StanFoster
Active Member
- Joined
- Nov 16, 2003
- Messages
- 17,139
- Location
- Paxton, Il
- Aircraft
- Helicycle N360SF
- Total Flight Time
- 1250
I love reading about rotorcraft aerodynamics. One area that used to baffle me was hovering in ground effect...and why less power, or AOA is really needed.
I had been reading several helicopter books...and one put out by the government you would think would be correct.
I initially learned that a helicopter flying IGE needed less power simply because the air was "packed" under the helicopter...more dense...and thus the blades needed less pitch to maintain a hover. This made sense in my newbie mind.....
Further reading in another helicopter book found myself learning that the air was being bottled up so to speak by not getting to exit out as fast from under the helicopter....and this caused the induced flow velocity through the rotor to decrease. Whenever the induced flow velocity decreases....this does indeed cause the AOA of the rotor blades to increase. Then the collective can be lowered to hover with less power required. This also made sense.
Close...but not the cigar. Justin Travis explained to me the best way I have seen...and have not read such a correct discription of this phenomena since.
He clearly explained to me what is going on when a helicopter is IGE. When a helicopter is within one rotor diameter of the ground....roughly.....you are entering ground effect. The rotor tips are generating very strong vortices...just like any wing tip does. These vortices developement however starts getting restricted IGE. The rotor disc area has some of this area consumed by these vortices...leaving the remainder disc area for the induced flow to travel through. When the helicopter is in ground effect...these vortices are restricted...are smaller...and thus consume less rotor disc area. This leaves MORE rotor disc area for the induced flow to travel through...and this automatically lowers the induced flow velocity. Any reduction in induced flow velocity results in the rotor blades seeing an increased AOA. This increased AOA can now be reduced by lowering the collective...and the end result is the helicopter requires less power to hover IGE.
Justin pointed out that it simply is wrong thinking that the air is denser, or packed under the helicopter. His reasoning was backed up by him saying if this was so....his altimeter would show a lower altitude if the air was indeed being packed tighter. He says he never sees any change at all.
I have been reading this topic being discussed right now on another helicopter forum...and I notice that some still believe what I first read.
I find several topics like this amazing ....and I learned you cant always believe what you read.
Stan
I had been reading several helicopter books...and one put out by the government you would think would be correct.
I initially learned that a helicopter flying IGE needed less power simply because the air was "packed" under the helicopter...more dense...and thus the blades needed less pitch to maintain a hover. This made sense in my newbie mind.....
Further reading in another helicopter book found myself learning that the air was being bottled up so to speak by not getting to exit out as fast from under the helicopter....and this caused the induced flow velocity through the rotor to decrease. Whenever the induced flow velocity decreases....this does indeed cause the AOA of the rotor blades to increase. Then the collective can be lowered to hover with less power required. This also made sense.
Close...but not the cigar. Justin Travis explained to me the best way I have seen...and have not read such a correct discription of this phenomena since.
He clearly explained to me what is going on when a helicopter is IGE. When a helicopter is within one rotor diameter of the ground....roughly.....you are entering ground effect. The rotor tips are generating very strong vortices...just like any wing tip does. These vortices developement however starts getting restricted IGE. The rotor disc area has some of this area consumed by these vortices...leaving the remainder disc area for the induced flow to travel through. When the helicopter is in ground effect...these vortices are restricted...are smaller...and thus consume less rotor disc area. This leaves MORE rotor disc area for the induced flow to travel through...and this automatically lowers the induced flow velocity. Any reduction in induced flow velocity results in the rotor blades seeing an increased AOA. This increased AOA can now be reduced by lowering the collective...and the end result is the helicopter requires less power to hover IGE.
Justin pointed out that it simply is wrong thinking that the air is denser, or packed under the helicopter. His reasoning was backed up by him saying if this was so....his altimeter would show a lower altitude if the air was indeed being packed tighter. He says he never sees any change at all.
I have been reading this topic being discussed right now on another helicopter forum...and I notice that some still believe what I first read.
I find several topics like this amazing ....and I learned you cant always believe what you read.
Stan