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Collective and antitorque pedal relationship


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Ok, I have 4.3 hours under my belt on a Schweizer 300. I can hold a hover, pedal turn in hover, set-down, and lift-off. I am just starting to get into the hang of patterns, but I seem to have a lot of difficulty with the antitorque pedals.

 

For this US Schweizer 300 (rotor turns to the left), when we lift-off to a hover, we apply left pedal, as the torque will be to the right, which will result in the need for left pedal to counter this torque. I understand this.

 

Moving from hover to forward flight, we have to ease off the left pedal, and apply right pedal to maintain a straight heading, to get into ETL. Why?

 

Also, when coming into a normal landing descent, with a decrease in collective, we are again aplying right pedal to compensate for the lowering of torque?

 

Explanations on the relationship between collective and pedals much appreciated.

 

Thanks!

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The short answer is that the tail rotor also experiences an increase of effectiveness (ETL) as airspeed increases. Since it is getting more effective, you don't need as much left pedal to counteract yaw. The opposite happens when you are transitioning to a hover at the end of a normal approach. The tail rotor becomes less effective, so you need more pedal to counter the yaw.

 

The stabilizers also play a part, since they also become effective as speed increases, by taking some of the "load" off the tail rotor. In other words, you don't need as much pedal because the stabilizers do some of the work in forward flight.

 

Hope this helps!

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What PhotoFlyer said is correct. The vertical stabilizer does become more effective as you gain airspeed. Part of ETL is also the horizontal flow across the main rotor. As you transition from hover to forward flight, the airflow transitions from vertical (Induced Flow) to horizontal (ETL). The down wash also becomes more horizontal. Instead of being in "dirty" turbulant air from the M/R down wash the T/R enters relatively "cleaner" air and therefore becomes more effective. Now the left pedal you had to hover becomes too much and you reduce the amount pedal you are using maintain heading.

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It's a complicated relationship, with the relative wind, airspeed, temperature, density altitude, pilot technique, and other things all having some effect. The pedals control the pitch in the tail rotor, counteracting torque. If you lower the collective, you're reducing torque, and if you raise it, you're increasing torque. How much it changes, and thus where the pedals need to be, depends on the interaction of all the factors, and the anti-torque, or pedal input required, changes from moment to moment. Thus you have to constantly move the pedals to keep the nose straight. As airspeed increases, stability increases, and as the stability increases, the amount of pedal movement required decreases. The short answer is that you have to use the pedals to keep the nose straight, whatever happens. Just keep looking ahead, and do whatever it takes to keep the nose straight. Generally you will need more right pedal as you reduce collective, and more left pedal if you increase collective, but how much is variable, and all you can do is anticipate the need for pedal changes, while being ready to reverse the inputs if necessary. Hovering, and all other low-speed work requires busy feet and steady hands. If your feet are dancing and your hands are barely moving, you're probably doing it about right.

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Gomer Pylot, rotormandan, PhotoFlyer - thanks for all the good advice.

 

Especially "If your feet are dancing and your hands are barely moving, you're probably doing it about right."

 

Will keep that one in mind!

 

Cheers!

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  • 2 months later...
  • 11 months later...

this one got me thinkin' :huh:

 

I've always wondered about the difference of left pedal needed between a 269C and a A & B in a hover. The C has a longer tail boom AND larger diameter--which i had always attributed to the less left pedal, but? now i am wondering how much the "dirty air" plays into it?

 

also just a little "tid-bit" about the TH-55 & its tail rotor: did you know that IF the T/R strap pack has seen any "military time" that its life limit is significantly reduced? :o (when one "civilianizes" a TH-55, part of the process is to re-rig the T/R) & the ones that i have done? have significantly MORE left pedal allowed in their stops ! (hence the more stress on the strap) maybe apiaguy has some input? :D

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I've never thought too much about the small difference between the two... I never even really thought that the A or B had less tail rotor authority.... I know, I know... it does.... a little.

 

That said... the C model has 1 degree more pitch available on a book setting. It also has 5 inches more on the disc diameter than the a/b. Funny... it turns slower 3091 vs 3175. I would tend to say that these conditions alone would account for the small difference in performance.... 5 inches alone on a 33 inch effective blade length is quite a bit. The a/b disc diameter effective blade length is 28 inches... that 5 inches more blade is almost 20%. Now add the 1 degree more pitch available (small I know) and that would probabally be countered for the slower turning rpm of the C anyway.

 

The dirty air syndrome..... doubt it has any influence... yes, the horizontal stab. is slightly narrower on the C... maybe allowing cleaner air flow...

 

Yes, I have heard the TH55 rigging stuff... never found anything that documented them setting them up that way.... I'm sure a few guys that are alot older than I could say... I always thought You would be able to find a military document that shows them setting them up a certain way...

 

I've always thought that they reduced the life limit possibly due to the military nature of use. Also the thought that the strap may have been used on a 369 in the military may have had something to do with it.

Edited by apiaguy
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  • 2 weeks later...

The vibrations

The vibrations are simple caused by all of the inequalities of the rotor disc transitioning into the cleaner parcel of air.

 

 

The vibrations are caused by blade vortex interaction (BVI). See Leishman Principles of Helicopter Aerodynamics pages 58, 488-489, 491-492, and 578-581. I think the FAA discussion is inadequate on this topic. Do a BVI Google search and then correlate BVI and vortex ring state.

Edited by Tom22
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