Ground effect in a gyroplane?

[Mayfield]

(I actually had a question on ground resonance on my gyroplane knowledge test (PRG)!)

A&S 18A and to a lesser extent the McCulloch J2. When I used to teach in an 18A one of the lessons would be making sure you pattern the blades before starting pre-rotation.

 

[bugflyer]

A&S 18A and to a lesser extent the McCulloch J2. When I used to teach in an 18A one of the lessons would be making sure you pattern the blades before starting pre-rotation.

"pattern the blades" ????

smiles,
Charles

 

[Mayfield]

"pattern the blades" ????

smiles,
Charles

J.R. (Waspair) would probably answer your question better. The A&S 18A has lead/lag hinges as well as flapping hinges. Physically moving the blade to the rear stop of the lead/lag hinges, before starting pre-rotation, ensures the CG of the disc is concentric with the hub as the blades come up to speed. The blades self center as RRPM increases during spin-up.

 

[bugflyer]

J.R. (Waspair) would probably answer your question better. The A&S 18A has lead/lag hinges as well as flapping hinges. Physically moving the blade to the rear stop of the lead/lag hinges, before starting pre-rotation, ensures the CG of the disc is concentric with the hub as the blades come up to speed. The blades self center as RRPM increases during spin-up.

Gotcha, thanks.
smiles,
Charles

 

[WaspAir]

As usual, Mayfield has it right. On the 18A, there is an adjustable lead-lag damper on each blade, consisting primarily of a stack of friction discs under compression. These can change slightly with conditions (especially after a big change in humidity). Periodically, I use a fish-scale spring, hooked to the blade tip, the see how much force it takes to break free and get lead motion while the other blades are held still. (Repeat for each blade.) If necessary, a little turn of a nut on top on any out-of-spec damper would set all the dampers to the proper range. Once the damper force is satisfactory, you preset the damper positions daily so that they're all the same before the first spinup of the day. Putting them all against the rear stop will keep good balance until the blades start to sort themselves out with natural lead-lag action.

The J-2 has little oil damper cylinders on each blade. I had little yellow paint stripes on mine that would line up with a cast-in numerical scale so they could all be set the same.

For either aircraft, you set the position by moving a blade in one direction and then rapidly jerking it the other way to make the dampers yield momentarily. With a bit of back and forth action like that you can set them easily to the desired pattern. If you follow this process faithfully, ground resonance is not an issue. If you are sloppy about such things, you might start a prespin with the blades way out of position, and the spin-up will feel like a washing machine spin cycle with all the towels on one side of the tub (signiicant rpm is necessary to get blades flying into their proper spacing naturally). Once the blades are flying nicely, it's all self-regulating and smooth.

 

[WaspAir]

Thanks for your help to all. Will play with the info and see if that helps. Still see the advancing blade rising creating a larger AOA due to relative wind. My sailing days perhaps interfering?

Just pretend you're a 5 year old who wants to fly like a bird. Run forward flapping your arms up and down, and you will definitely feel more AOA on the down strokes (and less lift on the up strokes).

Might be good to do this when nobody's watching, but it works.

 

[Doug Riley]

The apparent wind that an individual blade "sees" is the vector sum of its rotational velocity and the aircraft's velocity.

In sailboating terms, the advancing blade's rising path is like the path of a sailboat that's steered so close to the wind that its sails are past the "groove" angle and are starting to luff.

 

[WaspAir]

Thanks, Doug. Some of us landlubbers don't know the head from the poop deck.

 

[Sv.grainne]

Thanks to everyone, I was confusing and misinterpreting relative wind not accounting for the change in direction of the blades as they teetered.

 

[Abid]

Just pretend you're a 5 year old who wants to fly like a bird. Run forward flapping your arms up and down, and you will definitely feel more AOA on the down strokes (and less lift on the up strokes).

Might be good to do this when nobody's watching, but it works.

Yeah it will work. Duct tape some cardboard on your arms while doing it and you will really notice it fast

 

[Tyger]

Yeah it will work. Duct tape some cardboard on your arms while doing it and you will really notice it fast

Yes, and other pilots will also really notice it fast, shaking their heads and saying, "Another one of those gyro nuts..."

 

[kolibri282]

Recent discussion featuring relative wind for a sail boat.
https://www.rotaryforum.com/threads/el-mirage-lakebed-in-the-news-here.1146278/#post-1178783

 

[eidschun]

A very patient Raul and Jerry spent a very long time (an hour and thirty eight minutes) on the phone sorting through our divergence of opinions.

I gave Jerry my examples and he suggested we compromise with; “there is no tangible benefit from ground effect in a gyroplane.”

I agree, flying low enough to experience ground effect in a gyroplane is dangerous.

In my opinion using ground effect to manage a takeoff is poor aviation decision making.

Jerry wanted to go a step further and say any benefit from ground effect is not from the rotor and we did not reach agreement.

Raul is doing some interesting work with the FAA that may cause some changes.

According to slide 19 and in interpreting it as a depiction of an approach to a landing, it is unclear what happens to the autorotation inflow, u, or the rotational relative wind as the aircraft rounds out and then flies level or quasi-so, low to the ground. Therefore, it is unclear what happens to the resultant relative wind, since the latter is the vector sum of u, the rotational relative wind and the forward airspeed, as shown on the slide. This also means that the change in the angle of attack, alpha, is also unclear.

Moreover, the cross sectional area of the intake air column is depicted as getting smaller and smaller as the aircraft rounds out and then proceeds forward, although that is not explained either.

