Tail rotor position
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h.i.d, not so sure i do! It is just a case of packaging the tail rotor to suit the main rotor diam (which is generally as large as practical). TR will become more efficient in forward flight, but in hover often uses MR downwash to improve efficiency and avoid TR vortex ring state (forward upward rotation diverts some MR downwash over the TR).
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Hhmm, Graviman, I'm not sure I agree with anything you just posted. Perhaps I am misunderstanding you. The tail rotor doesn't become more efficient in forward flight, the vertical fin does though, which reduces the force required to be produced by the tail rotor, resulting in a reduction in pedal required. I was pretty sure that m/r downwash is also often a major cause of loss of tail rotor effectiveness in the hover (which can often be due to tail rotor VRS), which obviously doesn't improve its efficiency.
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Can't see why the tail rotor should be exempt from the same physics that give you translational lift from the main rotor, althought the fin effect is there also. A lot of the difficulties students have in learning transitions away are the onset of translational lift in the main and tail rotors at the same time giving a balloon and yaw left, followed by a fishtailing departure up in the avoid curve.
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BJC, i have no numbers, and can only go on what i've read elsewhere - perhaps i am in error. I gather that in many Sikorsky designs the MR downwash catches an advancing blade on the tail rotor. Prouty comments that vortex ring has affected some tail rotor designs in yaw against MR direction, and that bottom blade forward rotation helps. Translation effect must also affect TR.
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Here to learn
I'll freely admit that I'm no expert either, probably didn't help that my first helo aerodynamics lesson consisted of "it's all black magic, as long as you believe in magic the helo will keep flying". I know every helo design is different but I have seen wind tunnel videos of the 206 where the main rotor vortices impinge on the t/r essentially causing it to enter vrs, which I believe is the suspected cause of LTE in some wind azimuths in that series. So in that case it sure doesn't improve its efficiency. Since my background is in mostly Bells that has clouded my opinion since.
I can't wait until I get to work and Matthew can expain how my intrepretation of the laws of physics is all right or all wrong. I would love to have been there when the aero instructor used the black magic line on him (I think we had the same instructor for that) since he had the benefit of a physics degree! I was more then happy to accept the IP's description just as long as I still passed the exam and got my wings! Prouty has since cost me more then a few sleepless nights while I try to digest what he's saying.
I can't wait until I get to work and Matthew can expain how my intrepretation of the laws of physics is all right or all wrong. I would love to have been there when the aero instructor used the black magic line on him (I think we had the same instructor for that) since he had the benefit of a physics degree! I was more then happy to accept the IP's description just as long as I still passed the exam and got my wings! Prouty has since cost me more then a few sleepless nights while I try to digest what he's saying.
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BJC, that's what i like about the heli community - there just are no putdowns. I suspect that's because the heli is the most versatile machine ever invented, so heli pilots always end up approachable. It makes this forum a great place to be.
Interesting about the Bell tail rotor. I could see that in flight slipping to port the downwash might increase flow against tail rotor outwash, which could potentially set up VRS. I'll keep an eye out for any info i see on that.
I studied aerodynamics, and came to the conclusion that anything beyond simple models are a black art. In the powertrains side of the auto industry i was exposed to CFD for various flows in and around engines, and realised that beyond simple predictions you couldn't second guess the computer. I'm guessing Matthew just got scared of the Schrodinger equation, and figured helis were easier.
Interesting about the Bell tail rotor. I could see that in flight slipping to port the downwash might increase flow against tail rotor outwash, which could potentially set up VRS. I'll keep an eye out for any info i see on that.
I studied aerodynamics, and came to the conclusion that anything beyond simple models are a black art. In the powertrains side of the auto industry i was exposed to CFD for various flows in and around engines, and realised that beyond simple predictions you couldn't second guess the computer. I'm guessing Matthew just got scared of the Schrodinger equation, and figured helis were easier.
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These were the azimuths we taught back in the good ol' TH-57 (B206). These are far better pictures than the ones I and my students drew on the white boards (except the one guy who was an art major... his were gorgeous).
While undergoing familiarization, the students had the dubious pleasure of doing 360 degree clearing turns after every landing (several hundred each) unless the winds were higher than I believe 15 knots. This was less for actually clearing around the helicopter than it was for improving low-level handling ability. I suspect I've orbited in that thing counter-clockwise (always counterclockwise) somewhere on the north side of 10,000 times. You get a lot of opportunity to observe the effects of wind azimuth on the tail rotor going round-and-round about a point in this environment, as you probably can imagine.
I found these azimuths to be quite accurate, and if one looks to see where the red regions of the various diagrams superimpose themselves on one another, one might suspect that the effects were additive in those regions. One would be correct.
The article from which I extracted these diagrams is available here:
http://www.dynamicflight.com/aerodyn...loss_tail_eff/
While undergoing familiarization, the students had the dubious pleasure of doing 360 degree clearing turns after every landing (several hundred each) unless the winds were higher than I believe 15 knots. This was less for actually clearing around the helicopter than it was for improving low-level handling ability. I suspect I've orbited in that thing counter-clockwise (always counterclockwise) somewhere on the north side of 10,000 times. You get a lot of opportunity to observe the effects of wind azimuth on the tail rotor going round-and-round about a point in this environment, as you probably can imagine.
