Turbulators on Helicopter Blades ?
A slat is a leading edge high lift device - just like you'll find on all commerical airliners. (The bit that slides down at the leading edge of the wing for take of and landing). They are not practical for helicopters, but make for interesting studies for academics.
CRAN
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Hi CRAN
ok, i call it "flaperon". thank you Jcran
i agree that it would be hard to setup on a complete blade, but should be experimented at the root (25%) or so.
what do you think ?
thanks
ok, i call it "flaperon". thank you Jcran
i agree that it would be hard to setup on a complete blade, but should be experimented at the root (25%) or so.
what do you think ?
thanks
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Perhaps there may be a future for slats on the blades of a helicopter.
On January 11, 2005, Sikorsky was granted US patent 6,840,741. It is for 'Leading edge slat airfoil for multi-element rotor blade airfoils'.
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There must be at least one or two thousand helicopter patents. Sikorsky alone, has approximately 335 US patents.
It would be interesting to know;
On January 11, 2005, Sikorsky was granted US patent 6,840,741. It is for 'Leading edge slat airfoil for multi-element rotor blade airfoils'.
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There must be at least one or two thousand helicopter patents. Sikorsky alone, has approximately 335 US patents.
It would be interesting to know;
- What percentage of these patents ever made it into a production craft?
- What percentage of these patents ever deterred another party from putting that patent, or a derivative of it, into a production craft?
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Hey !
is the idea of a slat on an helicopter blade forbidden to use ? what the .
.
i think it is a good idea
is the idea of a slat on an helicopter blade forbidden to use ? what the .
.i think it is a good idea
Last edited by zeeoo; 29th Jan 2005 at 00:42.
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Just to throw two (hopefully relevant) ideas into this very interesting discussion.
The CH-47D has rotor blades that are significantly wider chord than the predecessor's - nearly 50% more chord. Being made of fiberglass, the area behind the spar was quite rough, certainly in comparison to the surface of the metal blades. I asked about the roughness, and was told it was actually beneficial for keeping flow attached.
Second point - I noticed on the tips of the Bell 212 blades recently that there was the 'normal' erosion on the leading edges, and also an area of erosion on the trailing edge near the tip - there appeared to be quite a large area between the two with no erosion, which indicated to this uneducated eye that perhaps there was quite a bit of flow separation going on here.
Saw the same things on the tail rotor blades of a Hughes 500 - leading edge erosion with a gap and then sand-blasting at the (reflexed) trailing edge.
Any comments?
And has anyone actually tried turbulator tape on a helicopter rotor blade?
The CH-47D has rotor blades that are significantly wider chord than the predecessor's - nearly 50% more chord. Being made of fiberglass, the area behind the spar was quite rough, certainly in comparison to the surface of the metal blades. I asked about the roughness, and was told it was actually beneficial for keeping flow attached.
Second point - I noticed on the tips of the Bell 212 blades recently that there was the 'normal' erosion on the leading edges, and also an area of erosion on the trailing edge near the tip - there appeared to be quite a large area between the two with no erosion, which indicated to this uneducated eye that perhaps there was quite a bit of flow separation going on here.
Saw the same things on the tail rotor blades of a Hughes 500 - leading edge erosion with a gap and then sand-blasting at the (reflexed) trailing edge.
Any comments?
And has anyone actually tried turbulator tape on a helicopter rotor blade?
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Hi Shawn,
i'm not an aero eng,so i just throw my opinion as a newbie.
about the rough area, maybe a clue : i think since a long time that some experiments on non slick surfaces could be applied to the wings.
I talk about surfaces like the new swimming suits whose small "turbulences" create an air cushion in the water.
People think that the shark skin is not good hydrodynamically, but studies have shown that their micro-structure made of thousands of streamlined micro-teeth have a benefic effect as ther unbound the water from the skin, making a turbulent thin cushion that improves the Cx.
The studies on birds, particularly nightbirds show that their smooth quills avoid turbulences and make their flight silent.
I don't know the effetc of a turbulator tape, but i think the thin tape limit should catch the bubble, making it more predictable.