In general, "momentum theory" and other assumptions used to model air flow, reaction forces, etc. (hence the indications on the slide of half the rotor diameter and twice the rotor diameter for the intake air column's diameter) are too simplistic to describe these situations, both for gyroplanes as well as for helicopters. Instead, computational fluid dynamics is necessary, which is just a fancy name for finite element analysis a.k.a. the method of moments, as applied to air.

We all know that a gyroplane requires substantially less power to fly level, close to the ground, and will float substantially farther after a lower round-out, and so on. The question though is, "why?"

For starters, any expansion of the rotor wake toward the ground while the gyroplane is in the float (just after the round-out) will be impeded by the ground, while expansion of the rotor wake upward and sideways will not be impeded by the ground. So in this sense, the ground certainly makes a difference -- and all the more so when considered as a boundary condition to the differential equations that would describe the motion of the air that is compressed somewhat as the rotor encounters it from above.

 

[Jean Claude]

Because of its proximity to the ground, airflow around the rotor is hindered, allowing the disc to work at a lower angle of attack. The result is reduced rotor drag.
There's no mystery about it, and it's quantifiable.
At the near-landing speed of our gyroplanes, say 50 km/h, the rotor drag when the wheels are close to the ground is about 85% of the drag when the ground is far away. (Using Nasa technical note D-234 http://babel.hathitrust.org/cgi/pt?id=uiug.30112106589309;view=1up )

 

[eidschun]

Because of its proximity to the ground, airflow around the rotor is hindered, allowing the disc to work at a lower angle of attack. The result is reduced rotor drag.
There's no mystery about it, and it's quantifiable.
At the near-landing speed of our gyroplanes, say 50 km/h, the rotor drag when the wheels are close to the ground is about 85% of the drag when the ground is far away. (Using Nasa technical note D-234 http://babel.hathitrust.org/cgi/pt?id=uiug.30112106589309;view=1up )

Oui, mais il me semble que ce traité concerne un rotor entraîné. D'ailleurs, il s'agit des solutions analytiques et non pas des solutions numériques comme celle de la CFD.

 

[Jean Claude]

This technical note applies to all " lifting rotors ", provided you take into account the angle of the wake in relation to the axis.
A gyrocopter in forward flight is characterized by a wake angle greater than 90° (say 100°), whereas the wake angle of a powered rotor is less than 90° (say 80° during forward flight).

It shows how to take angles greater than 90° into account

 

[eidschun]

This technical note applies to all " lifting rotors ", provided you take into account the angle of the wake in relation to the axis.
A gyrocopter in forward flight is characterized by a wake angle greater than 90° (say 100°), whereas the wake angle of a powered rotor is less than 90° (say 80° during forward flight).
View attachment 1160797
It shows how to take angles greater than 90° into account

While "For all conditions, ground effect is favorable; that is, the interference velocity is an upwash which will tend to reduce the induced power required by the rotor" in that technical note could be interpreted to mean that the rotor was auto-rotating in the wind tunnel, you have to look at the derivations in order to know the constraints associated with the modeling, i.e. modeling that may not correspond to, for example, the auto-rotative state after all. (Indeed, "hovering" is mentioned many times, "helicopter" a number of times and in the titles of the references too, etc.) However, that technical note contains few derivations and instead references another technical note that doesn't seem to be readily available. Anyway, as I alluded to previously, analytical solutions for these situations, such as those presented in that technical note, are often largely inaccurate. Only CFD would give reliable solutions.

 

[Jean Claude]

Yes, but what's the point of accurately calculating the ground effect? This effect is small because it only concerns the induced power, which is less than one third compared to the friction power of the blades plus the parasitic power of the airframe.
A very simple solution for calculating the ground effect of a gyrocopter, whose wake is always almost parallel to the ground, is to replace the ground by an identical, mirrored rotor. So, his marginal circulations cancel out the vertical components at ground level of the first, as would the true ground.
So all we just need to do is quantify the difference in induced velocity at the center of the real rotor with and without the image rotor, knowing that circulation gives a tangential velocity inversely proportional to tips distance
For example, if the real rotor flies at 0.6 R above the ground, then the edge of the mirrored rotor is 1.56 R from the center of the first, and the circulation it adds is inclined at 50° to the vertical. This gives a vertical induced speed 0.6 times that of the mirrorless rotor.

 

[eidschun]

Yes, but what's the point of accurately calculating the ground effect? This effect is small because it only concerns the induced power, which is less than one third compared to the friction power of the blades plus the parasitic power of the airframe.
A very simple solution for calculating the ground effect of a gyrocopter, whose wake is always almost parallel to the ground, is to replace the ground by an identical, mirrored rotor. So, his marginal circulations cancel out the vertical components at ground level of the first, as would the true ground.
So all we just need to do is quantify the difference in induced velocity at the center of the real rotor with and without the image rotor, knowing that circulation gives a tangential velocity inversely proportional to tips distance
For example, if the real rotor flies at R above the ground, then the edge of the mirrored rotor is 1.56 R from the center of the first, and the circulation it adds is inclined at 50° to the vertical. This gives a vertical induced speed 0.6 times that of the mirrorless rotor.
View attachment 1160799

Given that the ground effect is small, what's the point of an analytical solution that has an error of something like +/- 100% and that as such, isn't accurate enough to show that there even is a ground effect?

 

[Jean Claude]

What makes you believe that analytical solutions have 100% uncertainty?
Prandtl's lifting-line approach (#278) gives an induced velocity ratio in ground effect of 0.60. While with the same conditions, the NASA D-234 approach gives 0.56
You can see also Nasa TM- X 71951 where it says in conclusion 5: "The angle of attack of the rotor has a strong influence on ground effect in forward flight; tilting forward reduces ground effect and tilting backwards increases it"