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› Tail Theory - Discussion
05-13-2008 10:35 PM  9 years agoPost 1
stocky

rrKey Veteran

Taxachusetts

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No arguments please

There is nothing wrong with my tail setup (well maybe a little) just want to know for my education.

If I have mechanically set up my tail so that in rate mode the servo arm is 90deg and the tail rotor pitch is set to exactly counter the torque of the main rotor (Neutral position)

If I were to give left stick (nose left, tail right, with the direction of torque) so that I got an extra 5degrees of tail pitch from neutral.

Then I were to give right stick (nose right, tail left, against the direction of torque) so that I got an extra 5 degrees of tail pitch from neutral.

Would the resulting piro rates be the same in both directions ?

(Assume no outside influence, wind, constant head speed, main blade pitch etc)

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05-13-2008 10:48 PM  9 years agoPost 2
fenderstrat

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Aston,Pa

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no, your piro rates will not be the same because of torque...if you have EQUAL ammounts of tail pitch deg...this will come out to less piro speed going right because the heli has to fight torque...conversely...you can get faster piro rates going left with less tail blade pitch...this is why many set ups will have different EPA/ATV on ch 4 but have even piro rates...you use the ATV to counter the difference from torque..and get your piro rates even....I hope this makes sense

this process is the reason the motored tail heli works(tail motor only spins one way)...there is actually NO thrust(from the tail blade) helping the heli turn left..its all torque..the motor just slows down until the heli yaws ...when you give a full left rudder command..the motor stops and torque takes over

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05-13-2008 11:21 PM  9 years agoPost 3
stocky

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Taxachusetts

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I am not disagreeing, just discussing.

Forgetting about ATV, Dual Rates etc, just the 5 deg pitch either way with all else equal.

I can't see it

5 degrees against the torque you would be applying a force > than the torque to overcome the torque.
5 degrees with the torque you would be applying a force > than the force of the tail rotor offset (countering the torque) which must be the same as the torque. Therefore wouldn't the forces left and right be the same and therefore give you the same piro rate in both directions?

In other words you would be pushing against either the torque or the tail rotor both of which are equal??

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05-13-2008 11:28 PM  9 years agoPost 4
ragtop

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salt lake city, utah, usa

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ya im with fender my left piros are pretty fast compared to my right piros. i think this phenom is more apperent with the king as opposed to the belt. its for that reason that my atv are lopsided.

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05-13-2008 11:44 PM  9 years agoPost 5
stocky

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Taxachusetts

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ya im with fender my left piros are pretty fast compared to my right piros
Yes mine too, but is your left input converted to an angle of tail blade attack exactly the same as your right?
More angle will also give you faster piro speed.
So take that out of the equation and say for arguments sake that you had exactly 5 degrees either way from neutral would the piro be different then? I can see (maybe wrongly) why they would be, just can't grasp it why they would not.

Torque>>> [0 Rotation] <<< Blade N Angle
Torque>>> + 1deg angle> [> rotation] <<< Blade N Angle
Torque>>> [< rotation] <<< Blade N Angle + 1deg angle<

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05-13-2008 11:48 PM  9 years agoPost 6
fenderstrat

rrProfessor

Aston,Pa

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if you remove everything else from the equation and just consider blade (tail pitch)..then the answer is still the same....NO..your piro rates will not be the same...if you have the same ammount of air pushing the heli left and right..the heli is going to spin left faster as its working with torque

say you have 5 ft/lbs of thrust at 5 deg pitch(arguments sake)...you piro left and get the full 5 lbs of thrust...when you go right you have to overcome torque...if it takes 1lb of thrust to over come torque then you only have 4 lbs of thrust to create yaw...and a slower piro

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05-14-2008 12:03 AM  9 years agoPost 7
stocky

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Taxachusetts

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the heli is going to spin left faster as its working with torque
Yes but against the thrust of the tail rotor you set to counter the torque in the first place.

OK
So with the Heli switched off on the workbench tail in.
use your left hand index finger to push the tail left. The tail moves left of course. Now use the index finger to counter the pressure you are applying with the left, the tail stay in the same place. Now if somebody else were to push either side Left or Right of the tail, which direction would be harder to move? Both the same right?
So call your left hand torque and your right hand blade counter torque and the other person left or right blade angle.

Maybe I should just accept it

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05-14-2008 06:03 AM  9 years agoPost 8
Gregor99

rrElite Veteran

Western Wa

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Interesting thread. I love it when the "accepted" is questioned and deeper understanding results.

About the issue, I think on paper you are correct, the result should be equal with the "centered" offset. As I think about what is different between the theory and practice, the only thing I could come up with is that the "on-paper" torque is linear. Meaning that is the same regardless of which way the heli is spinning. I suspect this is not the case.

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05-14-2008 06:52 AM  9 years agoPost 9
shizack

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Aiken, SC USA

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Speculating...
...
When piro'ing left, some of the power that was being leeched to provide tail rotor thrust will be returned to the main rotor, increasing torque and speeding up the yaw/piro.

When piro'ig right, even more power is leeched from the motor, lowering headspeed and thus lowering tail rotor speed. More pitch would be required from the tail rotor to provide the desired thrust. This is also why the heli may drop altitude with right piros - the mains need more pitch as well due to lower headspeed.
...

Right...?

A soul in tension is learning to fly

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05-14-2008 01:34 PM  9 years agoPost 10
Jerry In Maine

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"Downeast" Maine

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Possibly related question -

I've seen some full scale helicopters that have their entire tail rotor canted at at upward angle...maybe 15* or so. Anyone know why this is?

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05-14-2008 02:25 PM  9 years agoPost 11
fenderstrat

rrProfessor

Aston,Pa

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I understand this discussion......a question I cant seem to grasp is the torque itself...why does the heli spin opposite of the main blades???...you would think the blades would "pull" the heli around with it..but I seem to remember "every action has an equal and oppisite reaction" from physics class.......but if the frame of the heli is basically independant of the blades due to bearings...what is pushing the heli to rorate left???

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05-14-2008 02:40 PM  9 years agoPost 12
Jerry In Maine

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"Downeast" Maine

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what is pushing the heli to rorate left
Engine shaft torque transmitting power to the rotorhead. Engine is solid to the frame so where it goes the frame goes.

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05-14-2008 02:53 PM  9 years agoPost 13
fenderstrat

rrProfessor

Aston,Pa

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AAAAAHHHHH...makes perfect sense...thank you....so the torque is really from the motor,not the main blades???

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05-14-2008 03:04 PM  9 years agoPost 14
Sgt Heli

rrVeteran

Remlap, Al USA

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Yes, it all starts at the motor. Just like a floor buffer that wants to spin opposite the motor shaft. An outboard boat motor has a tab just above the prop to help offset this affect. Same affect you get with a hand drill.

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05-16-2008 03:53 AM  9 years agoPost 15
billcoltharp

rrNovice

Austin,TX

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Jerry:

I think some machines tilt the tail rotor up because its thrust is below the center of lift of the main rotors. Witout tilt, the tail rotor will tend to roll the aircraft to the right. Tilting it up a little can counter this roll.

I played with this a little while I was flying the Honeybee CP

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