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Model Forum / Radio Controlled / Helicopters / October 20032 degrees tilt in hover?
I noticed the heli models in Realflight G2 seem to tilt to the right when you hold them in a perfect hover with no wind. An experienced RC heli pilot at the local RC club told me this is realistic as all helis tilt 2 degrees in hover. When I asked why, he told me it was due to the main rotor torque. I didn't press him for a detailed explanation but I'm curious if anyone here can help me understand the physics behind what he's talking about. Thanks. He's partially correct. All clockwise rotationg helis have to tilt slightly to the right. This is because of the side force applied by the tail rotor. The tail rotor is trying to pull the heli to the left somewhat so in order for the heli to hover staionary some right cyclic trim is needed, hence the 2 degree or whatever tilt to the right. The reverse happens with counter-clockwise rotationg rotorheads. Will > He's partially correct. > [quoted text clipped - 5 lines] > > Will Ahh, makes perfect sense. Thanks for the info. P.S. Nice try Beav but Will beat you to it... > I noticed the heli models in Realflight G2 seem to tilt to the right when > you hold them in a perfect hover with no wind. An experienced RC heli pilot > at the local RC club told me this is realistic as all helis tilt 2 degrees > in hover. When I asked why, he told me it was due to the main rotor torque. > I didn't press him for a detailed explanation but I'm curious if anyone here > can help me understand the physics behind what he's talking about. As Will says, it's all because we have tail rotors. If we didn't have a tail rotor to stop the heli rotating, it wouldn't be shoving the heli sideways down the field and YOU wouldn't need to trim with right cyclic to stop it drifting off. Ask your local helidude about translating tendency and all should become clear. He'll clearly know something, or he'll clearly not.  SignatureBeav Please note my E-mail address is "beavis dot original at ntlworld dot com" (with the obvious changes) Beavisland now lives at www.beavisoriginal.co.uk Hi there all. Some good, clear replies to the original question. I would like to add another point which may be of interest to beginners. Some helis have the tail rotor mounted on a pylon, which brings the tail rotor's axis up higher, in line with the main rotor. Look at helicopters like the Westland Sea-King, and Bolkow BO105, you will see the idea. These machines hover much closer to 'level'. The BO105 is very popular as in air-ambulance operations, and the tail rotor design helps stop the paramedics being obliterated when loading patients in through the back doors! The tail rotor is conveniently above head-height. Happy learning/flying! Joe Landy Peterborough UK. > > I noticed the heli models in Realflight G2 seem to tilt to the right when > > you hold them in a perfect hover with no wind. An experienced RC heli [quoted text clipped - 5 lines] > here > > can help me understand the physics behind what he's talking about. ALWAYS press EVERYONE for a full explanation! It's the quickest way to learn, and also helps cut the crap from the fact. >Hi there all. >Some good, clear replies to the original question. [quoted text clipped - 3 lines] >Look at helicopters like the Westland Sea-King, and Bolkow BO105, you will >see the idea. These machines hover much closer to 'level'. With respects, Joe, the position of the tail rotor relative to the plain of the main rotor has nothing to do with the helicopters tilt in a hover. The tail rotor is producing a thrust to counter the torque of the main rotor. If the main rotor is perfectly level, the helicopter will slide sideway in the direction of the tail rotor thrust. Full size pilots refer to this as translating tendancy. The main rotor is tilted opposite the tail thrust in order to maintain position. It doesn't matter if the tail rotor is below the main rotor or "in-plane" with the main rotor. That tail thrust is there either way and the main rotor must be tilted opposite that force if the helicopter is to maintain a specific point over the ground in a hover. Fly Safe, Steve R. Hi there Mr Rhodesst. Thanks for the polite correction. The books I have read have led me to believe the aerodynamics work as I stated in my original post, but I can see, like you say, that there is still a sideways force which will shift the helicopter when in the hover. From watching the helicopters which I mentioned in action, they do seem to hover much closer to the horizontal though. Is there another factor coming into this? I'll keep learning! Cheers. Joe. P.S Guess I'm in for a bollocking for not bottom-posting now? Bad luck, the subject has taken a different direction further down! > >Hi there all. > >Some good, clear replies to the original question. [quoted text clipped - 17 lines] > Fly Safe, > Steve R. >>Some helis have the tail rotor mounted on a pylon, which brings the tail >>rotor's axis up higher, in line with the main rotor. [quoted text clipped - 17 lines] >Fly Safe, >Steve R. Joe has made a good point, and John Fay, one of the leading authorities on how a helicopter flies, refers to it in his book "The helicopter and how it flies". He (John Fay) calls it a 'rolling couple'. In his book, he says (paraphrased for briefness):- "it may be noticed that some helicopters hover with the fuselage tilted laterally. This