Why right pedal when descending?

[Bearfoot]

I notice that when lowering the collective to descend at above 40-60 knts, I have to feed in right rudder (from neutral) to keep her straight.

 

What are the dynamics behind that?

 

Something to do with retreating blade effect?

 

Of course, when taking off, we increase the collective, we increase the torque and we need to increase left rudder to counter the inertial spin. I get that. But when reducing collective, it's not like the blade starts to spin the other way, so this cannot be the reason we need right rudder?

[AlphaOneSix]

You could always think of it not so much as "adding right ruder", but instead think of it as "removing left rudder". More collective = more torque = more left pedal. Less collective = less torque = less left pedal.

 

Bear in mind that at "neutral" (that is, with the pedals centered in the cockpit), there is still positive pitch in the tail rotor blades. On the Mi-8, for example, full left pedal is just over 6 degrees of pitch while full right pedal is just over 23 degrees of pitch in the tail rotor blades. So centering the pedals in the cockpit is still about 8 degrees of positive pitch in the tail rotor blades.

[b00ce]

The tail is shaped like a wing to reduce pilot work load and counter a bit of torque. It also allows the pilot to continue to fly (with a lower power setting mind you) somewhat in the event tail rotor effectiveness is lost, provided there is enough forward airspeed.

[hog_driver111th]

In Helicopter training, you learn that left is positive collective, right is negative collective. What that means is, even in a full down collective, you will have to add right pedal to keep the helicopter going straight. Any pull on the collective and you will have to add right. Any loss of power where you might be centered, you'll have to add right pedal.

[j0nx]

I think you meant to say "Any pull on the collective and you will have to add LEFT". Not right :)

[Bearfoot]

Thanks, all.

 

I am now working my way through the FAA helicopter handbook, and it confirms what you said AlphaOneSix:

 

"""

With the antitorque pedals in the neutral position, the tail rotor has a medium positive pitch angle. In medium positive pitch, the tail rotor thrust approximately equals the torque of the main rotor during cruise ight, so the helicopter maintains a constant heading in level flight.

"""

 

I guess I am surprised at the amount of right pedal I need when descending. It seems to be more then the left pedal I need in a hover!

[Jafferson]

You ask about the physics, i try to explain...

Blades rotating left

 

Collective up = The blades have drag working against the air, fuselage turns right. Imagine a toy huye, hold him at his blades and start engine, fuselage turns right.

 

Collective down = The blades have no drag, the autorotation is giving the fuselage a extra spin, fuselage is turning left. Imagine a toy huey floating on the water, spin the blades with your finger left, fuselage will follow left.

Edited July 18, 2016 by Jafferson

[WildBillKelsoe]

I notice that when lowering the collective to descend at above 40-60 knts, I have to feed in right rudder (from neutral) to keep her straight.

 

What are the dynamics behind that?

 

Something to do with retreating blade effect?

 

Of course, when taking off, we increase the collective, we increase the torque and we need to increase left rudder to counter the inertial spin. I get that. But when reducing collective, it's not like the blade starts to spin the other way, so this cannot be the reason we need right rudder?

 

You need right rudder because there is more RPM transferred to the tail rotor (and conversely stripped off the main rotor) hence the fuselage tends to slightly yaw into the opposite direction and therefore more right needed.

 

This is true for overloaded takeoffs because main rotor needs more RPM so you push the right pedals in to transfer RPM from tail rotor to main rotor except in this case you dont aim for directional control but you aim for a right handed takeoff (also tilting disc to the right and forward) thus getting into ETL as quickly as possible from the left hand (described in Masons book).

[Robert31178]

I think I got this lol.....retreating blade stall is an issue for conventional helicopters when you go too fast. Basically your retreating blades are making the same angle as your advancing blades, but since they have less relative wind they stall before the advancing blades at critical angle, causing asymmetrical lift. This is a part of why Russians have been putting out helicopters with counter-rotating twins, they always have advancing blades on both sides of the helicopter. It's also why the Chinook is one of the fastest helicopters in the inventory.

[Chic]

You need right rudder because there is more RPM transferred to the tail rotor (and conversely stripped off the main rotor) hence the fuselage tends to slightly yaw into the opposite direction ....

 

At the risk of nit picking, I would submit that RPM of the main and tail rotor are proportionately linked through the transmission gear ratio. Any droop will effect both rotors "equally".

The yaw produced by increased load demands on the main rotor is the result of the Newtonian response to the abrupt increase in the torque transmitted through the main rotor mass to the fuselage.

 

 

This is true for overloaded takeoffs because main rotor needs more RPM...

 

 

The main rotor needs more lift which is achieved by increasing blade pitch resulting in increased torque demand. Droop aside, this is accomplished without change in rotor RPM.

Edited June 6, 2017 by Chic

[FragBum]

 

The main rotor needs more lift which is achieved by increasing blade pitch resulting in increased torque demand. Droop aside, this is accomplished without change in rotor RPM.

 

Unless you need to tap into more inertia from the rotor and make the cranky light come on. :lol:

[Robert31178]

I was experiencing this last week lol...we ran an op and I was building a radar site on the highest peak near Vlad.....ran out of left pedal authority and was barely able to get my crates down safely. We also had a few downed pilots in the MT's there and I had to use a different technique to ensure I didn't wind up smashing myself all over the place.

[Razor18]

At the risk of nit picking, I would submit that RPM of the main and tail rotor are proportionately linked through the transmission gear ratio. Any droop will effect both rotors "equally".

The yaw produced by increased load demands on the main rotor is the result of the Newtonian response to the abrupt increase in the torque transmitted through the main rotor mass to the fuselage. The main rotor needs more lift which is achieved by increasing blade pitch resulting in increased torque demand. Droop aside, this is accomplished without change in rotor RPM.

 

On the other hand the governor is there to (try to) keep the RPM at 100% all the time anyway, so as you said, it is the main rotor pitch (causing drag) change by the collective, and not any RPM change that causes the yaw. Cheers: Blade ;)

[Udat]

I think I got this lol.....retreating blade stall is an issue for conventional helicopters when you go too fast. Basically your retreating blades are making the same angle as your advancing blades, but since they have less relative wind they stall before the advancing blades at critical angle, causing asymmetrical lift. This is a part of why Russians have been putting out helicopters with counter-rotating twins, they always have advancing blades on both sides of the helicopter. It's also why the Chinook is one of the fastest helicopters in the inventory.

 

If that was true, that the blades have the same angle at the retreating and advancing side, the helicopter would produce 2-3 times the lift on the advancing side at around 100 kn if the blades spin at around 400 kn tip speed. Not good for level flight.

 

What happens is that the blades starts producing more lift than it would be at a hover, as soon as it passes the 6 o'clock position on the rotor disk. This happens due to the increased velocity of the relative wind. Now an increase in lift makes the blade want to go up, and since helicoper blades can flap, the blade flaps up. This upward motion causes the relative wind to come from higher up, thereby reducing the AoA, and therefore also lift. The opposite happens at the retreating side. The higher the speed, the more the blades will flap to compensate. If this were not true, helicopters would not be able to accellerate out of a hover. And they would not be able to hover over a point on the ground in windy conditions. What i just explained is disymmetry of lift.

 

The aerodynamic hazard called retreating blade stall occurs when the forward speed of the helicopter is so great that the retreating blade has to flap so much that the AoA to the realtive wind exceed its critical angle of attack, and stalls, eliminating lift and dramatically increasing drag for the affected section of the rotordisk.

Edited June 21, 2017 by Udat

[Robert31178]

Nice!! Thanks for the correction!!!

[SnowTiger]

Ya ... What Udat said !

I'm going with him on this one.