Turbulators on Helicopter Blades ?

[zeeoo]

Hi eng people.
do you know if a system similar to the "turbulators" on fixed wings has been tested on helicopter blades ?
if not what is your opinion on that ?
thank you.

Victor

[CRAN]

I haven't heard of a 'Turbulator', did you mean Vortex Generator?

CRAN

[zeeoo]

hi J.Cran
sorry for the word "turbulator".

I don't mean a vortex generator or a fence.
I'm trying to explain : on a wing, a line at the stall limit that "delimits" the stall region and catches the stall "bubble", or the early turbulence.

http://www.mh-aerotools.de/airfoils/turbulat.htm

this shows a pneumatic turbulator, but it can simply be a V groove.
From my little understanding,the are generally used to "drive" and delimit the stall zone , avoiding some vibrations.

i was thinking about that while i was thinking on a high-lift - airfoil , with a late but brutal stall.

has it been experimented ?
thank you

[sprocket]

Turbulators would probably cause mechnical stresses over and above the forces of an already highly stressed component (radially).

The aerodynamic side would also be complex because of the varying airspeeds and directions over the blade surfaces (each revolution of the rotor) when in forward flight.

[NickLappos]

sproket is right, the awful world of a helicopter blade is a far cry from that of an airplane wing in cruise. The properties of the airfoil are often selected as a compromise beyween the need for low drag in hover, high lift in high speed maneuvering, and low induced drag near stall. It is possible that the kinds of things that work on steady flow on an airplane have little positive effect when the blade goes from near transonic to near low speed stall in 150 milliseconds, and does that alternatively for hours at a time!

[zeeoo]

Thanks,
i also agree with you sprocket.
i was comparing drag and lift polars of a VR7 and an ONERA OA213 and wondering what solutions had been tested to improve lift in the rev-flow zone or, at least, to control the stall.

The funny is that all numbers about the airfoils are given for a straight perpendicular airflow, but this "ideal" airflow just happens at the 90 deg azimut.. the rest of the time a blade being submitted to a mix of front and lateral flow. in fact i guess the air flow on an airfoil is more curved. or am i wrong?
If i'm correct how is it managed in the theories?

thanks.

[slowrotor]

zeeoo,
Turbulators are used mostly on very high performance sailplanes that have laminar flow airfoils.The purpose is for tripping the laminar bubble.
A strip of zig-zag tape is applied, usually aft of 50% chord.
A vortex generator is usually a piece of aluminum about 1"x .375" bonded every 6" or so.

Helicopter blades cannot maintain laminar flow, because for laminar flow an extremely smooth surface must be maintained.
The sandblasted finish of heli blades is far too rough. Even dust can destroy laminar flow.

slowrotor

[Thomas coupling]

Turbulators when they blow tangential to vortex generators can sometimes trigger induced oscillations which perpetuate a resonance approximately 3 generations to the first harmonic. When this becomes fully established laminar fatigue envelopes are propgated at the genesis of the flow and allow boundary flow to disrupt.

It could reduce your Vne by up to 1 knot on a still wind day.

[CRAN]

Zeeoo,

Turbulators are essentially boundary layer trips, which are used to induce laminar to turbulent transition further forward than it would have occured naturally. Boundary layer transition is a very complicated process for which there is no known general theory - even for simple geometries like a parallel flow flat plate case. A number of empirical relations are available but they have very limited applicability and cannot be applied willy-nilly.

The most important point to understand here is the way in which transition occurs on a 2D steady aerofoil at different Reynolds numbers. Let's take for example the venerable NACA0012 aerofoil, with which I have a lot of experience.

