Tail rotor position
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Not doubting that for an instant, Um... lifting...
So the flat Earth goes around the Sun, eh?
So the flat Earth goes around the Sun, eh?
Last edited by Graviman; 20th Oct 2007 at 09:18. Reason: A good nights sleep and a stong coffee often help!
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If you look at helicopter like the CH-53. The tailrotor is mounted almost inline with the mainrotor. So why do they offset the vertical stabilizer to the left tilting the tailrotor to one side and almost looks like it could create a small amount of lift in the same direction as the mainrotor?
Choppie - because that is exactly what it does
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Choppie,
Crab is right, the canted tail rotor on the CH-53E does provide significant lift. It serves to offset the aft CG of the baseline aircraft, since the E model added an aft engine and a big cabin plug as compared with the D model, which had a normal tail rotor configuration.
Crab is right, the canted tail rotor on the CH-53E does provide significant lift. It serves to offset the aft CG of the baseline aircraft, since the E model added an aft engine and a big cabin plug as compared with the D model, which had a normal tail rotor configuration.
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Interesting. I would guess that splitting the CH-53E cabin plug between fore and aft was considered, but rejected due to need to redesign control system linkage. I guess that's not such an issue on a fixed wing with cables or hydraulics.
I'm guessing Chook cg is a little port of centreline.
Originally Posted by Role1a
Just to expand the argument, why does the Chinook hover left wheel low?
Last edited by Graviman; 19th Oct 2007 at 19:42.
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Choppie, with a 20 degree cant angle, the upward thrust is about 1200 lbs, at the tail. This is pure lift, but also helps shift the lift balance to offset 4000 lbs just aft of the mast.
Please note that the tail lift acts as pure lift at the CG, as well as a nose down rotation due to the strong moment from the tail lift. So much for the "stick the moment anywhere you'd like" school of bizarro-physics.
Please note that the tail lift acts as pure lift at the CG, as well as a nose down rotation due to the strong moment from the tail lift. So much for the "stick the moment anywhere you'd like" school of bizarro-physics.
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Love these arguments where everyone looks to find who's right when, depending on your point of view, the truth is either everybody's right or nobody's right.
Demanding a degree in physics prior to learning how to fly a helicopter is impractical, yet understanding why things happen the way the do with helicopters is necessary. To make the concepts understandable, answers are diluted quite a bit in most cases, and in some cases are complete lies. But they still get the point across.
This idea of looking at the difference in height between the tail rotor thrust and the horizontal component of main rotor thrust has huge merit. If they act at the same height, then when there is no net lateral force, there is also no net rolling moment. Thus you hover with skids level. (At least within the many approximations that have to be made.)
Is that accurate with physics? Yes, but it is far from the whole story. Just knowing the difference in height doesn't help much in designing helicopters, but it does help in understanding a small part of helicopter flight.
So is the right answer that those two forces create a moment about the center of gravity and the vector sum of those moments is the net? Well, that is a right answer, but there are others. Because of all the approximations still being made, it would be valid to call this a wrong answer as well.
What's right really depends on why you need the information. If you're designing helicopters you need way fewer lies and much more accuracy. If you're trying to learn to fly helicopters, a general understanding is sufficient.
Demanding a degree in physics prior to learning how to fly a helicopter is impractical, yet understanding why things happen the way the do with helicopters is necessary. To make the concepts understandable, answers are diluted quite a bit in most cases, and in some cases are complete lies. But they still get the point across.
This idea of looking at the difference in height between the tail rotor thrust and the horizontal component of main rotor thrust has huge merit. If they act at the same height, then when there is no net lateral force, there is also no net rolling moment. Thus you hover with skids level. (At least within the many approximations that have to be made.)
Is that accurate with physics? Yes, but it is far from the whole story. Just knowing the difference in height doesn't help much in designing helicopters, but it does help in understanding a small part of helicopter flight.
So is the right answer that those two forces create a moment about the center of gravity and the vector sum of those moments is the net? Well, that is a right answer, but there are others. Because of all the approximations still being made, it would be valid to call this a wrong answer as well.
What's right really depends on why you need the information. If you're designing helicopters you need way fewer lies and much more accuracy. If you're trying to learn to fly helicopters, a general understanding is sufficient.
Mainly true Matthew, the P of F that is taught in UK is essentially a convenient explantion of something we know happens but usually isn't the truth, the whole truth and nothing but the truth.
However useful simple concepts are for teaching, there is no point letting people try to convince others that these simple concepts are the whole story.
However useful simple concepts are for teaching, there is no point letting people try to convince others that these simple concepts are the whole story.
