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HomeAircraftHelicopterHelicopter Main Discussion › rotor:tail_rotor ratio
02-09-2008 03:53 AM  10 years agoPost 1
wlfk

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uk

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When I turn the main rotor on either of my helicopters (logo 600 and T-Rex 450) until the main blades are in line with the tailboom, the position of the tail rotor changes with each revolution of the mainblades. Now if the orientation was fixed, you could set it up so that whenever the main blades were in line with the boom, the tail rotor was well clear of the main rotor - I make the ideal position with the rear blade oriented upwards at about 45 degrees. This way you could fit much longer main blades without having to stretch the tailboom.

Or are the designers worried about downdraft/tailrotor interactions? I know this is a problem with some Bell helicopters.

K

A bit like a kite, but 500 times more expensive

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02-09-2008 04:40 AM  10 years agoPost 2
Lamar Cooley

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Houma,La.,U.S.A.

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rotor;tail-rotor ratio
wlfk,
O.K. I'm in.
Count the tail rotor revolutions to one main blade revolution.
On my raptors the ratio is 4 tail rotor to one main blade rev.
I would think that to acheive the tail blade position you inquire about. The ratio would have to be one to one. ( No ? )
That said, then the tail rotor would not have sufficeint thrust for
tail control. Given our head speeds range from 1700 to around 2000
rpms. ( Example: head speed 1700 rpm = 6800 tail rpm )
Just remembered why I shouldn't stick my finger in there.
That's my two cents.
Anyone care to raise ?
Regards
From Louisiana; flat land, with lots of water and very large
mosquitos


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02-09-2008 04:42 AM  10 years agoPost 3
wlfk

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uk

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No, it wouldn't have to be 1:1. 2:1, 4:1, 8:1.. would all work fine because the tail rotor would always be safely clear when the main blades passed overhead. The problem comes when it's something like 1.1:1 because the tail rotor is always in a slightly different position.

K

A bit like a kite, but 500 times more expensive

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02-09-2008 05:01 AM  10 years agoPost 4
Lamar Cooley

rrApprentice

Houma,La.,U.S.A.

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tail rotor
wlfk
So you answered your own question.
Now what about tail rotor thrust ?


Here when wanted.
There when needed.
Everywhere when looked for.

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02-09-2008 07:40 AM  10 years agoPost 5
wlfk

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uk

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I'm not sure whether it's a question, an observation or an invention. Just strikes me as a pity that on my logo 600 the ratio is almost, but not quite, correct. I already worked out that if the ratio was correct, then direct rotor:tailrotor interference wouldn't happen.

Doing a bit more maths, I reckon that the 'danger' section is about 10 degrees wide as each blade passes over the tailboom. Now if the tailrotor was to make 8 full revolutions for each revolution of the mainblades (360/10) divided by 2 mainblades divided by 2 tailrotor blades then you'd still run the risk of a blade/blade strike. But if you were using a ratio of 4:1 then you could safely increase the main blade length by the radius of the tailrotor, which on a logo 600 would mean safely carrying 740mm blades rather than 650s. Maybe even a bit more.

I guess I'm wondering whether there is a reason people don't do this. I suppose it would involve a bit more care setting things up, and would be lethal on the T-Rex 450 where the belt slips all the time. But if you just wanted a small blade-length increase of a few cm you would actually have quite a lot of latitude.

K

A bit like a kite, but 500 times more expensive

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02-09-2008 12:17 PM  10 years agoPost 6
"Cam"

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UK

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Lead and lag of the baldes probably makes the 'danger' angle a lot bigger.

I once put 335 blades on a small TRex and when inverted the tail woudl kick badly sometimes - after landing I could see the tail blades were a mess!

I think your idea is sound, except most helis do not have a main-to-tail rev ratio that gives a 'whole' number. It's often 1:4.5 or some other 'point something. Meaning on those helis you are always looking at blad interaction with oversized blades.

F3C guys run oversized blades but then they don't yank back on the cyclic!

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02-09-2008 03:58 PM  10 years agoPost 7
GMPheli

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W. Bridgewater, MA USA

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And all you would have to do is touch the tail rotor to the ground and the belt slips and you don't know it. Or the belt is slightly loose because it is cooler that day and your boom contracts enough so when you put in hard tail rotor inputs your belt skips a couple teeth. It would work, but for everyday flying I would think you would have problems eventually.

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