Theorical question

[Codralta]

"When turning left, the nose tends to drop. when turning right, the nose tends to go up."

"When pushing down the collective, the nose goes down. When rising the collective, the nose goes up."

 

Why ?

 

I understand the explanation of retreating blade, but don't understand these effects.

 

Thank you

[kodoz]

"When pushing down the collective, the nose goes down. When rising the collective, the nose goes up."

 

Why ?

 

 

It's the horizontal stabilizer. Collective goes down, the helicopter "drops" but the horizontal stabilizer makes the tail lag behind.

 

Out of curiosity, how far along are you in your training?

[iChris]

"When turning left, the nose tends to drop. when turning right, the nose tends to go up."

"When pushing down the collective, the nose goes down. When rising the collective, the nose goes up."

 

Why ?

 

I understand the explanation of retreating blade, but don't understand these effects.

 

Thank you

 

 

1. Helicopter pitch changes during left and right rolls are the result of “pitch-roll coupling”. It’s a matter of how the rotor system flaps in response to lateral cyclic.

 

In a left roll, blade pitch is reduced over the nose and increased over the tail. This unbalances the lift forces and causes the rotors to flap down on the left side and up on the right. The result is a shift of the trust vector causing the fuselage to follow, rolling left and pitching down (as the blades flap down over the nose). After the aircraft reaches a steady rate of turn, the fore and aft lift forces balance.

 

In a right roll, blade pitch is increased over the nose and reduced over the tail. This unbalances the lift forces and causes the rotors to flap up on the left side and down on the right. The result is a shift of the trust vector causing the fuselage to follow, rolling right and pitching up (as the blades flap up over the nose). After the aircraft reaches a steady rate of turn, the fore and aft lift forces balance.

 

Because of the static stability designed into the aircraft these effects are not normally noticed. The addition of a horizontal stabilizer is also part of most designs to improve the longitudinal stability (pitch stability). The pilot automatically corrects these small out-of-trim conditions.

 

Also note that blade coning and asymmetrical induced velocity distribution across the rotor disc results in less induced drag over the nose and more induced drag over the tail in forward flight. These asymmetrical induced velocities over the nose and tail causes a transient torque effect were left rolls require more torque than level flight and a right roll requires less torque than level flight.

 

2. Collective inputs in forward flight also induce some blade flapping that causes the nose to pitch up or down. The advancing and retreating blades receive the same degree in pitch angle change. However, the advancing blade has higher velocities and develops more lift (dominating blade) than the retreating blade.

 

An up collective input causes the advancing blade to start flapping up from its trim position at three o’clock resulting in maximum pitch-up over the nose. The reverse input, down collective will cause the advancing blade to start flapping down from its trim position at three o’clock resulting in maximum pitch-down over the nose.

 

Again, because of the static stability designed into the aircraft these effects are not normally noticed. The addition of a horizontal stabilizer also improves the longitudinal stability (pitch stability) by countering these effects. Unless rapid changes in collective are made, the pilot automatically corrects these small out-of-trim conditions without even thinking about it.

 

If you’re new to this, an aircraft has static stability, when it tends to return to its trim flight condition following any external disturbance.

Edited October 29, 2011 by iChris

[Codralta]

Thank you for your answers.

 

For the 2. , I don't see why the helicopter pitch changes during the flight. During take-off (and landing), I understand ; but when the helicopter is already in flight, the nose has already pitch up during the take-off and I can't visualize how it can pitch up again as the lift added is the same everywhere in the rotor disc. (Or is the lift not proportional to the velocity of the blade ?)

 

"Also note that blade coning and non-uniform induced velocity distribution across the rotor disc results in less induced drag over the nose and more induced drag over the tail in forward flight. These non-uniform induced velocities over the nose and tail causes a transient torque effect were left rolls require more torque than level flight and a right roll requires less torque than level flight."

 

I don't want to abuse but I don't understand.

 

I'm not (yet) in training, I'm just a passionate. Could you advise me a book about the theory of helicopters ?

 

Thanks,

[iChris]

Thank you for your answers.

 

For the 2. , I don't see why the helicopter pitch changes during the flight. During take-off (and landing), I understand ; but when the helicopter is already in flight, the nose has already pitch up during the take-off and I can't visualize how it can pitch up again as the lift added is the same everywhere in the rotor disc. (Or is the lift not proportional to the velocity of the blade ?)

 

"Also note that blade coning and non-uniform induced velocity distribution across the rotor disc results in less induced drag over the nose and more induced drag over the tail in forward flight. These non-uniform induced velocities over the nose and tail causes a transient torque effect were left rolls require more torque than level flight and a right roll requires less torque than level flight."

 

I don't want to abuse but I don't understand.

 

I'm not (yet) in training, I'm just a passionate. Could you advise me a book about the theory of helicopters ?

 

Thanks,

 

Lift is proportional to the Angle of Attack times Velocity squared (V²).

L ≈ a x V²

 

If the blade rotational speed is 300kts and the helicopters forward airspeed is 100kts, the relative airflow across the advancing blade is 400kts and only 200kts across the retreating blade.

 

 

Below is a reference covering this post:

Download: Blade_Flapping_and_Feathering.pdf

 

The two references below cover basic aerodynamics. (With respect to aerodynamics, these two are more than enough to take you from zero time – CFII)

 

Download: Rotorcraft Flying Handbook.pdf

 

Principles of Helicopter Flight By: W. Wagtendonk

 

If you want more, look into these three:

Helicopter Aerodynamics Volume I - Ray-Prouty

 

Helicopter Aerodynamics Volume II- Ray-Prouty

 

Cyclic and Collective - Shawn Coyle (Author)

 

May be of interest to know what’s really going on around the rotor disk with all that flapping and feathering:

 

Edited October 31, 2011 by iChris

[Codralta]

Thank you very much Chris !

 

I also find "Private Helicopter Pilot Studies".