View Full Version : ATPL Theory question!

16th Mar 2007, 08:51
Some help please!

In autorotation the tail rotor thrust on a helicopter with anti-clockwise main rotor rotation as seen from above is:

to the left

In a helicopter as stated above, the fuselage wants to rotate with the main rotor in autorotation. To stop this we apply lots of right pedal, surely we must be directing thrust to the right to prevent it rotating in such a way?



16th Mar 2007, 09:01
There is no t/r thrust required in autorotation as there is no torque to be countered. in the flare at the bottom, there can be a tendancy for the fuselage to rotate in the same direction as the main rotor, due to friction in the gearbox, which will require some pedal to stop.

16th Mar 2007, 09:08
Wonder if this is a classic CAA play on words?

There must be a reaction if not torque reaction in autorotation hence the need for lots of right pedal?
But doesn't explain why the answer is to the left. Pushing the tail to the right would further agrivate the situation?

Am I confusing something??? :ugh:


16th Mar 2007, 09:12

1/ Nose to the right requires TR thrust (they are referring to the net reaction i.e. vector) to the left where I am sitting, don't know about you.

2/ NO torque - well not that you would see on the dials anywhere. So whats keeping the following going - MGB oil pump, Hyd pumps, TR drive train etc?

16th Mar 2007, 09:17
So whats keeping the following going - MGB oil pump, Hyd pumps, TR drive train etc?
How about the main rotor?? And what's driving the main rotor?? Autorotational airflow. So, no torque, so no torque reaction. :ugh:

16th Mar 2007, 09:22
OK new plan here...

lets call this an R22. As we autorotate we apply lots of right pedal, Why? So why is thrust directed left (as per question)?

16th Mar 2007, 09:25
Right pedal, nose right (or at least not left), tail left (or at least not right), tail rotor thrust left? As seen from above... Am I missing something?

16th Mar 2007, 09:35
I think,the tailrotorthrust on a cc-helicopter is always pointed to the left. In normal flight lots of thrust, in autorotation nearly no thrust, just a little bit for compensation of friction???

Camp Freddie
16th Mar 2007, 09:40
back to basics here I think as per R22

viewed from above in autorotation with zero tail rotor pitch

*nose yaws left, tail moves right (transmission drag)

apply right pedal to stop yaw (negative pitch)

*nose yaws right, tail moves left, tail rotor thrust from right to left

thats it, I think it is a reasonable question



Note 1: by tail rotor thrust the question master means, Which way will the tail rotor move when pedals are moved from zero pitch viewed from above, NOT which way the tail rotor physically moves the air passing through it !

Note 2: in powered flight tail rotor thrust from left to right

16th Mar 2007, 10:08
Thanks Camp Freddie, well explained...:D

I think the confusion is with think of the tail rotor like a fan. You nailed it with Note 1 and 2!



16th Mar 2007, 10:11
Mighty Gem,

For you to extract power (energy) from the main rotor to drive the things I mentioned before there MUST be torque!

"Friction" in the gearbox is an oversimplification. Does this "friction" suddenly appear from nowhere or "increase" when you flare? See above

16th Mar 2007, 10:29
As we autorotate we apply lots of right pedal, Why
Because you were flying with left pedal, which you now no longer need because you don't have any torque to counter.

16th Mar 2007, 11:07
so when we fly with the pedals in the netural position what position are the TR blades in (pitch angle)... power on and off? Are they at zero alpha, or at an angle to keep the helicopter pointed in the same direction by balancing against torque reaction at operational RPM?

16th Mar 2007, 11:08

Sticking with the quoted directions of rotation.

1/ On most (not all) helicopters there is an offset in the vertical fin which is trying to turn you to the left as well. This effect will vary with airspeed.

2/ Torque between the mast and the fuselage due to "friction" - which as actually more than friction, it is the energy required to drive the accessories, friction in the powertrain, the TR etc. etc. This will vary with Rotor RPM.

Unless of course your flying MG's perpetual motion helicopter which evidently has NO torque reaction.

Ref your most recent post - would you care to refine the question.

Head Turner
16th Mar 2007, 12:42
I see the question as fair one to ensure that you understand what is required of you during an autorotation.

If you mentally or by drawing a picture of a helicopter from behind and above so that the rotor blade is passing over the tail boom. This rotor blade will be rotating from left to right(anti-clockwise), so the tail boom will be moved from left to RIGHT due to , lets summerise it as 'Friction'. Therefore the T/R Thrust required to offset the 'Friction' will have to be to the LEFT

16th Mar 2007, 13:19
I think I understand it now.
The issue I had was that the tail wants to go to the right (nose left) which is why we put right pedal in during auto.
If you think of the vecotor diagram on its side we want to create lift, in this case to the left to make the nose go right.
I was trying to imagine it like a fan. If you want the tail to go left you need to blow air right (almost like a notar)
however I think I understand it now thanks for the Help/posts!!!

PS (the picture is not what I think is happening in this case!!!)

16th Mar 2007, 13:33
RVDT has the precisely correct explanation. Vertical fin offset to the left has to be fought in many helos, and driving the main transmission in autorotation means that the rotor must torque the mast, and the torque relationship switches sign as compared with powered flight, a little right thrust is needed from the tail rotor (typically about 3 degrees of tail blade pitch angle). This also means that the TR can run out of poop if there is a crosswind at the bottom that also needs right pedal - a classic case for Bell 47 drivers, where the last bit of an auto landing with a left crosswind is often on the right pedal stops as the helo scrubs itself along the ground in a left turn.

16th Mar 2007, 14:27

Can I get a job a SK?

The phenomena on a 47 was way worse on a Hiller 12E. What do you want to have sir, enough Left or Right pedal but unfortunaely we can't give you both.


Not the best example in the picture - a not too widely publicised quirk of the Notar is - when you pull pitch at the bottom of an autorotation, you get anti-torque thrust from the Coanda effect around the boom just when you DON"T need it. When you pull pitch you really have to feed in right pedal without reacting to the aircraft yaw as it is possible for it to yaw left to a point were you may not recover. It is definitely different to a tail rotor!

16th Mar 2007, 17:20
Us older(not old) guys ,who had to sit in the hover for more than an hour chasing submarines with our S55s in the 60s with lots of torque applied, remember that we had difficulty walking when we got out of the helicopter as one leg seemed longer than the other. So we know that the anti-torque was applied to the advancing side.
Now we spend most of our time in the cruise we like to be comfortable and have the pedals set level most of the time. So the pedals must have cruise power anti-torque applied when they are level.
Go into autorotation, take away the torque and we finish up with right pedal forward for zero thrust. Then the friction in the transmission and the tail rotor drag will cause the nose to turn to the retreating side. So we need a negative pitch range on the tail rotor to prevent the nose turning with the rotor and thrust needs to be applied to the retreating side. Hence extra right pedal - well forward.
With the JAA exams always think American helicopters(anti-clockwise from above) unless otherwise stated.
16th Mar 2007, 18:23
Pofman - in the cruise, the pedals are about level because the fin is doing some of the anti torque work for you - if you took off the fin you would have to fly with more left pedal (in CC-wise helo). Enter auto and that lift from the fin is still trying to turn the nose left/tail right (even at reduced speed for normal auto) so TR thrust must be used to counter it - right pedal in this case.
That lift from the fin is what helps you if you have a TR failure in the cruise - most helos are capable of maintaining straightish flight at high cruise speeds in this condition.
I know this is effectively what Nick and RVDT said but I hope in a less technical form.