If i'm not wrong, In boats propellers, this phenomena is called cavitation and is known to cause metal fatigue.
I wonder if this phenomena in air if responsible for some delamination problems.
just throwing wild thoughts.
Thanks
i'm not an aero eng,so i just throw my opinion as a newbie.
about the rough area, maybe a clue : i think since a long time that some experiments on non slick surfaces could be applied to the wings.
I talk about surfaces like the new swimming suits whose small "turbulences" create an air cushion in the water.
People think that the shark skin is not good hydrodynamically, but studies have shown that their micro-structure made of thousands of streamlined micro-teeth have a benefic effect as ther unbound the water from the skin, making a turbulent thin cushion that improves the Cx.
The studies on birds, particularly nightbirds show that their smooth quills avoid turbulences and make their flight silent.
I don't know the effetc of a turbulator tape, but i think the thin tape limit should catch the bubble, making it more predictable.
If i'm not wrong, In boats propellers, this phenomena is called cavitation and is known to cause metal fatigue.
I wonder if this phenomena in air if responsible for some delamination problems.
just throwing wild thoughts.
Thanks
Shawn,
Those certainly are two interesting points indeed.
If we think about the erosion on the Bell 212 blades first. To start let me ask a question...If flow separation causes erosion to the surface of rotor blades wouldn't we expected to see rather a lot of erosion near the root - i.e. in the reversed flow region? I don't really know the answer to your original question I can only offer an educated guess. As we all know the erosion on the leading edge of rotor blades near the tip is essentially impact damage, from debris thrown around by the rotor wake, insects and other bits and bobs. It is most noticeable near the tips since this is where the velocities are greatest. I would argue that the erosion that you see on the trailing edge is likely to be related to the entrainment of that same debris etc, by the role-up of the tip vortex. I can't say whether or not the flow there will be separated, but it will certainly be highly three-dimensional.
If you are interested in the boundary layer behaviour on the Huey series then Tanner and Yaggy did a fascinating study on this subject using a UH-1B in the hover. The reference for this is:
Experimental boundary layer study on hovering rotors, Journal of the American Helicopter Society, Vol.11(3) pp. 22-37, 1966.
One of the figures in the report clearly shows the 'affected' area of the upper surface of the blade, over which the vortex lies, if this is approximately the same region in which you have noticed the erosion then we have validated my argument. The report also shows the extent of the laminar and turbulent regions on the rotor, which is essentially what this thread has been all about.
So the short answer is that I think the erosion on the aft of the blade in the tip area is likely to be the result of debris entrainment into the tip vortex flow field, which may or may not involve local flow separations.
The point about the CH47D blades is similarly interesting, but I feel that saying that surface roughness helps delay separation is way too simplistic to be particularly helpful. I have never seen any convincing evidence that the general rule is that separation location is delayed with increasing surface roughness levels for a fully turbulent flow. However, as we have discussed above and as you will see in the paper I have highlighted - even on relatively large helicopters operating at high tip Reynolds numbers, large regions of laminar flow exist on both the upper and lower surfaces of the main rotor. The report cited, illustrates a laminar region of approximately 10% chord on the upper surface and 40% chord for the lower surface for a severely eroded blade in steady hover. Under these conditions then surface roughness can make a significant difference. However, the roughness must exist ahead of the transition location, in order to trigger laminar-to-turbulent boundary layer transition prior to the location at which it would naturally occur. If this is done (on the upper surface only) then yes, leading edge laminar separation can be eliminated. However, I find it quite hard to believe that it can be demonstrated that surface roughness on a region of the blade that will be turbulent anyway will have a significant effect on the separation characteristics. All that I can see this doing is increasing the viscous drag.
So in this case I feel quite strongly that you should take that explanation with a pinch of salt. I think it is a classic case of simplifying things to the point that they don't actually make sense any more!