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Worthwhile discussion...
Correct me if wrong, but what Nick is getting at is that for helicopter to hover (or cruise) total lift must balance total weight. Ship rotates about it's cg until tail rotor thrust and main rotor lift vector sum to a vertical force above the cg. All the while the pilot is trimming the cyclic to keep references/string/ball in position. It's as Matthew says, a different perspective.
At a guess Comanche low tail rotor would hover left wing low, but with high vertical stabiliser would cruise right wing low. That powerfull bearingless rotor would then sort out any pitch attitude changes, from tail rotor cant.
Is it fair to assume centre of drag acts at similar position to cg?
Actually, designers often use tricks to get quick answers.
I'll revise my earlier posts appropriately...
At a guess Comanche low tail rotor would hover left wing low, but with high vertical stabiliser would cruise right wing low. That powerfull bearingless rotor would then sort out any pitch attitude changes, from tail rotor cant.
Is it fair to assume centre of drag acts at similar position to cg?
Actually, designers often use tricks to get quick answers.
I'll revise my earlier posts appropriately...
Last edited by Graviman; 20th Oct 2007 at 09:36.
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I have flown a few hours on the EC130 (didnt finish my conversion though), but what I did notice is that you fly with quite a bit of left pedal when in the cruise. Is this then the same because of the huge vertical stablizer like the Comanche? But I can't say that I felt the helicopter fly left skid low. Maybe I didn't notice it.
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Yes that I know. But in the EC130 it actually feels strange because it's so much left pedal. What I would like to know is does it cruise with the left skid low or level?
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Matthew
It looks to me that some scientific based reasoning would probably have cut short many of these discussions. Over and over again some topics resurface...
If you argue that simplification is OK for a pilot that -just- has to fly the thing, then that is OK with me, there are more important things to learn. As long as these pilots know that when they start making great theories, they may be way out of bounds off from what they have been thought.
d3
If you argue that simplification is OK for a pilot that -just- has to fly the thing, then that is OK with me, there are more important things to learn. As long as these pilots know that when they start making great theories, they may be way out of bounds off from what they have been thought.
d3
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Humble pie...
Well, this thread has knocked my thinking into shape. Read "Cyclic & Collective" by Shawn Coyle, in particular P220 covering tail rotors in advanced helicopter aerodynamics. Good read.
An important tail rotor function is to provide yaw control. If it is above or below the cg then you get a yaw-roll coupling. The vertical stabiliser can be positioned to perfectly counteract main rotor torque, but tail rotor is by necessity more of a compromise. I've stuck in a line in my earlier post.
Choppie, fly with string centred and see what ball says about trim.
Maxtork, interesting i wondered what was happening on this. 177kias is a good start, i imagine power limits level flight to 165kias. The only information i can find on X2 development is this frustratingly slow update web vid:
http://uk.youtube.com/watch?v=E-rDl8lbVcc
An important tail rotor function is to provide yaw control. If it is above or below the cg then you get a yaw-roll coupling. The vertical stabiliser can be positioned to perfectly counteract main rotor torque, but tail rotor is by necessity more of a compromise. I've stuck in a line in my earlier post.
Choppie, fly with string centred and see what ball says about trim.
Maxtork, interesting i wondered what was happening on this. 177kias is a good start, i imagine power limits level flight to 165kias. The only information i can find on X2 development is this frustratingly slow update web vid:
http://uk.youtube.com/watch?v=E-rDl8lbVcc
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Sorry for the slight change in thread!
I was wondering what determines the length of moment arm for the tail rotor, If the tail rotor was further away surely you would need less thrust to counteract the main rotor coupled with less TR drift, I am aware a heli would look pretty silly with an extra long boom but you guys seem to know what you are talking about.
Hope it has not been covered before
I was wondering what determines the length of moment arm for the tail rotor, If the tail rotor was further away surely you would need less thrust to counteract the main rotor coupled with less TR drift, I am aware a heli would look pretty silly with an extra long boom but you guys seem to know what you are talking about.
Hope it has not been covered before
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AW139 tail rotor is angled 11 degrees from vertical to provide lift to offset aft CG which is pronounced in this aircraft.
In north sea fit with 2 pilots and with help from tail tail rotor not much more than 1000 kg of fuel can be carried without ballast in cabin, bearing in mind that it can carry I think 1676 kg with aux tank !
CF
In north sea fit with 2 pilots and with help from tail tail rotor not much more than 1000 kg of fuel can be carried without ballast in cabin, bearing in mind that it can carry I think 1676 kg with aux tank !
CF
Last edited by Camp Freddie; 21st Oct 2007 at 20:10.