as long as rotor and prop area are constant. In some flight conditions puller prop will allow a slightly greater total area of induced flow, ie prop & rotor streamlines are kept apart, reducing power requirement. In power hungry conditions of hover and cruise-dash i doubt power requirements are any different with puller or pusher prop. Transition is one example of a single flight condition, but it a transient event requiring perhaps a brief blip in power increase for a given envelope.
Last edited by Graviman; 12th May 2009 at 18:28.
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Regarding X2, it does not matter where the prop is located relative to the rotor.
WIND-TUNNEL TESTS 0F SINGLE- AND DUAL-ROTATING PUSHER PROPELLERS HAVING FROM THREE TO EIGHT BLADES
"The single-rotating pusher propellers were from 0 to 7 percent less efficient than the corresponding tractor propellers and from 0 to 10 percent less efficient than the tractor propeller tested with the wing. The dual propellers provide about the same efficiency, within a few percent, irrespective of whether they were tested as tractors or as pushers and whether or not the wing was present."
The first sentence suggests that a pusher propeller is less efficient than a tractor propeller.The second sentence suggests that Kamov knows what they are doing when they put contra-rotating propellers on their proposed Coaxial-ABC ~ [post #362]
Dave
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Dave,
Are we discussing rotor-propeller interaction, fuselage-propeller interaction, or wing-propeller interaction here? The objective of the discussion, hence the reason for my explanations, seems to be constantly moving.
The results in the cited NACA report do not even begin to suprise me. This is the difference between drawing in laminar air and turbulent air. Turbulent air is caused by the fuselage boundary layer. The counterrotating prop gains efficiency by not producing downstream swirl, but the penalty is the rear prop is already running in turbulent air off the front prop. Hence fuselage turbulence becomes insignificant, so pusher or puller makes no difference.
Non of this says anything about how rotor effects the prop inflow. You will notice that in both designs the prop is pushed as rearwards as possible so as to minimise the cyclic variation as main rotor blade passes overhead. Cyclic loading from uneven inflow distribution is the main cause of fatigue failure in propellers. A simplistic evaluation of streamlines just can't capture this, but it must too be considered.
Like i stated earlier pusher prop is a compromise, mostly chosen from deciding just how far into the unknown a new design should venture. Since X2 is gently accumulating flight time i would say they got it right...
Are we discussing rotor-propeller interaction, fuselage-propeller interaction, or wing-propeller interaction here? The objective of the discussion, hence the reason for my explanations, seems to be constantly moving.
The results in the cited NACA report do not even begin to suprise me. This is the difference between drawing in laminar air and turbulent air. Turbulent air is caused by the fuselage boundary layer. The counterrotating prop gains efficiency by not producing downstream swirl, but the penalty is the rear prop is already running in turbulent air off the front prop. Hence fuselage turbulence becomes insignificant, so pusher or puller makes no difference.
Non of this says anything about how rotor effects the prop inflow. You will notice that in both designs the prop is pushed as rearwards as possible so as to minimise the cyclic variation as main rotor blade passes overhead. Cyclic loading from uneven inflow distribution is the main cause of fatigue failure in propellers. A simplistic evaluation of streamlines just can't capture this, but it must too be considered.
Like i stated earlier pusher prop is a compromise, mostly chosen from deciding just how far into the unknown a new design should venture. Since X2 is gently accumulating flight time i would say they got it right...
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Are we discussing rotor-propeller interaction, fuselage-propeller interaction, or wing-propeller interaction here? The objective of the discussion, hence the reason for my explanations, seems to be constantly moving.
The report in post #491 make mention of a 3% advantage for the tractor. Two previous aerodynamic arguments for the tractor are mentioned by other people in the middle of this page.
There is no argument with your basic algorithms related to thrust. However, it appears that the interaction of these two streamtubes does more than just re-vector the thrust.
Coincidentally, that report also discusses contra-rotating propellers. The 0-7% advantage due to the fuselage is of related interest, since the smaller ABC helicopters will have their propulsors located on the centerline of the craft. It is also of related interest since Kamov appears to be moving in that direction.
Mart, you have previously criticized me for hopping from concept to concept. I'm not building a helicopter, therefore there shouldn't be a bias. Personally, I would prefer the pusher for a number of reasons unrelated to aerodynamics.
Since X2 is gently accumulating flight time i would say they got it right...
Dave
Edited, in red, to correct error, 'tractor' was typed by mistake.
Last edited by Dave_Jackson; 14th May 2009 at 15:21.
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Well - OK, Dave. I'm just a little concerned since the discussion is moving away from the thread title Sikorsky X2 coaxial heli development (which is now actually accumulating flight time) towards various unspecified concepts (which are not). As i've commented before, i find it very difficult to provide constructive feedback, since i am never sure which concept i am meant to be considering. This is part of the engineer mentality: figure out the best solution using available data, then make it work - it is a well proven method of getting things done. That does not disallow future improvements, but often does mean that the solution is a compromise of keeping all of the designers the least unhappy. That's just the way of things...