With regards your question about trying turbulator tape on rotors, the answer is no. The reason is that only in the last few years have we been able to predict boundary layer transition accurately on helicopter rotors in the hover. We still can't do it for forward flight! If you can't predict the transition behaviour then you can't simulate the transitional flow. If you can't simulate the transitional flow then you won't predict the bubble behaviour correctly. And if you can't predict the bubble behaviour correctly how will you know where to put the tape to avoid the laminar separation that causes the bubble? Remembering that because of the vast array of aerodynamic conditions any balde section will be subjected to these transitional features will be moving all over the place and if you put the tape in the wrong place it will make things worse not better!
I hope this helps and keeps this interesting discussion going...
All the best,
CRAN
Those certainly are two interesting points indeed.
If we think about the erosion on the Bell 212 blades first. To start let me ask a question...If flow separation causes erosion to the surface of rotor blades wouldn't we expected to see rather a lot of erosion near the root - i.e. in the reversed flow region? I don't really know the answer to your original question I can only offer an educated guess. As we all know the erosion on the leading edge of rotor blades near the tip is essentially impact damage, from debris thrown around by the rotor wake, insects and other bits and bobs. It is most noticeable near the tips since this is where the velocities are greatest. I would argue that the erosion that you see on the trailing edge is likely to be related to the entrainment of that same debris etc, by the role-up of the tip vortex. I can't say whether or not the flow there will be separated, but it will certainly be highly three-dimensional.
If you are interested in the boundary layer behaviour on the Huey series then Tanner and Yaggy did a fascinating study on this subject using a UH-1B in the hover. The reference for this is:
Experimental boundary layer study on hovering rotors, Journal of the American Helicopter Society, Vol.11(3) pp. 22-37, 1966.
One of the figures in the report clearly shows the 'affected' area of the upper surface of the blade, over which the vortex lies, if this is approximately the same region in which you have noticed the erosion then we have validated my argument. The report also shows the extent of the laminar and turbulent regions on the rotor, which is essentially what this thread has been all about.
So the short answer is that I think the erosion on the aft of the blade in the tip area is likely to be the result of debris entrainment into the tip vortex flow field, which may or may not involve local flow separations.
The point about the CH47D blades is similarly interesting, but I feel that saying that surface roughness helps delay separation is way too simplistic to be particularly helpful. I have never seen any convincing evidence that the general rule is that separation location is delayed with increasing surface roughness levels for a fully turbulent flow. However, as we have discussed above and as you will see in the paper I have highlighted - even on relatively large helicopters operating at high tip Reynolds numbers, large regions of laminar flow exist on both the upper and lower surfaces of the main rotor. The report cited, illustrates a laminar region of approximately 10% chord on the upper surface and 40% chord for the lower surface for a severely eroded blade in steady hover. Under these conditions then surface roughness can make a significant difference. However, the roughness must exist ahead of the transition location, in order to trigger laminar-to-turbulent boundary layer transition prior to the location at which it would naturally occur. If this is done (on the upper surface only) then yes, leading edge laminar separation can be eliminated. However, I find it quite hard to believe that it can be demonstrated that surface roughness on a region of the blade that will be turbulent anyway will have a significant effect on the separation characteristics. All that I can see this doing is increasing the viscous drag.
So in this case I feel quite strongly that you should take that explanation with a pinch of salt. I think it is a classic case of simplifying things to the point that they don't actually make sense any more!
With regards your question about trying turbulator tape on rotors, the answer is no. The reason is that only in the last few years have we been able to predict boundary layer transition accurately on helicopter rotors in the hover. We still can't do it for forward flight! If you can't predict the transition behaviour then you can't simulate the transitional flow. If you can't simulate the transitional flow then you won't predict the bubble behaviour correctly. And if you can't predict the bubble behaviour correctly how will you know where to put the tape to avoid the laminar separation that causes the bubble? Remembering that because of the vast array of aerodynamic conditions any balde section will be subjected to these transitional features will be moving all over the place and if you put the tape in the wrong place it will make things worse not better!
I hope this helps and keeps this interesting discussion going...
All the best,
CRAN
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Interesting.
check this out : http://barreau.matthieu.free.fr/publ...ostic-aero.pdf
there are some visual tips to examinate the transition (french language).
check this out : http://barreau.matthieu.free.fr/publ...ostic-aero.pdf
there are some visual tips to examinate the transition (french language).
