Why does it do that?
 

Ok, here my invitation to be educated. Why do these helos fly at a given altitude? What I mean by that is the pull of gravity is the same at 3 feet as it is at 50 feet (32 ft/sec) So why does it take more throttle to hover just a little higher that the previous setting (once your out of ground effect) Is it the small difference in air density?

Ted

T-Rex, Blade CP

RE: Why does it do that?
 

Ground effect adds a 'cushion' of air under the whole heli. This can make the heli very unstable or conversely very stable depending on the given situation. The TRex is a very light heli easily effected by air currents as well as wind. A good pilot can overcome this, it is a challenge.

We can carry this out further with this elaboration. I like scale flight. I happened upon some big electric wire spools, that are about 3' in diameter when used as a table. In calm conditions, as I come in to land over the pad, things get funky as the front of the rotor disc begins to be directly above the table and the whole heli dances all around until I get the entire rotor disc over the table. The part ground effect and part out of takes great amounts of fast pitch changes to keep the heli stable.

Now, with wind/breeze this may not happen because it is the same as being in forward flight where you get translational lift which you perceive as less collective. Helis like to fly in forward flight much more than they like to hover. Extra lift. \

hope that helped

RE: Why does it do that?
 

heat radiation from the ground also plays a big part in ground effect, i fly primarily off concrete (hate a dirt/grass field) and you can notice a considerable lift from the heat rising off the concrete as you fly over at low altitude, it's almost like a bump where the heli rises a few inches as it passes over the hard top.

kc

RE: Why does it do that?
 

That's interesting, that's never ocurred to me before, I rarely fly over concrete, i'm gonna try this out the next time I go flying, weather is shaping up real nice real quick over here!

RE: Why does it do that?
 

wish i could say the same for the weather here, it's near 90* and very humid, not exactly my ideal flying conditions and not normal for this early in the year, gonna be a HOT summer.

kc

RE: Why does it do that?
 

I don't think he is just talking about the ground effect.

It takes more power to fly at 200 ft than at 50ft because of the change in air density. There is more air at ground level than high up. This means there is more air your helicopter can grab and push down. Also your engine is also getting less air and becoming richer. Gravity is decreasing. g=MG/r^2 ( I might be a little off since I haven't taken physics in 5 years).

The reason jets fly at 30-40k feet is because there is less air density which causes less drag on the airplane. The thrust from the turbines also decreases, but not as quickly as drag.

RE: Why does it do that?
 

Interesting theory. However I don't think that the altitude changes we experience with the typical model helicopter would have enough air density change to be noticeable.

RE: Why does it do that?
 

the only air density difference i notice are from winter to summer, cold dense air of winter gives tremendous lift and power. and requires a 3-4 click turn in on the main needle for most of my birds.

kc

RE: Why does it do that?
 

When i first read this i was going to reply about air density as well, but then i thought about it in practical conditions.

Changing from 10 feet to 20 feet will not have ground effect, air density or air temperature affecting flight, but it still does take more power\pitch to fly at 20 feet versus 10 feet, correct?

I am not sure, i know it takes power to get up there, but once up there can you put it back in hover power\pitch range and stay up there?

RE: Why does it do that?
 

It's been my personal experience that hover pitch is still mid stick, wether i'm at 10 ft or 100 ft.
If I understand the question you're asking, why does it take more power to make the helicopter ascend to a different altitude, it has to do with thrust vs weight ratio. When you're hovering, the helicopter is providing only enough vertical thrust to keep the weight of the model in the air (so you're basically at a 1:1 thrust vs weight ratio). If you want to go higher you need to increase overall vertical thrust so that the thrust being applied is more than the weight of the helicopter it's self, allowing it to climb. But with our models, once you're actually at the altitude you wish to be at, you should be able to hover in the same spot with your stick in roughly the same area. (mid-stick, usually).

I've never noticed my hover point on the sticks increasing when i'm higher in the air. I doubt air density changes much at the altitudes our models fly it.