One of my favourite quotes comes from Aaron Cohen, the program manager who kept Space Shuttle development within budget:
"Better is the enemy of good."
Let us agree that in principle the tractor does offer some aerodynamic advantages in certain parts of the flight envelope. However, a tractor would not suit a machine which is after all still a helicopter. Powertrain packaging would be my main reason. But, putting engineering issues asside, visibility of touchdown point will also never be as good with a tractor.
Let X2 prove that 250kts is possible in a helicopter - that is already a challenge. Once this is done the we can discuss improvements until the cows come home...
One of my favourite quotes comes from Aaron Cohen, the program manager who kept Space Shuttle development within budget:
"Better is the enemy of good."
Let us agree that in principle the tractor does offer some aerodynamic advantages in certain parts of the flight envelope. However, a tractor would not suit a machine which is after all still a helicopter. Powertrain packaging would be my main reason. But, putting engineering issues asside, visibility of touchdown point will also never be as good with a tractor.
Let X2 prove that 250kts is possible in a helicopter - that is already a challenge. Once this is done the we can discuss improvements until the cows come home...
Last edited by Graviman; 14th May 2009 at 18:22. Reason: Typo
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Graviman & CEFOSKEY
Sorry, a BIG mistake was made in my previous post when the word 'tractor' was typed instead of the word 'pusher'. It has now been corrected.
Dave
Sorry, a BIG mistake was made in my previous post when the word 'tractor' was typed instead of the word 'pusher'. It has now been corrected.
Dave
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Dave, CEFOSKEY,
It sounds like we are all in agreement then about X2 being the best compromise configuration for this application.
Dave, you have a tendancy to take my comments personally - please don't. They are always intended to provide constructive feedback, and never intended as a putdown. As a design analyst, i am generally under pressure so am very used to providing a response that i feel accurately sums up the situation. If i comment that you would benefit from reading up on aerodynamics that is exactly what i mean, and i am in no way having a poke at your considerable general knowledge about helicopters. You have put together a great web-resource for helicopter design, and i enjoy the fact that you challenge current thinking. By accurate use of aerodynamic and materials terminology your arguements would carry considerable weight, which would make the "experts" have to think much harder to dismiss them.
Mart
It sounds like we are all in agreement then about X2 being the best compromise configuration for this application.
Dave, you have a tendancy to take my comments personally - please don't. They are always intended to provide constructive feedback, and never intended as a putdown. As a design analyst, i am generally under pressure so am very used to providing a response that i feel accurately sums up the situation. If i comment that you would benefit from reading up on aerodynamics that is exactly what i mean, and i am in no way having a poke at your considerable general knowledge about helicopters. You have put together a great web-resource for helicopter design, and i enjoy the fact that you challenge current thinking. By accurate use of aerodynamic and materials terminology your arguements would carry considerable weight, which would make the "experts" have to think much harder to dismiss them.
Mart
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Mart, your remarks are well taken.
That said, some people strive to be experts by focusing their field of vision. Others strive for a holistic perspective, albeit with a reduced depth of understanding. Different strokes for different folks.
I 'assume' (due to limited knowledge) you will agree that it would be ineffective to present an oral dissertation on quantum mechanics to a Buddhist convention.
___________ Back to the Future __________
This thread appears to be the last of the technical ones on PPRuNe-Rotorheads. Fortunately, it is kept alive because of its relationship with an active helicopter development program.
Mart, you are correct when you insist that the postings must be tightly coupled to the Sikorsky-X2. However, if all technical discussion had been excluded, the posts would have been little more than links to infrequent promotional releases, and subsequent platitudes.
Perhaps this long running thread also serves to debate technical subjects that are directly associated with the X2 and its ABC feature.
One, or more, directly related subjects might be;
The current tractor vs. pusher arguments,
Single vs. contra rotating propellers,
Airfoils and flight controls for operating in reverse velocity,
Variable speed rotor vs. Stepniewski's fixed slow speed / large chord ABC rotor.
Variable speed propeller vs. constant speed / variable pitch propeller.
None of the above can be considered as a critique of the X2 since they are little more then hypothetical considerations until proven, as is the developing X2.
Then again, perhaps those who participated in and enjoyed the earlier rotorcraft technical discussions have moved on.
Dave
That said, some people strive to be experts by focusing their field of vision. Others strive for a holistic perspective, albeit with a reduced depth of understanding. Different strokes for different folks.
I 'assume' (due to limited knowledge) you will agree that it would be ineffective to present an oral dissertation on quantum mechanics to a Buddhist convention.
