hueyman

My Feedback: (2)

Is it normal to slowly lose altitude while hovering just out of forward flight. If i leave the throttle at a setting to hover at 3' then progress to forward flight the heli starts to climb as it picks up speed. when i pull back cyclic to slow into a hover (it is maybe 10 - 15' feet by this time) it will slowly start to lose altitude while hovering until it touches the ground then comes back up to the original 3' hover. all this without touching throttle/pitch control. I know collective control should be used to prevent this but the slow sinking of the heli once it is in a stable hover @ 10-15' doesn't seem right.

-pkh-

My Feedback: (2)

While you're moving, the airflow over the blades due to forward motion generates more lift, like a fixed wing plane generating lift with forward motion. When you stop for a stationary hover, you need to replace the lost lift with more rotor speed/pitch. After awhile, you'll get the feel for how much throttle/collective to add when you do this transition, and the heli won't even drop... just takes some practice...

hueyman

My Feedback: (2)

the way it gradually dropped i though maybe the collective was moving around then sticking with no input but it works smooth with no load or under load. Just have to start working in the proper throttle management i guess. just learning so i got my thumbs full as it is. Thanks.

Maxime

This is called translationnal lift. When you hover, the rotor blows directly under the chopper, and that air is very turbulent. The rotor isn't very efficient at that point. When you start moving, this turbulent air will start getting behind the rotor, and the rotor will bite into some clean air, becoming more efficient. At this point this is where you get translationnal lift. The helicopter will start climbing if you don't correct for it.


The reverse is also true, when you slow down you will lose translationnal lift... then you need to add power to avoid falling to the ground.



There are other phenomenons too, when you start moving, the center of lift will start moving towards the advancing blade ( in relation to the direction the helicopter is flying ), and the helicopter will have a tendency to bank on its own. ( side opposite the advancing blade ). To make this clear: If the chopper is hovering, then everything is balanced and the center of lift will be at the center of your rotor. When you start moving forward, there is a blade that will "get more wind" than the other... The advancing blade in relation to the direction of flight. This blade will generate more lift than the retracting blade ( again in relation to the direction of flight ), so the rotor disk will tend to bank towards the retracting blade because the center of lift will not be in the center of the disk anymore, but towards the blade that generate more lift.



I do experiment these phenomenons everyday, as I fly full scale choppers....




Hope this make sense.... English isn't my first language!

BarracudaHockey

My Feedback: (11)

Same exact answers he got on RunRyder.

voltron1011

Excellent explaination Maxime. This transitional lift is also the reason helicopters will only go so fast. I used to fly full scale Blackhawks and Jetrangers, and as you approach 180 knots the retreating blade is approaching the speed of sound!

BarracudaHockey

My Feedback: (11)

Retreating blade or advancing blade? The retreating blade should be approaching blade stall (Tip speed - TAS) where the advancing blade should be approaching the speed of sound (Tip Speed + TAS).

Isn't retreating blade stall what limits the speeds of full scale? And isn't that why the Apache's and such have a wider blade chord?

Just wondering.