situation will occur in a helicopter when the tail rotor is considerably lower than a line which is normal to the axis of rotation and which passes through the main rotor hub. A rolling couple is therefore created. The magnitude of it is dependent on the tail rotor thrust and the vertical distance between the tail rotor hub and the main rotor hub" So maybe you are both right :-) >Joe has made a good point, and John Fay, one of the leading authorities on >how [quoted text clipped - 14 lines] > >So maybe you are both right :-) Hi Mike, I've heard of this too. The tail rotor being located at some distance below the plane of the main rotor will impart some amount of roll coupling to the fuse when tail rotor commands (pirouettes) are given. I've been told that on full size birds like the Bell 206 Jet Ranger, which has an underslung, teetering two bladed rotor system, the effect can be quite pronounced although I've obviously not had any personal experience with this. I think that our models also exhibit this trait as there's a distinct difference in the cyclic inputs needed to do a stationary pirouette to the right as apposed to the left on my X-Cells. I've not flown a model helicopter with the tail rotor mounted up closer to the plain of the main rotor. I'd be real interested to try pirouetting one to see how much difference there is. Anyone out there who's flown both, care to comment on the difference?? :-) As for "normal" tilt in hover, it really is a function of tail rotor thrust. The tail rotor being below the main rotor may cause the fuse to sit in a position such that it appears to be banked over more but I'd bet that if you could measure the actual tilt of the rotor system itself (forget how the fuse looks) you'd find that the angles would be "very" close either way. Fly Safe, Steve R. > >Joe has made a good point, and John Fay, one of the leading authorities on > >how [quoted text clipped - 32 lines] > Anyone out there who's flown both, care to comment on the difference?? :-) There's no REAL difference to speak about Steve. I've flown a few model heli's where the tail is in line with the mains (Huey and the Lockheed 286) and the Huey leans right skid low, the Lockheed left skid low (the Lockheed's a left rotator). I'm sure there's a mathematical formula to show just how wrong I am and that the differences are massive, but I've not seen any evidence of it:-) > As for "normal" tilt in hover, it really is a function of tail rotor thrust. Yep. > The tail rotor being below the main rotor may cause the fuse to sit in a > position such that it appears to be banked over more but I'd bet that if you > could measure the actual tilt of the rotor system itself (forget how the fuse > looks) you'd find that the angles would be "very" close either way. I'd go with that too. I'm wondering if the full sized machine where everything rotates more slowly show this difference better? SignatureBeav Please note my E-mail address is "beavis dot original at ntlworld dot com" (with the obvious changes) Beavisland now lives at www.beavisoriginal.co.uk >I'd go with that too. I'm wondering if the full sized machine where >everything rotates more slowly show this difference better? Good question, Beav. I think it would depend on the aircraft in question. I think that, overall, our model fuselages are much more "connected" to the main rotor system and follow it's lead in pitch (attitude) and roll than their full size brothers. I'm not a full size heli pilot but based on observation and conversations with pilots of the big birds, my impression is that heli's like the Jet Ranger can tilt the tip path plain of the rotor system a fair distance before the fuse wakes up and follows along. That would also mean that the fuse has more freedom to shift positions under the rotor system in a hover, depending on the aircrafts CG location, tail thrust and wind direction. Aircraft with more rigid rotor systems like the BK117 or BO105 are more tightly coupled (fuse to rotor system) and (I would think) behave more like our models do. I'd also point out one other little phenomenon that may confuse some of our less experienced pilots when they first encounter it. If you're hovering on a calm day, will see the slight tilt of the machine that we've been discussing. That's normal. Now come back on a day with a nice 10 mph breeze and hover sideways to the wind with the tail thrust pointing into the wind. You'll see that the model hovers almost perfectly level because wind and tail thrust cancel each other out and there's no need to tilt the main rotor to any significant degree. Now (!), hover the model sideways to the wind with the tail thrust pointing "with" the wind. You'll find that you have to lean the model over pretty hard to maintain point because the rotor now has to fight not only the tail thrust but also the wind at the same time. I've flown on windy days where the model had to be tilted over as much as 30 to 40 degrees to hold point in a hover. It's very disconcerting at first, especially when you don't understand fully, what's going on. Just know that it's normal and don't get too up tight about it. FWIW, Fly Safe, Steve R. > >I'd go with that too. I'm wondering if the full sized machine where > >everything rotates more slowly show this difference better? [quoted text clipped - 3 lines] > rotor system and follow it's lead in pitch (attitude) and roll than their full > size brothers. Yeah, heli's