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laminar flow
Laminar flow will exist only if the airfoil is smooth. If a small spec of dirt or bug is stuck near the leading edge it will trip the laminar flow. Nobody has figured a way to avoid bug impacts as far as I know, so laminar flow is not really found in actual operations(sailplanes can benefit from laminar flow lower drag, but only if the pilot cleans the wing before flight and flies through the lower atmosphere without striking many bugs. Each bug leaves an expanding wake.
Also the airfoil profile must be designed for laminar flow and I dont think helicopter airfoils are designed for laminar flow.
And the airfoil must be carefully sanded to provide a smooth flow.(no waves in the surface more than .003" in two inches)
If you can feel any imperfection with your hand, it will not support laminar flow.
I would forget about laminar flow. The studies mentioned were probably made with smooth airfoils in a laboratory.
Turbulent separation is something to think about however, and vortex generators are normally employed to prevent the reverse flow of turbulent separation and the high drag and loss of lift.
Another idea would be something 3M company was working on years ago. They invented something called "riblet skin" (I think) a plastic skin with micro grooves that reduced drag similar to the shark skin zeeoo mentioned.
But it could be big problem if the tape starts to disbond in flight, think about that before you modify a blade. I applied some duct tape once to the tip of my airplane prop to experiment with balance. Then I flew it. The tape came off part way and made a buzzing noise and I could barely climb. Little things can have a big effect.
slowrotor
Also the airfoil profile must be designed for laminar flow and I dont think helicopter airfoils are designed for laminar flow.
And the airfoil must be carefully sanded to provide a smooth flow.(no waves in the surface more than .003" in two inches)
If you can feel any imperfection with your hand, it will not support laminar flow.
I would forget about laminar flow. The studies mentioned were probably made with smooth airfoils in a laboratory.
Turbulent separation is something to think about however, and vortex generators are normally employed to prevent the reverse flow of turbulent separation and the high drag and loss of lift.
Another idea would be something 3M company was working on years ago. They invented something called "riblet skin" (I think) a plastic skin with micro grooves that reduced drag similar to the shark skin zeeoo mentioned.
But it could be big problem if the tape starts to disbond in flight, think about that before you modify a blade. I applied some duct tape once to the tip of my airplane prop to experiment with balance. Then I flew it. The tape came off part way and made a buzzing noise and I could barely climb. Little things can have a big effect.
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Slowrotor,
you're very well informed, that's precious
can i have some infos on that 3M tape ?
i feel, since sometimes that a kind of "brushed velvet", reproducing the quills micro-hairs could be interesting to try.
more simply, i was wondering if a brushed surface could add some value.
Thanks
you're very well informed, that's precious
can i have some infos on that 3M tape ?
i feel, since sometimes that a kind of "brushed velvet", reproducing the quills micro-hairs could be interesting to try.
more simply, i was wondering if a brushed surface could add some value.
Thanks
Slowrotor,
With all due respect, I strongly disagree with your comments regarding laminar flow and the extent of roughness required to transition to turbulence for the case of a hovering rotor. The article that I referred to describe actual flight tests on a real, full scale Bell UH-1B, with SEVERELY eroded blades. I have (although I cannot describe the specifics) similar reports for flight tests on a Eurocopter AS-365 Dauphin and the BO-105 that show similar extents of laminar flow.
There are a host of complex, interconnected reasons why laminar flow CAN exist on hovering rotor blades - even in the presence of some roughness, not least the relatively low Reynolds numbers seen on rotors. The key to all of this is the fact that in hover, the rotor experiences a steady, largely two-dimensional flow. In addition, the majority of the rotor does not have sweep and therefore the modes by which transition can occur are limited to the two-dimensional modes; Tollmien-Schlichting instability and transition in the free shear layer following a laminar separation. Notice that the troublesome three-dimensional modes, cross-flow and attachment line do not play a part in hovering rotor transition. This is the crux of why laminar flow can exist on a rotor. The three-dimensional modes are generally far more unstable than the two dimensional ones and as a consequence it is much more difficult to sustain laminar flow on a fixed wing. (For small GA aeroplane at low speed I would expect attachment-line contamination to be the main problem, though I have never looked at it in detail.) The attachment line in particular is renowned for its sensitivity to bugs and dirt and hence this, I believe, is where the origins of your views on the effect of surface roughness lie.