Firstly, for a medium size helicopter the tip Reynolds number in the hover is around 3,000,000. Reynolds number is simply the ratio of inertial and viscous forces in the flow. At these sorts of Reynolds numbers transition on isolated steady aerofoils generally occurs as the result of the Tollmien-Schlichting instability up until relatively high incidence when separation can become important. The NACA0012 does have a small separation bubble under these conditions, but it usually plays no part in the stall process. (See NASA TP1100) In addition for this Reynolds number and assuming a very low speed, say Mach 0.15, the transition on the upper surface will move from 0.21c to the leading edge between 3-12 degrees, much further forward than the low Reynolds number cases on the web site linked.

Now, at very low Reynolds number such as those found on low pressure compressor and turbine blades, glider wings and model aircraft, things change considerably. At very low Reynolds numbers transition would naturally occur a very long way aft. This results in the existence of a large region of laminar flow. As has rightly been pointed out; laminar flow is relatively unstable and cannot tolerate adverse pressure gradients or surface roughness. Therefore, a laminar separation often occurs. When this happens two events can follow. If transition occurs in the free shear layer, then the resulting turbulent boundary layer will be able to reattach creating a laminar separation bubble - as described in the web page zeeoo linked to. If transition, doesn't occur early enough then the flow will remain completely separated. This highlights the key difficulty in producing efficient aerodynamic shapes at low Reynolds numbers and is one of the key reasons why small scale gas turbine engines are inefficient and why model aircraft require such high power-to-weight ratios to make them work effectively.

In low Reynolds number flows, the turbulator provides a boundary layer trip that causes transition to occur much earlier than it would have done naturally. By triggering transition early, the laminar separation cannot occur and so the flow behaves much more like a high Reynolds number flow.

Clearly then, this technique can only add value in situation where the rotor is operating at very low Reynolds numbers, such as a Martian rotorcraft, and has been designed with an aerofoil that is completely inappropriate for that aerodynamic condition. At low Reynolds numbers, aerofoils must be carefully designed to control the large laminar region without triggering large separations. At high Reynolds numbers these problems seldom occur because transition occurs much closer to the leading edge (on the upper surface) and so transition occurs before laminar separation anyway.

In my opinion, turbulators are not appropriate for real helicopter/gyro plane flows, because the Reynolds numbers and Mach numbers will be too high for laminar separation be a serious problem - provided the aerofoil choices are sensible. At low Reynolds numbers they are a bit of a 'bodge-fix' and are a kind of admission of defeat in terms of designing a suitable aerofoil for the flight condition...we couldn't do it properly so we tripped the flow early to avoid the problem. Certainly not elegant engineering!

The other important point as Nick rightly pointed out is that there is a whole world of difference between the transition behaviour in steady and unsteady flows. I can tell you for certain that at typical helicopter Reynolds numbers and flight conditions that the transition behaviour in the leading edge region of the upper surface of common rotor aerofoil has a dramatic effect on the dynamic stall behaviour and the unsteady loads produced...but that's a heck of a lot more complicated!

So in answer to your question; has it been experimented with…yes, but not for the reasons you want. Will it help? No, not unless you are building a model gyrocopter! Will it help control stall? No, because stall is not a result of premature laminar separation on most helicopter aerofoils at realistic Reynolds numbers.

Hope this helps
CRAN

:D

[zeeoo]

Thomas, Slow, J.Cran

great explanations.

I fully agree with you cran when you say it's a defeating engeneering ;) a kind of "driving the worse"..

I still having questions in the choice of a OA213 and a VR7 or NACA0012 , the first has, appearently a design made for higher AOA , similar to the VR4.

My questions would be what choice for thinner chords (lets say 16 cm). and what are the wayto reduce the reverse flow and gain efficiency.

I'm also wondering in the sanded surface on the upper surface... it sould be very slick... the sanded should be under, to slow the flow.. or is it a way to deal with a non-laminar but averaged up flow ?

Thanks to all, this really helps.

[Thomas coupling]

Zeeoo: you could consider postulating the disturbed flow under the nacelles and wingform.
This gives optimised non laminer flow without degradation of theta2.
try a reynolds number equivalent to the latest NACA series amendment?