like a model Jet Ranger have comparitively VERY stiff teeter restraints, where a full size has none at all. That must allow the head on the full size to lean without it making the fuse lean as much. > I'm not a full size heli pilot but based on observation and conversations with > pilots of the big birds, my impression is that heli's like the Jet Ranger can > tilt the tip path plain of the rotor system a fair distance before the fuse > wakes up and follows along. Yeah, but the Jet Ranger also has a forward tilt AND a sideways tilt built into the mechanics. IIRC it's 2 degrees to the side and 4 forwards. It makes for a much less painful experience piloting one because when you're cruising, the mast and blades are tilted forwards, but the fuse is level. That means your HEAD is level and you don't get "Bolkow neck". (Bolkows have a "staright up" mast. That would also mean that the fuse has more > freedom to shift positions under the rotor system in a hover, depending on the > aircrafts CG location, tail thrust and wind direction. Aircraft with more > rigid rotor systems like the BK117 or BO105 are more tightly coupled (fuse to > rotor system) and (I would think) behave more like our models do. They respond to cyclic control more immediately than "say" a Jet Ranger. I'd LOVE to see what a Huey feels like with it's built in stabiliser. I've flown a Bell 47 with stab and one without and the difference was chalk and cheese. > I'd also point out one other little phenomenon that may confuse some of our > less experienced pilots when they first encounter it. If you're hovering on a > calm day, will see the slight tilt of the machine that we've been discussing. > That's normal. It's also normal for FNG's to try to STOP the lean, with the resuling sideways fly-off. > Now come back on a day with a nice 10 mph breeze and hover sideways to the wind > with the tail thrust pointing into the wind. You'll see that the model hovers [quoted text clipped - 4 lines] > point because the rotor now has to fight not only the tail thrust but also the > wind at the same time. That difference in lean is really noticeable whan you fly a "left hooker" after not flying one for a good time. They lean the other way and our brains ave cmpensated for the "right" lean and we see the new "left" lean as twice as bad as it really is. Now take one of THEM and hover with that same downwind:-)) I've flown on windy days where the model had to be > tilted over as much as 30 to 40 degrees to hold point in a hover. It's very > disconcerting at first, especially when you don't understand fully, what's > going on. Just know that it's normal and don't get too up tight about it. And don't try to stop it either. SignatureBeav Please note my E-mail address is "beavis dot original at ntlworld dot com" (with the obvious changes) Beavisland now lives at www.beavisoriginal.co.uk > >Hi there all. > >Some good, clear replies to the original question. [quoted text clipped - 14 lines] > way and the main rotor must be tilted opposite that force if the helicopter is > to maintain a specific point over the ground in a hover. Indeed it is Steve... but.... the position DOES have an effect. The tail also "provides" a roll couple but the closer to the main rotor centre line, the less the roll couple "hurts". Translating tendency is still the main reason for the lean though SignatureBeav Please note my E-mail address is "beavis dot original at ntlworld dot com" (with the obvious changes) Beavisland now lives at www.beavisoriginal.co.uk > > Fly Safe, > Steve R. I was wondering how much this condition affected RC helis: When in forward flight, the main rotor blade advancing side (the left side in CW rotating helis) sees a higher freestream velocity than the retreating blade side. This would mean that the left side would have more lift, causing a rolling moment on the heli to roll to the right. I know this is accounted for in full scale helis, but was wondering if it is a big deal in RC helis as well. Are gyros used to counteract this? -Jonathan- >I was wondering how much this condition affected RC helis: > [quoted text clipped - 6 lines] > >-Jonathan- Hi Jonathan, The airspeed differential you mention between the advancing and retreating blades produces what is known as dissymmetry (sp?) of lift. It's common to all rotorcraft and our models compensate for it in the same way that the full size bird do. There is no need to use gyros to control this problem. It's compensated for by the flapping motion that the rotor blades are allowed to make while they rotate. As a rotor blade moves into the advancing half of it's cycle, it gains lift and tends to flap upwards. Because of this, it sees a slightly reduced angle of attack. On the opposite side of the rotor disk, the other blade is moving into the retreating half of the disk and looses lift. As this happens, it flaps downward which slightly increases the angle of attack of the rotor blade. These very small changes in the angle of attack produced by the flapping motions tend to equalize the lift patterns between the advancing and retreating sides of the rotor disk. Check out: http://www.thaitechnics.com/helicopter/heli_principle.html and http://www.aviationtoday.com/reports/rotorwing/previous/0702/0702flightdyn am.htm for similar descriptions. Hope this helps, Fly Safe, Steve R. Thanks! That makes much sense. -Jonathan- |
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