Ground effect in a gyroplane?

Vance

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Jerry is a 100 hour gyroplane pilot and a 20 hour gyroplane flight instructor.
He is a fixed wing and helicopter flight instructor too.
Raul and Jerry were both very generous and cordial with their time despite our divergent opinions.
 

Doug Riley

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As someone who self-taught in the old Bensen way, I logged a lot of hours under power at under 10 ft. off the runway.

Adding to my scores of hours in ground effect was the fact that I started out with a 1500 cc VW, which would not fly out higher than ground effect at any airspeed! Yet it would fly all day IN ground effect (at under 10 ft.). I'd call that a "tangible" (noticeable) benefit, though not a very useful one.

Before Vermont's winter climate moderated in recent years, Lake Champlain would freeze over, all the way across to New York state most years. Buddy Bill Raub and I would layer up our clothing in February and go blasting about, just off the ice, for miles. This was a real treat. You could throttle back significantly and still maintain speed and altitude, just as Vance describes. The reduced noise from lower engine RPM was pleasant.

Some incoming air, of course, spills up through a gyro rotor disk; there is not a vacuum just above the disk. The physics of lift cannot work, however, without there being a net surplus of air molecules ending up below the level of the disk and a net shortage of them above it. Overall, the rotor deflects air down-and-back. The resulting downwash is quite visible (or, well, visualizable) behind a gyro flying low over grass or crops.

A rotor blade is a very ordinary wing. It makes lift the same way as any other wing. All wings create tip vortices, as some of the air pushed down by the wing squirts out and around the end of the wing. Making these little horizontal tornadoes soaks up power in an un-useful way. Partially blocking them adds to the efficiency of any wing, including a rotor blade.
 

Resasi

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They are of course high aspect ratio wings and I wonder if there is a practical form of wingtip winglet that can further reduce these tip vortices on rotors. This has been discussed on the Forum before.

While winglets do not seem common on gliders, sailplanes and do can utilise winglets to improve gliding efficiency. While originally used to enhance the performance of gliders in the 15 meter class, they are now found on unlimited class sailplanes.
M.D. Maughmer had an article in the November-December 2003 issue of Journal of Aircraft (Vol. 40, No. 6).

Winglets must decrease in size with an increase in aspect ratio and in higher speeds the drag rise may cancel any gain. There are however various tip mods being done of helicopter rotors like the BERP rotors mentioned by WaspAir and Joe Nelson so it would seem an open field for some inspired rotor manufacturer. Wonder if there are any computer programs that would allow experimentation to be explored in this particular field

But anyway it seems another experienced pilot has experienced appreciable ground effect.
 
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WaspAir

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This is what modern sailplanes look like. We have one of these with 20 meter span and jet sustainer engine (retracts behind the cockpit) among the ships owned by members of my glider club (made in South Africa by Jonkers).JS1-1.jpg

Check out those wingtips . . .
 
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Jean Claude

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To give an improvement, the "winglets" must be positioned at a precise angle relatively to the direction of flight.
But a blade sees this direction changing constantly during a revolution. More or less 30 degrees
Therefore "winglets" are not compatible with blade tips.

In any case, it is the friction of the air on the blades that is the main cause of the power absorbed by a rotor, due to the high speed. The tip vortex is for very little in this story.
Also, the ground effect acts not on the blade tip vortex , but on the vortex of the disc tips.

Sans titre.png
 
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Resasi

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Thank you Jean Claude, explains well why they haven’t been seen.
 

Jean Claude

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I quantified the benefit in ground effect with a 7.3 x 0.18 m rotor carrying 248 kg at 45 mph:
Without ground effect, the rotor drag is 565 N, in steady flight
But when the wheels are just above the ground ( Zrotor = .6 R), then the rotor drag is only 514 N, calculated with the Naca TN D-234 indications
 
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XXavier

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I quantified the benefit in ground effect with a 7.3 x 0.18 m rotor carrying 248 kg at 45 mph:
Without ground effect, the rotor drag is 565 N, in steady flight
But when the wheels are just above the ground ( Zrotor = .6 R), then the rotor drag is only 514 N, calculated with the Naca TN D-234 indications
That's interesting... At first sight, it looks like a relatively small gain. In terms of power, and assuming 72 km/h, the savings are perhaps two hp in terms of shaft power...

I always believed that the savings in power when flying a very low pass above a smooth runway were much higher...
 
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Jean Claude

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With a 8.5 x 0.2 m rotor carrying 400 kg at 45 mph:
Without ground effect, the rotor drag is 1020 N, in steady flight
But when the wheels are just above the ground ( Zrotor = .6 R), then the rotor drag becomes 925 N,
So, the benefit is 95 N * 20.1 m/s = 1910 watts produced by the propeller.
Taking acount the propeller efficiency of .6 about, it is a benefit of 3180 watts on the engine shaft (4.26 hp)
 

Vance

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Thank you Jean Claude, I learn so much from you.
It appears to me I am getting more benefit in The Predator than 4 horsepower.
I am not sure I would even notice four horsepower.
30 foot eight an a half chord blades, 1,100 pounds gross weight and 50kts in ground effect from 2,150 propeller RPM to 1,720 rpm to fly level. The rotor is about 11 feet above the ground at the center. I don't know what the disk angle is.
This seems like a bigger power change than you have calculated.
I am getting some lift from the ground effect on the horizontal stabilizer.
Based on fuel consumption I am guessing The Predator uses around 80 horsepower at 50kts at altitude.
 