I found these azimuths to be quite accurate, and if one looks to see where the red regions of the various diagrams superimpose themselves on one another, one might suspect that the effects were additive in those regions. One would be correct.
The article from which I extracted these diagrams is available here:
http://www.dynamicflight.com/aerodyn...loss_tail_eff/
Given the choice, the best way for the TR to rotate is so that the advancing blade goes from the bottom up into the MR vortex rather than from the top down through it. The idea is that the TR blade sees the vortex and the downwash as a headwind rather than a tailwind thereby getting more Vsquared and thus more lift.
Tail rotors do gain translational lift and they also flapback - initially they become more efficient as IAS increases but eventually drag reduces this efficiency.
On models where the vertical stabiliser is designed to produce all the anti-torque thrust in the cruise, the net force is still lateral and TR drift exists in forward flight just as it does in the hover (inherent sideslip is, I believe, the correct name for it). Inherent sideslip exists regardless of whether you have a big vertical stab or not.
Tail rotors do gain translational lift and they also flapback - initially they become more efficient as IAS increases but eventually drag reduces this efficiency.
On models where the vertical stabiliser is designed to produce all the anti-torque thrust in the cruise, the net force is still lateral and TR drift exists in forward flight just as it does in the hover (inherent sideslip is, I believe, the correct name for it). Inherent sideslip exists regardless of whether you have a big vertical stab or not.
Originally Posted by Graviman
The only information i can find on X2 development is this frustratingly slow update web vid:
Too bad they didn't show any hover, would have been interesting to see if it hovered left or right wheel low.
-- IFMU
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Um...lifting:
Remember the FAA's Rotorcraft Flying Manual?
These azimuth' are still beeing tought these days:
http://www.faa.gov/library/manuals/a...-h-8083-21.pdf
I don't know who "adapted" his drawings from whom... but in the section about LTE the FAA uses pretty much the same pictures.
Not trying to be nitpicking, just found it curious how similar those drawings look?!
Remember the FAA's Rotorcraft Flying Manual?
These azimuth' are still beeing tought these days:
http://www.faa.gov/library/manuals/a...-h-8083-21.pdf
I don't know who "adapted" his drawings from whom... but in the section about LTE the FAA uses pretty much the same pictures.
Not trying to be nitpicking, just found it curious how similar those drawings look?!
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I'm pretty sure the diagrams in my Naval FTIs are identical as well, though I'm not at home to check them. My guess is that the FAA or some other agency, via calculated data from Bell or somebody came up with those diagrams. I would venture to guess they're at least 35 years old and the hand that drew them had no idea they'd still be kicking around today.
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Interesting read, Um.. Lifting.
So the TR VRS is caused by high right yaw, which can be caused by MR vortex interference requiring potentially large pedal inputs. I'm starting to see the benefits of using yaw stability augmentation designed into the machine, to limit yaw rate.
IFMU, X2 is another internal Sikorsky project that looks set to fly sometime in 2008...
So the TR VRS is caused by high right yaw, which can be caused by MR vortex interference requiring potentially large pedal inputs. I'm starting to see the benefits of using yaw stability augmentation designed into the machine, to limit yaw rate.
IFMU, X2 is another internal Sikorsky project that looks set to fly sometime in 2008...
I think Nick might point out to you that those azimuth graphs for the 206 were simply a result of putting a 'big girl's blouse' of a TR on it instead of a proper manly one.
Any helicopter that struggles with a 15 kt crosswind should never have been certified in the first place.
Any helicopter that struggles with a 15 kt crosswind should never have been certified in the first place.
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The R22 and R44 Raven II's tailrotor blades spin in opposite directions and you can feel a difference regarding stability in the hover.
All helicopter can struggle with a 15kts wind if not used correctly. If you had to do a fast right pedal turn in a B206L the chances of getting LTE/VRS is quite big.
But what is the max wind for say the R22? I haven't looked through their manual for a while but I think is something like 30 or 40kts?
Any helicopter that struggles with a 15 kt crosswind should never have been certified in the first place.
But what is the max wind for say the R22? I haven't looked through their manual for a while but I think is something like 30 or 40kts?
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where the main rotor vortices impinge on the t/r essentially causing it to enter vrs, which I believe is the suspected cause of LTE
Choppie - the point is that a decent helicopter should not suffer from TR handling problems (much as I hate the R22, it seems to have a more than adequate TR). Bell invented LTE to cover up the fact that theirs was the only helo to suffer from it, simply because it wasn't man enough for the job.
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I do agree with you Crab, I've flown quite a bit on 206's and Squirrels and there is a huge difference between the two. I don't think I've ever run out of pedal on the Squirrel. But what I'm trying to say is that you can introduce LTE on any helicopter if you had to do the wrong maneuver at the wrong time.