Of coarse, in forward flight the rotor is heavily yawed and all modes become important, so for the most part I would expect to see far less laminar flow, but I would be very surprised if 'pockets' of laminar flow were not still present.
I hope this helps
Kind Regards
CRAN
With all due respect, I strongly disagree with your comments regarding laminar flow and the extent of roughness required to transition to turbulence for the case of a hovering rotor. The article that I referred to describe actual flight tests on a real, full scale Bell UH-1B, with SEVERELY eroded blades. I have (although I cannot describe the specifics) similar reports for flight tests on a Eurocopter AS-365 Dauphin and the BO-105 that show similar extents of laminar flow.
There are a host of complex, interconnected reasons why laminar flow CAN exist on hovering rotor blades - even in the presence of some roughness, not least the relatively low Reynolds numbers seen on rotors. The key to all of this is the fact that in hover, the rotor experiences a steady, largely two-dimensional flow. In addition, the majority of the rotor does not have sweep and therefore the modes by which transition can occur are limited to the two-dimensional modes; Tollmien-Schlichting instability and transition in the free shear layer following a laminar separation. Notice that the troublesome three-dimensional modes, cross-flow and attachment line do not play a part in hovering rotor transition. This is the crux of why laminar flow can exist on a rotor. The three-dimensional modes are generally far more unstable than the two dimensional ones and as a consequence it is much more difficult to sustain laminar flow on a fixed wing. (For small GA aeroplane at low speed I would expect attachment-line contamination to be the main problem, though I have never looked at it in detail.) The attachment line in particular is renowned for its sensitivity to bugs and dirt and hence this, I believe, is where the origins of your views on the effect of surface roughness lie.
Of coarse, in forward flight the rotor is heavily yawed and all modes become important, so for the most part I would expect to see far less laminar flow, but I would be very surprised if 'pockets' of laminar flow were not still present.
I hope this helps
Kind Regards
CRAN
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Cran,
You are quite correct. Thanks for setting me straight.
I had never considered laminar flow with a rough surface to be possible, but indeed, in Prouty's book he states "a rotor blade, even one with leading edge erosion, can maintain laminar flow more easily than a wing, possibly because built-in surface imperfections are usually less and also because pitting is less detrimental than protrusion".
That's good to know.
One does'nt learn while shooting their mouth off, but sometimes it gets a response that they will remember.
Thanks Cran.
slowrotor
(now I have to figure out what a supercritical airfoil is)
You are quite correct. Thanks for setting me straight.
I had never considered laminar flow with a rough surface to be possible, but indeed, in Prouty's book he states "a rotor blade, even one with leading edge erosion, can maintain laminar flow more easily than a wing, possibly because built-in surface imperfections are usually less and also because pitting is less detrimental than protrusion".
That's good to know.
One does'nt learn while shooting their mouth off, but sometimes it gets a response that they will remember.
Thanks Cran.
slowrotor
(now I have to figure out what a supercritical airfoil is)
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Talking about my interest : autorotation
i think that i won't bet on a laminar flow given the shape of the airflow when in autorotation :
http://www.aero.gla.ac.uk/Research/Fd/Project15.htm
that illustrates how poor are the studies in pure autorotation.
IMHO The helicopter model doesn't work in that case.
Thanks
i think that i won't bet on a laminar flow given the shape of the airflow when in autorotation :
http://www.aero.gla.ac.uk/Research/Fd/Project15.htm
that illustrates how poor are the studies in pure autorotation.
IMHO The helicopter model doesn't work in that case.
Thanks
Slowrotor,
Thanks for taking the time to engage in the discussion, transition on helicopter rotors is something I have spent a long time working on and so I have quite a good understanding of what’s involved! ...I hope!