XXavier

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With a 8.5 x 0.2 m rotor carrying 400 kg at 45 mph:
Without ground effect, the rotor drag is 1020 N, in steady flight
But when the wheels are just above the ground ( Zrotor = .6 R), then the rotor drag becomes 925 N,
So, the benefit is 95 N * 20.1 m/s = 1910 watts produced by the propeller.
Taking acount the propeller efficiency of .6 about, it is a benefit of 3180 watts on the engine shaft (4.26 hp)

I see...

It would be interesting to know what would be the power savings (possibly much higher) or a low-wing FW of the same mass flying a low-pass at a similar speed...
 

Tyger

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Is there any ground effect attributable to the fuselage itself?
 

fara

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With a 8.5 x 0.2 m rotor carrying 400 kg at 45 mph:
Without ground effect, the rotor drag is 1020 N, in steady flight
But when the wheels are just above the ground ( Zrotor = .6 R), then the rotor drag becomes 925 N,
So, the benefit is 95 N * 20.1 m/s = 1910 watts produced by the propeller.
Taking acount the propeller efficiency of .6 about, it is a benefit of 3180 watts on the engine shaft (4.26 hp)
That sounds about right Jean.
There is a benefit but its not a lot. However, I am quite sure there is also some combined effect from body fairings and landing gear fairings shaped as an airfoil as well for gyroplanes equipped with those
 

Jean Claude

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At 45 mph, the lift of a horizontal stabilizer such as Magni M16 (10 sq ft) is less than 50 N, ie less than1% of that of the rotor.
This is negligible.
 

StanFoster

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Jean Claude...I have to laugh on your "negligible" comment. While I totally agree...sometimes "negligible" can be a major determining factor. Let me explain. My dad was a pilot and was at the airport watching a spray plane filling up for another very hot and muggy August spray run. Dad noticed a thick layer of smashed bugs all along the leading edge of the airplane wing. Dad offered to go get a 5 gallon bucket of water to scrub that airfoil clean....beings the pilot that he was. The spray pilot declined the offer and away he went. Dad went home, but a few hours later that airplane was attempting to take off fully loaded and very hot out....and he "just" didnt quite have enough lift. He mushed it out as the end of the runway came up ...and his wheels were greeted with skimming the viney tops of a soybean field. Those soybean plants soon tenaciously sucked that airplane into the ground...flipping the plane over. Pilot was ok, but had he allowed dad to earlier have washed those bugs off....that extra ""negligible" difference would have saved his plane!!
 

Resasi

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Would definitely agree with that Stan.

Doing a charter flight in a Cessna 402 up at a grass/mud strip called Kericho in Kenya a big tea area. It was 7,165 feet msl and 3,885 feet long. Durning the rainy season would get pretty soggy and with the density alt invariably higher with any sort a load, the ability to lift off early then accelerate in ground effect was vital as I am very grateful for. I was younger then but it was a memorable lesson.

Also believe in getting those summer bugs washed off the rotors before every flight. It’s the ‘neglible’ differences that can have massive effects.
 

Jean Claude

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Xavier,
We have to compare the benefit to the power required OGE on the engine shaft .
Parasit drag 175 N (if S*Cd = 0.7)
Rotor drag 1020 N OGE
Total 1195 N OGE

Power on engine shaft = 1195 N * 20.1 m/s /0.6 = 39.8 kw
Hence a benefit of 8 %.

Now, I did this evaluation for a Cessna 150 (700 kg) at the same speed (flaps down) when the wheels are almost touching the ground (z/b = 0.19)
I get an benefit of 40% (assumed parasit S*Cd = 0.5 included wings)
 
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XXavier

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Xavier,
We have to compare the benefit to the power required OGE on the engine shaft .
Parasit drag 175 N (if S*Cd = 0.7)
Rotor drag 1020 N OGE
Total 1195 N OGE

Power on engine shaft = 1195 N * 20.1 m/s /0.6 = 39.8 kw
Hence a benefit of 8 %.

Now, I did this evaluation for a Cessna 150 (700 kg) at the same speed (flaps down) when the wheels are almost touching the ground (z/b = 0.19)
I get an benefit of 40% (assumed parasit Cd =0.5 included wings)

I expected a big difference between FW and gyros concerning GE, and you're confirming that with your figures. Perhaps the reason that some pilots simply deny the existence GE in gyros is that they may be used to the much stronger and perceptible GE in FWs. Of course ground effect is present in gyros, but is less noticeable...

As Tyger and fara write above, there may be also some contribution, in the case of faired bodies like those of ELAs and Magnis, as they may work as lifting bodies, benefiting from the ground effect...
 

Tyger

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Even if the lift contributed by the fuselage and tail are tiny, compared to the rotor, these components will be much closer to the ground at lift-off.
 

Jean Claude

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If the trailing edge were to rub on the ground, so that air could not escape under the horizontal stabilizer, then the dynamic pressure (1/2 Rho V2) applied over the entire bottom surface would give a maximum of 250 N at 45 mph.
But the air is in reality never completely blocked and adding the leaks at the ends, I can't reasonably imagine this lift to be more than 100 N, which is still "negligible".
 
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