[zeeoo]

Thomas, thanks.

I misunderstand your first recommendation :
what disturbed flow ? you mean sucking it ?

what are the latest NASA series ? high lift ?
i heard about Re about 20M to 30M , is it right ?

thank you

[CRAN]

Zeeoo,

Thomas is making fun of you/us.

CRAN
:suspect:

[zeeoo]

Oh.. thanks Cran.
thanks for not having and ego so inflated by theta ;).

I'm not interested in studying the vaccum inside the skulls.
excuse my poor english.

BTW i woud be grateful to someone able to tell if there is an algorithm or approx. to calculate a Coeff of lift (Cz in my language), for a given airfoil.

Thanks.

[Dave_Jackson]

zeeoo,

I looked for the very same information a few years back, but with little luck. There is an economical book called 'Theory of Wing Sections' by Abbott and Doenhoff, but it is from 1959. In addition, Prouty's main book gives quite a bit of information on the basic airfoils, such as the NACA 0012.

If you come across a newer or better source for this information, please advise.

Here is the coding, in Basic, for the lift for the NACA 0012. If you want, I can email you the whole boring module. http://www.unicopter.com/Sleep.gif
______________________________

Public Function coefficient_of_lift_NACA_0012(Alpha, AlphaL0, M, aa As Single) As Single
'NACA 0012
Dim AlphaL As Single 'Angle of attack where the lift coefficient
' first shows the effects of stall.
Dim K1 As Single 'Coefficient
Dim K2 As Single 'Coefficient
On Error GoTo coefficient_of_lift_NACA_0012_Err
'The basic simple equations.
'cl = aa * Alpha
'cl = 0.1 * Alpha 'Alpha (angle of attack) in degrees.
'cl = 6.0 * Alpha 'Alpha (angle of attack) in radians.

If (M < 0.725) Then ' Lift coefficients below 0.725M
AlphaL = 15 - 16 * M 'First effects of stall.
If (Alpha > AlphaL) Then 'If above stall.
K1 = 0.0233 + 0.342 * M ^ 7.15
K2 = 2.05 - 0.95 * M '## PROBLEM IF BELOW IS NEGATIVE & BELOW IS NOT A INTEGER
coefficient_of_lift_NACA_0012 = (aa * Alpha) - (K1 * ((Alpha - AlphaL) ^ K2))
Else 'If below stall.
coefficient_of_lift_NACA_0012 = aa * Alpha
End If
Else '(M < 0.725) Then 'Lift coefficients above 0.725M
K1 = 0.575 - 0.144 * (M - 0.725) ^ 0.44
K2 = 2.05 - 0.95 * M
coefficient_of_lift_NACA_0012 = ((0.677 - 0.744 * M) * Alpha) - (0.0575 - 0.144 * (M - 0.725) ^ 0.44) * ((Alpha - 3.4) ^ (2.05 - 0.95 * M))
End If

coefficient_of_lift_NACA_0012_Exit:
Exit Function
coefficient_of_lift_NACA_0012_Err:
MsgBox Err.Description
Resume coefficient_of_lift_NACA_0012_Exit
End Function:

[CRAN]

Zeeoo,

There is, but its not simple! To calculate the behaviour of the aerodynamics around any given shape for a viscous, compressible, unsteady flow you need to solve the Navier-Stokes equations. The implementation of a 2D Navier-Stokes solver for aerofoils, involves about 100-200 pages of code depending on its complexity. Once you have your code you will need to solve the equations on a suitable computational grid - you will need grid generation software to produce the grid also.

Once you have a code and a grid for the aerofoil, you can expect the calculation to take about half a day to accurately compute lift and drag, for a single design point on a desktop PC. (Less if you accept lower accuracy.)

If you accept lower accuracy you could also choose to solve reduced forms of the equations, which may, for example, limit you to attached incompressible flows. For this I suggest you do a search for XFOIL on the internet, it's free and should get you started! Plus it runs much, much quicker!