With regards the supercritical aerofoils, they are simply aerofoils that have been designed with low upper surface curvature in order to minimise the strength of the shocks created a high subsonic mach numbers without having to resort to very low thickness-to-chord ratios, which are impractical for real aircraft. I have attached a picture from Georgia Tech's website that show the differences between the geometry and pressure distributions of a normal NACA aerofoil (Left) at high subsonic mach numbers and a supercritical one (Right).
Hope this helps
CRAN
Zeeoo,
2D is shorthand for two-dimensional and 3D is shorthand for three-dimensional. In aerodynamic terms we usually refer to things as being two-dimensional phenomena when they don't vary with span, such as the pressure distribution around a constant section, constant chord infinite span wing. A three-dimensional phenomenon is one which does vary with span, such as the roll-up process of the tip vortex around a wing or rotor tip (its effect becomes less as you move inboard).
With regards laminar flow in autorotation, there will probably not be much. But this will be due to the fact that the rotor is in forward flight (see my earlier post) not because it is in autorotation. Visualisations of the rotor wake in hover and steady forward flight look just a scary! Have a look at Richard Brown’s work at Imperial: http://www.ae.ic.ac.uk/research/rotorcraft/
Hope this helps
CRAN
Thanks for taking the time to engage in the discussion, transition on helicopter rotors is something I have spent a long time working on and so I have quite a good understanding of what’s involved! ...I hope!
With regards the supercritical aerofoils, they are simply aerofoils that have been designed with low upper surface curvature in order to minimise the strength of the shocks created a high subsonic mach numbers without having to resort to very low thickness-to-chord ratios, which are impractical for real aircraft. I have attached a picture from Georgia Tech's website that show the differences between the geometry and pressure distributions of a normal NACA aerofoil (Left) at high subsonic mach numbers and a supercritical one (Right).
Hope this helps
CRAN
Zeeoo,
2D is shorthand for two-dimensional and 3D is shorthand for three-dimensional. In aerodynamic terms we usually refer to things as being two-dimensional phenomena when they don't vary with span, such as the pressure distribution around a constant section, constant chord infinite span wing. A three-dimensional phenomenon is one which does vary with span, such as the roll-up process of the tip vortex around a wing or rotor tip (its effect becomes less as you move inboard).
With regards laminar flow in autorotation, there will probably not be much. But this will be due to the fact that the rotor is in forward flight (see my earlier post) not because it is in autorotation. Visualisations of the rotor wake in hover and steady forward flight look just a scary! Have a look at Richard Brown’s work at Imperial: http://www.ae.ic.ac.uk/research/rotorcraft/
Hope this helps
CRAN
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JCRAN,
that helps, like always, thank you.
I was wondering is the use of a supercritical airfoil could be justified at the very tip of a blade, combined with a vortex fence (short anhedral).
The APACHE and Black Hawh blades seem to have that kind of airfoil or am i wrong ? at least those blades have a sudden change in twist at the tip.
what do you think ?
thanks for the link, indeed the airflow shape is scaring !
i really wonder how a 2d study is usefull , unless you apply a lot of empirical corrections..alas i have not acces to a full 3D airfliw simulation software.
Thank you
that helps, like always, thank you.
I was wondering is the use of a supercritical airfoil could be justified at the very tip of a blade, combined with a vortex fence (short anhedral).
The APACHE and Black Hawh blades seem to have that kind of airfoil or am i wrong ? at least those blades have a sudden change in twist at the tip.
what do you think ?
thanks for the link, indeed the airflow shape is scaring !
i really wonder how a 2d study is usefull , unless you apply a lot of empirical corrections..alas i have not acces to a full 3D airfliw simulation software.
Thank you
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From the profile of the supercritical airfoil, it looks like the pilot must roll and then fly the helicopter in the inverted position during high speed flight
Zeeoo,
Supercritical aerofoils are justified for use on rotor systems, but as with everything else, the application of this technology to helicopters is very difficult. While we seek to exploit the benefits of the supercritical aerofoil's performance in the high speed high speed flow found on the advancing blade, we also need the aerofoil to have a high Cl_max capability for the retreating blade, which is not what supercritical aerofoils do best. Therefore, real helicopter aerofoils are a compromise between the type of supercritical aerofoils you might use on a fixed wing aircraft and the thicker, more cambered aerofoils you require for higher incidence operations at lower Mach numbers.