___________ Back to the Future __________
This thread appears to be the last of the technical ones on PPRuNe-Rotorheads. Fortunately, it is kept alive because of its relationship with an active helicopter development program.
Mart, you are correct when you insist that the postings must be tightly coupled to the Sikorsky-X2. However, if all technical discussion had been excluded, the posts would have been little more than links to infrequent promotional releases, and subsequent platitudes.
Perhaps this long running thread also serves to debate technical subjects that are directly associated with the X2 and its ABC feature.
One, or more, directly related subjects might be;
The current tractor vs. pusher arguments,
Single vs. contra rotating propellers,
Airfoils and flight controls for operating in reverse velocity,
Variable speed rotor vs. Stepniewski's fixed slow speed / large chord ABC rotor.
Variable speed propeller vs. constant speed / variable pitch propeller.
None of the above can be considered as a critique of the X2 since they are little more then hypothetical considerations until proven, as is the developing X2.
Then again, perhaps those who participated in and enjoyed the earlier rotorcraft technical discussions have moved on.
Dave
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Exactly. Think P-39 with a rotor system. I wouldnt want to be on that transmission design team.
Then again, perhaps those who participated in and enjoyed the earlier rotorcraft technical discussions have moved on.
Bob
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X2 vs. Bell HTR
So what's a better overall compromise- Sikorsky's X2 coaxial or Bell's Hybrid Tandem Rotor configuration?
http://www.flightglobal.com/blogs/th...em%20rotor.jpg
http://www.flightglobal.com/blogs/th...em%20rotor.jpg
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Relyon,
That Bugatti looks well thought out. Very low frontal area and as simple a profile change as possible to minimise velocity hot spots. I'm slightly suprised that supine position didn't catch on more in powered fixed wings. The risk with a drive shaft is torsional resonance, particularly with direct drive. With a gearbox the inertia of the propellor is reduced on the shaft (proprtional to ratio^2), shifting the frequencies up (proportional to ratio). That said i imagine this would only be a concern when the prop was at low thrust pitch, since there would be damping at high thrust pitch.
riff_raff,
That is the million dollar question - actually the market will likely be billions.
I'm not wanting to upset our tilt-rotor friends here, but for me it all comes down to lowest disk loading and smallest landing area planform. I notice that this new concept reduces from full tilt to just enough tilt to achieve cruise. That goes a long way towards answering the concerns about autorotation from transition, particularly if the tilt mechanism is passive using rotor to vector tilt (would need a damper rather than an actuator). I would imagine that the wings make up for the retreating blade loss of lift, so basically a laterally seperated ABC. Looks like the influence of one particular highly regarded engineer and TP.
My own view is that the market will be big enough for both concepts to have a share. That said X2 ABC is a purist concept. The engineers started from a machine designed to hover and scratched their collective heads to figure how to make it go faster. A wing is always going make life awkward for the hover: they want to take up room and will always stall as speed reduces. To my way of thinking a rotor capable of reacting low rpm lift at high speed will be lighter.
Time will tell...
That Bugatti looks well thought out. Very low frontal area and as simple a profile change as possible to minimise velocity hot spots. I'm slightly suprised that supine position didn't catch on more in powered fixed wings. The risk with a drive shaft is torsional resonance, particularly with direct drive. With a gearbox the inertia of the propellor is reduced on the shaft (proprtional to ratio^2), shifting the frequencies up (proportional to ratio). That said i imagine this would only be a concern when the prop was at low thrust pitch, since there would be damping at high thrust pitch.
riff_raff,
That is the million dollar question - actually the market will likely be billions.
I'm not wanting to upset our tilt-rotor friends here, but for me it all comes down to lowest disk loading and smallest landing area planform. I notice that this new concept reduces from full tilt to just enough tilt to achieve cruise. That goes a long way towards answering the concerns about autorotation from transition, particularly if the tilt mechanism is passive using rotor to vector tilt (would need a damper rather than an actuator). I would imagine that the wings make up for the retreating blade loss of lift, so basically a laterally seperated ABC. Looks like the influence of one particular highly regarded engineer and TP.
My own view is that the market will be big enough for both concepts to have a share. That said X2 ABC is a purist concept. The engineers started from a machine designed to hover and scratched their collective heads to figure how to make it go faster. A wing is always going make life awkward for the hover: they want to take up room and will always stall as speed reduces. To my way of thinking a rotor capable of reacting low rpm lift at high speed will be lighter.
Time will tell...
Last edited by Graviman; 16th May 2009 at 15:48.
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Just in terms of size the htr concept will always have the disadvantage of taking up more space, which is a concern when planning to ship these by air to bases around the world. That is one aspect of the coaxial design that the russians have capitalized on.
Last edited by bat1; 16th May 2009 at 20:32.