Incidentally, before you get too excited, the best practical useful methods available today (Reynolds Averaged Navier-Stokes Solvers) cannot accurately predict Clmax, i.e. the static stall point. Nor can they predict drag particularly well! It takes a very long time to learn how to tease useful engineering data out of these methods and if not applied very carefully will produce nonsense for results! Like I said...it's not easy!

Hope this helps
CRAN

PS: If I were designing a recreational gyrocopter I probably wouldn't be attempting to design new aerofoils - I would use established aerofoils for which there is lots of EXPERIMENTAL test data over a broad range of conditions. Perhaps the NACA0012...:) Like Dave said 'The Theory of Wing Sections' by Abbott and Von Doenhoff is your best bet.

[zeeoo]

Hi,

JCran, this helps and makes sense.
I don't try to resolve the Navier thing. as yous said, i just need a common, united database from whitch i could get the data to include in my routines. a kind of CLift (airfoil, AoA).
not much, as you said, i believe i must not go into a comlex simulation but re-use existing data but Whitch data.
XFoil and JAVAFoil are great but i can't get them interfaced with my code yet.

Dave, thanks for the code, interesting, but very particular to the naca0012, if i understand it correctly. i asked the same on rotary and got an approx solution for a 3D wing , i think it is for a very approx aprox :
---------------------------------- from Gabriel Hugh Elkaim
- rotary
Cl = 2*pi*AR/(AR+2)*alpha

where alpha is the angle of attack from the zero lift line in RADIANS
AR is the aspect ratio (span/chord for a rectangular wing)
____________________________________

using the AR to calculatet hat is a kind of simple , how "un-acurate" can it be ?


for your understanding : i try to code a simulator that will compute the lift along a blade, with a transition between different airfoils and different twist values. Then i will compute the lift on the disk at different angles and airfoil combinations.
Since I'm not a math man, i slice the blade in N small elements to plot the lift/drag values.

QUESTIONS :
I was wondering if i can interlopate an average CLift betwen 2 airfoils CLifts at the same AoA.

How to consider the airflow on the 0 degree azimut (blade at the front), the blade being submitted to an angular speed + a frontal speed, the air flow passes through a distorted airfoil. could i introduce a kind of coefficient of distortion ? or can i consider the lift is the same.

Thanks for your quality help.

---------------------------------- from Geneweber - rotary
The Department of Aerospace Engineering at the University of Illinois has a large airfoil coordinate database. http://www.aae.uiuc.edu/m-selig/ads.html I ran some of these through both JavaFoil http://www.mh-aerotools.de/airfoils/javafoil.htm and Xfoil http://raphael.mit.edu/xfoil/ and was able to generate lift and drag plots. It was interesting. Hope this helps.

[Dave_Jackson]

zeeoo,

" i try to code a simulator that will compute the lift along a blade, with a transition between different airfoils and different twist values. Then i will compute the lift on the disk at different angles and airfoil combinations.
Since I'm not a math man, i slice the blade in N small elements to plot the lift/drag values."

What you are looking for is in Prouty's book 'Helicopter Performance, Stability and Control". He provides 21 steps, and their algorithms, to calculate the lift of rotors and their blades, in hover. The calculations are based on 'Combined Momentum and Blade Element Theory with Empirical Corrections'.

The inputs are;
Rotor geometry; - number of blades, radius, chord, twist, cutout and airfoil data.
Test conditions;- tip speed, atmospheric density, speed of sound.

I have coded it in Basic in Microsoft's Access database. The previous post was the NACA 0012 lift calculations Module from this program.

The problem was that I could only find valid algorithms for the NACA0012 and the 8-H-12 airfoils. The algorithms for other airfoils could only be guesstimated.