I can't comment on the aerofoils used on either blackhawk or apache as I am not familiar with the rotor systems. Perhaps Nick could help here.
In practise, designing a rotor system involves the careful integration of the characteristics of the aerofoils used along the rotor span and the rotor planform adopted. There are many ways in which the planform behaviour can be used to compensate for the limitations of given aerofoils. However, this is only really important when one is trying to squeeze out the maximum performance that contemporary technology will allow and it is certainly not something one would consider doing for a recreational aircraft as the cost and complexity of design, development and testing would far outweigh the benefits.
With regards the usefulness of 2D approximations of helicopter aerodynamics, I think you would be surprised! Although the rotor wake is actually highly unsteady and three-dimensional, this behaviour can very easily be broken down into the influence that it has on local 2D elemental aerodynamics. Indeed, this is what is done in every comprehensive analysis used by every manufacturer! This is perfectly acceptable for performance calculations and flight dynamics studies. The real limitations of the approach have been highlighted in vibration prediction. For this much higher order aerodynamic simulations are required and most people generally opt for three-dimensional computational fluid dynamics approaches here. The problem with this is that the run-time for the calculations, even on multi-processor supercomputers are huge; many weeks in fact. This of-coarse limits the usefulness of these powerful techniques to design verification rather than design synthesis.
Hope this helps
CRAN
Supercritical aerofoils are justified for use on rotor systems, but as with everything else, the application of this technology to helicopters is very difficult. While we seek to exploit the benefits of the supercritical aerofoil's performance in the high speed high speed flow found on the advancing blade, we also need the aerofoil to have a high Cl_max capability for the retreating blade, which is not what supercritical aerofoils do best. Therefore, real helicopter aerofoils are a compromise between the type of supercritical aerofoils you might use on a fixed wing aircraft and the thicker, more cambered aerofoils you require for higher incidence operations at lower Mach numbers.
I can't comment on the aerofoils used on either blackhawk or apache as I am not familiar with the rotor systems. Perhaps Nick could help here.
In practise, designing a rotor system involves the careful integration of the characteristics of the aerofoils used along the rotor span and the rotor planform adopted. There are many ways in which the planform behaviour can be used to compensate for the limitations of given aerofoils. However, this is only really important when one is trying to squeeze out the maximum performance that contemporary technology will allow and it is certainly not something one would consider doing for a recreational aircraft as the cost and complexity of design, development and testing would far outweigh the benefits.
With regards the usefulness of 2D approximations of helicopter aerodynamics, I think you would be surprised! Although the rotor wake is actually highly unsteady and three-dimensional, this behaviour can very easily be broken down into the influence that it has on local 2D elemental aerodynamics. Indeed, this is what is done in every comprehensive analysis used by every manufacturer! This is perfectly acceptable for performance calculations and flight dynamics studies. The real limitations of the approach have been highlighted in vibration prediction. For this much higher order aerodynamic simulations are required and most people generally opt for three-dimensional computational fluid dynamics approaches here. The problem with this is that the run-time for the calculations, even on multi-processor supercomputers are huge; many weeks in fact. This of-coarse limits the usefulness of these powerful techniques to design verification rather than design synthesis.
Hope this helps
CRAN
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jCRAN,
your advise is helpfull.
i definitely don't try to invent an Xblade. Just to add a little improvement with no risk. I want to enshure my choices won't lead to dramatic problems.
I will use a VR7 airfoil as main airfoil and a VR8 for the last last section of the tip.
Your advise on 2D calculations sound like an encouragement to go on with that despite of it's unperfection.
Thanks for your help
your advise is helpfull.
i definitely don't try to invent an Xblade. Just to add a little improvement with no risk. I want to enshure my choices won't lead to dramatic problems.
I will use a VR7 airfoil as main airfoil and a VR8 for the last last section of the tip.
Your advise on 2D calculations sound like an encouragement to go on with that despite of it's unperfection.
Thanks for your help
