Ascending while turning

[Looney]

After noticing this behaviour, I can't really wrap my head around the physics involved.

 

Here's the deal, when you are flying straight and level and turn with both cyclic and rudder, you eventually begin to ascend when in the turn.

 

Now, I would expect the aircraft to drop because you are increasing your force vector in the direction of the turn so the resulting vector from all the forces would lead to a decrease in lift vector.

I am wrong however, as I need to drop collective when in a turn with the Mi-8.

 

Can anyone explain?

 

 

From the FAA site:

 

As the angle of bank increases, the total lift force is tilted more toward the horizontal, thus causing the rate of turn to increase because more lift is acting horizontally. Since the resultant lifting force acts more horizontally, the effect of lift acting vertically is deceased. To compensate for this decreased vertical lift, the angle of attack of the rotor blades must be increased in order to maintain altitude. The steeper the angle of bank, the greater the angle of attack of the rotor blades required to maintain altitude. Thus, with an increase in bank and a greater angle of attack, the resultant lifting force increases and the rate of turn is faster.

Edited June 20, 2016 by Looney

[NeilWillis]

The answer has to be the same as all facts regarding helicopters - witchcraft!

 

I'm sure the real reason must be to do with gyroscopic effects, because I agree, it is totally the opposite of what it would be reasonable to expect. I'd even go as far as to say you might expect turning the other way to make the nose drop far more than reason would demand too, due to the advancing/retreating blade phenomenon.

 

I'd be very interested to hear too about what causes this, and lots of the other unfathomable stuff that occurs in helicopter aerodynamics.

[Cibit]

Hey Looney, I don't have this problem I am afraid but perhaps try the autopilot altitude channel. :) I don't use rudder too much in the turns, only left or right stick as soon as rudder inputs are made they reduce or add lift

Edited June 20, 2016 by Cibit

[Weta43]

Maybe see if you ascend when turning one way, and descend when turning the other (just sit in a stationary hover, and add or remove rudder).

It could just be that as you decrease the amount of power needed by the tail rotor (because you're allowing the aircraft to slowly turn with the rotor), the main rotor has more power available to it for a given net engine power output and the aircraft climbs.

If it is that, turning the other way should require more power to the tail rotor & result in a descent.

[Looney]

I'll do some tests later tonight to see if the rudder input is the main culprit orsomething else.

[Slazi]

If you're not at 90 degrees during your turn, you might be trading air speed for height. That would be my guess.

[Shahdoh]

When you bank over, are you pulling back on the cyclic for the turn? If so, you might be doing it to much, inducing the climb.

[Home Fries]

When you bank over, are you pulling back on the cyclic for the turn? If so, you might be doing it to much, inducing the climb.

Either that, or the act of banking the aircraft helps to force the pitch back to a neutral position. This could be the just because of the joystick, where an offset in the X axis pushes the Y axis slightly to neutral if you are going with the same spring tension, because that tension is now applied to two springs instead of one.

 

Move the joystick around its limits in the Control Panel to see if this is the culprit.

[Cibit]

Did some flying in the hip tonight. I can't do level turns using pedals. I have the ball centred and use left or right stick to turn level.

[Flamin_Squirrel]

Try it again, left and right, and see if you notice a difference.

 

A possible reason is the following:

 

With a clockwise turning rotor, a right hand turn will lead to the air encountered by the advancing blade to increase in velocity, increasing lift, which applies an upward force on the blade. Gyroscopic precession means the affect of the applied force is seen 90 degrees later when the blade is pointing forward; in other words, the rotor disc has been raised at the front (and lowered at the back) causing the nose to pitch up.

 

A left hand turn has the opposite affect, with the disc lowering at the front causing the nose to drop.

[Mayo2017]

Could it also be the pitch/bank autopilot trying to pull the nose back up?

[kuki767]

Simply, it is caused by the total sum of velocities on the rotor disc (relative velocity of air because of helicopter rotation + velocity of air because of rotor rotation).

 

Go into a hover and then rotate the helicopter only by using pedals. You'll notice that when you turn left, your altitude will decrease and when turning right, increase. The same thing happens when you're turning in horizontal flight.

 

To sum up:

in left turn -> rotor speed - helicopter turning speed = reduced absolute blade speed (decreased lift)

in right turn -> rotor speed + helicopter turning speed = increased absolute blade speed (increased lift)

[Weta43]

I suggested that in post 4, & keep coming back to see if they tried...

 

 

To sum up:

in left turn -> rotor speed - helicopter turning speed = reduced absolute blade speed (decreased lift)

in right turn -> rotor speed + helicopter turning speed = increased absolute blade speed (increased lift) [

 

With the direction you get lift from depending on which helicopter your flying

[Braeden108]

How do you ascend normally in a helicopter? More collective, and/or pulling back on the cyclic. In the latter you're making the front side of the rotor disk higher than the back, and you go up. When you enter a turn you're making one side of the rotor disk higher than the other, however in this case it's simply the right or left side, higher or lower. Coupled with the fact that you still have inertia in the direction you were just going. If this doesn't make sense consider this. You're flying sideways, your nose is north but you're travelling east? What happens if you move the cyclic left and raise the eastern side of the rotor disk? You go up. This is the same thing as the turn, except in the turn you have forward inertia.