# Ground Effects on the rotor

#### Jean Claude

##### Junior Member
The White Cavalon I am flying (509PH) at 6,500 feet is showing between 400 and 410 rotor rpm.
The Predator at sea level typically flies level at 315 rotor rpm but I have seen close to 400 rpm at 12,500 feet. At a density altitude of 6,500 feet she flies around 360 rotor rpm.

Taking into account 6500 ft indicated by Vance, it appears that the aerodynamic pitch during the flight is 4.1° for Predator, and 3.1° for Cavalon.
Good value is from 3° to 3.5°. >4° is less safe in my opinion.

#### Vance

##### Gyroplane CFI
my best measurement.

my best measurement.

It seemed such a good idea to prescribe rrpm and backwards calculate pitch, Jean-Caude, that I couldn't resist and ran such a calculation...;-) The mean rotor drag coefficient I used was 0.01 for both cases. The results are:
Blade pitch = 4.63° (rotor 30 'x 8.5" for 1100 lbs and 315 rpm at see level )
Blade pitch = 1.93° (rotor 28 'x 8" for 950 lbs and 400 rpm at see level )
These results are so close that I think we've got it right to within the limitations of the theories we are using. I also thinkg that our approaches are so different that we can exclude the possibility that we both make the same mistakes.
I do hope that Vance will provide values for the pitch settings, that'll be very interesting indeed!

I did my best to measure the angle Juergen.

I have the rotor off of the Predator so I decided to measure the flat on the bottom of the Sport Copter blade compared to the flat on the hub bar. The rotor is level on two sawhorses.

I calibrated my smart level for zero on the hub bar and then measured the flat on each blade.

I measured 1.5 degrees on each blade.

I don’t feel I can accurately measure the airfoil so I won’t try to translate that into anything other than a simple measurement of the flat in relation to the hub bar.

#### Rowdyflyer1903!

##### Newbie
Hi Bryan

While landing in the sport copter trainer I realized our intended touchdown point was covered with leaves and as we came in; the leaves were actually going out in front of us. To see that while feeling the braking lift of the high angle and the loss of flight speed gave me some good insight into just what was going on.

It is a really cool image to recall.

Gilbert
No joke? You witnessed this? Well then that seems to be something which could be very reproducible and could end a lot of debate. I would love to see a video of this done.

#### Brian Jackson

##### Platinum Member
No joke? You witnessed this? Well then that seems to be something which could be very reproducible and could end a lot of debate. I would love to see a video of this done.
I recall when James Randi busted a "psychic" who claimed he could turn a book page telepathically. Randi simply sprinkled talcum powder in front of the book to reveal he was actually blowing on it. I wonder if a similar approach would work for demonstrating. Engine-off of course.

#### Rowdyflyer1903!

##### Newbie
ohhhh thats funny. I like the way you think.

#### C. Beaty

##### Gold Supporter
Ground effect tales remind me of a joke:

A truck was in line, waiting to go through the road department’s scales and the driver was beating on the side of his truck with a stick.

Asked why, the driver replied; “I’m over weight with a load of chickens so I’m keeping them in flight to lighten the load.”

Unfortunately, if the chickens were in a totally enclosed area, in flight or roosting would have made no difference in truck weight.

#### Rowdyflyer1903!

##### Newbie
What a hoot. I spent the last 15 years at sea 9 weeks at a time on a geology research vessel. I brought a dinky drone on board. Now the ship was pretty big at 470 ft long but it not large enough not to be tossed by the ocean swells. It was a drill ship and you would stand and watch the drill stem moving up and down sometimes as much as 25 feet. Well it was NOT the drill stem moving up and down it WAS THE SHIP!

The oddest thing would happen and please no one doubt me on this because I saw it and I was intrigued by it and I experimented to prove it. If I flew the drone indoors the drone would hold altitude in relation to the room. ( It would fly a few feet off of the table and hold that few feet) This would happen even though the ship was moving up and down of the ocean swells. Now conversely, if I took the drone to the out side and flew it, I would have to be mindful to take the drone up to an altitude beyond what the ship was moving to keep it safe. I would have to launch the drone at the peak of the swell and climb or the ship would rise up and smack the drone out of the air.

So basically the drone was flying in a captured air mass inside the ship and other than the inertia of mass (which was very little) the drone moved up and down with the captured air mass. Take that same drone out side and the ship moved independently of the atmosphere.

#### XXavier

##### Member
Interesting... The behavior of the drone under those conditions would be probably the same as that of a small hydrogen-filled balloon ballasted to match exactly its weight, so that it would remain stationary. Inside the ship, it would keep its altitude constant in relation the the floor, but outside the ship, it would keep that constant altitude in relation to the sea...

#### Rowdyflyer1903!

##### Newbie
Its puzzling but it seems that is the case. If you look at it from a pilots point of view, we too inflight will move up and down with the air mass, for this is the reason we use the Kollsman dial to pressure compensate so we can all be on the same altitude page, so to speak.

#### C. Beaty

##### Gold Supporter
All objects, in flight or not, are ultimately supported by the Earth’s surface.

I used to operate a business a mile of so from the end of an active runway of the Tampa airport. When a 747 flew over during a landing, the leaves on the trees would flutter even though the 747 was still at several hundred feet of altitude. The total pressure rise equals the weight of the aircraft but the aircraft’s behavior is essentially unaffected because it is outrunning its downwash.

Just before touchdown, the pressure rise is quite high because the area affected is much smaller, this being what we call ground effect.

A 747 at 30,000 ft still increases the pressure on the Earth’s surface to equal the aircraft’s weight but it’s spread over such a vast area as to be undetectable.

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