If you can find the algorithims for other airfoils, I'll trade the coding and forms for them. ;)

_______

These pages will have some information, which you know of, and perhaps some that is new to you.
OTHER: Aerodynamic - Airfoil (http://www.synchrolite.com/B356.html)
OTHER: Aerodynamic - Blade Profiles (http://www.synchrolite.com/B325.html)


Dave

[zeeoo]

Hi Dave,
for airfoil data, i plan to plot myself some values, coming from an array, like you did on the vR7 data on your site (yes i read it sometimes ;)).

I dont need an universal alrorithm, only some airfoils like VR7b, VR9, VR5, OA213(similar to VR12),B29-root , so, i think i will "hard" code the data, based on Bezier splines, i will "build" the curve my self trying to be as close as the original, doing so, i can have an interpolated (quite precise) data.

do you think i can interpolate between the CL of 2 given airfoils at the same AoA ? for example from 0.50 to 0.58, giving 0.54 ?

What i want tyo do is calculate the lift not in hover, but with a disk pitch and a relative wind (speed).
then , if i get it working, i will compute the vectors for each "slice" of the blade and will have an approx of the autorotation capabilities.

I can easily have a 3D interface, could be interesting.

then why not the calculation of the moments, centrifugal loads, and maybe compute the coning and the sissors load at the root.
and why not imput a kind of hinge elasticity , and why not a pitch imput... and why not...
i can get busy thill the summer :D

what do you think ?

I will never be 100% acurate (no method is), so i will deal with some approx.

BTW do you know if Prouty's book has been released in french?

thanks

[zeeoo]

hi engies,
i found this , it is not a turbulator but looks like a similar device, could anyone explein please ? Thanks


NUMERICAL STUDY OF MULTI-ELEMENT AIRFOIL
AERODYNAMICS
CLIN M. WANG Georgia Inst. of Tech., GA, US and CHEE TUNG In
Developments in theoretical and applied mechanics; Southeastern Conference
on Theoretical and Applied Mechanics, 16th, Nashville, TN, April
12-14, 1992. A95-93700 Tullahoma, TN The Univ. of Tennessee Space
Inst. (SECTAM, Vol. 16) 1992 p. III.II.32-III.II.39
(ISBN 1-879921-01-4) Copyright

Unsteady flowfields around oscillating Boeing VR7 airfoil with and
without a leading-edge slat were numerically investigated by a novel zonal method using a conformal mapping technique. Numerical aero-dynamic hysteresis loops show that the leading-edge slat prevents the airfoil dynamic stall at reduced frequency of 0.15, Reynolds number of 1 million

Question : what is this "slat" ? it's shape ?

[CRAN]

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

[Thomas coupling]

CRAN: If I may correct you there:

A "slat" is a tart from London.

:uhoh:

[zeeoo]

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

[Dave_Jackson]

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'.
__________________

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? :confused: What percentage of these patents ever deterred another party from putting that patent, or a derivative of it, into a production craft? :confused:

[zeeoo]

Hey !
is the idea of a slat on an helicopter blade forbidden to use ? what the .:mad:.

i think it is a good idea

[Shawn Coyle]

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?

[zeeoo]

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

[CRAN]

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
:ok:

[zeeoo]

Interesting.

check this out : http://barreau.matthieu.free.fr/publications/diagnostic-aero/2004-01-diagnostic-aero.pdf

there are some visual tips to examinate the transition (french language).

[slowrotor]

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

[zeeoo]

Slowrotor,
you're very well informed, that's precious :D
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

[CRAN]

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
:ok:

[zeeoo]

Cran,
as a pure newbie, what do you mean by 2D and 3D ?
thank you.

[slowrotor]

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)

[zeeoo]

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

[CRAN]

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).

http://www.ae.gatech.edu/research/windtunnel/classes/hispd/hispd04/Image402.gif

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

[zeeoo]

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

[Dave_Jackson]

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
;)

[CRAN]

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

[zeeoo]

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