Gyroplane Thrustlines vs. Center of Gravity

rtfm

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Hi,
My understanding is that if one powers the rotor to (say) 95% of the usual flying RRPM, then there is very little left for the airflow to do, and in fact, very little airflow is required to add the extra 5%. The rotor is still unpowered since it would run on an over-run clutch. Since so little additional assistance is required from the airflow, the rotor flying angle is almost (but not quite) horisontal, reducing drag considerably.

Another benefit is that there is now no danger of blade flap should one lose RRPM, because as soon as the RRPM drops, the sprag clutch engages, and maintains the RRPM at a healthy level. This application of rotor power will inevitably cause some yaw, but that's what the rudder is for. And besides, it is a whole lot better than the alternative of chopping off one's tail feathers.

Well, that's my understanding from reading about Dick De Graw's experiments, anyway. The trick is to tap enough power from the motor in an elegant fashion. Again, according to De Graw, a motor/rotor split of about 7:1 does the trick nicely.

But this still begs the question: if the rotor is flying at nearly horisontal (maybe 2 deg), this will significantly change the rotor thrust vector, and I am concerned about how to calculate this without having to take the gyro for a fly to find out... Vance had some rule-of-thumb numbers, which is a great place to start. I doubt that Hollerman's book will be of any help, since I don't think he discusses partially powered rotors. My big question is: if I get the positioning of the RTV wrong by a bit, will this result in a fun-ending day for me and my family

Regards,
Duncan
 

Vance

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Hello Duncan,

Your idea of a partially powered rotor is divergent from mine.

What you describe sounds more like a powerful pre-rotator.

In my concept of a partially powered rotor the rotor will fly at close to the same rotor rpm and thrust will still be provided by the propeller.

In normal gyroplane operations the air provides the power for the rotor and air is not a very efficient drive.

By going to a partially powered rotor the mechanical drive is more efficient creating less drag and allowing the same air speed using less horsepower or a slightly higher top speed.

In my opinion the disk does fly flatter for a given speed and that does move the rotor thrust vector.

My feeling is that the difference in disk angle is not enough to make a significant difference in the location of the rotor head because the disk in not that far back at maximum speed in all of the gyroplanes I have flown.

Most gyroplanes I have flown have aerodynamics that I feel would tend to become problematic if she flew too much nose down.
 

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Duncan,
You mention '7:1' ratio and 'sprag clutch', but then mention that your concept maintains a minimum RRPM.
That seems to imply that your partial power is not always powering the rotor, rather than always providing some % of power.
Full disclosure: I'm a fan of 'partial power' experiments.
I think that I'm with Vance on this one; it would seem that your partial power plan will not affect the alignment too much.
For sure, that is what the test period is for.
Perhaps design for easy adjustment based on flight test data.
I would think you would fly it in autorotation first and make sure that it flys fine that way since (if the partial power fails) you want the machine to still fly when the rotor is unpowered, too.
Ease into the partial power and see how it affects balance.
A pure mechanical set-up may limit your ability to 'ease' into that territory.
I'd be concerned about wear on the sprag clutch if your plan is for it to slip the majority of the time.
Somewhere I read about someone who planned to be able to move the mast fore/aft to trim the machine's balance.
Brian
 

Jean - Claude

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Ducan, We can try quantify with this assumptions:
One seat, 600lbs in flight, rotor 24’ x 8.5”, pitch setting 3 degrees, cruise 55 mph, 353 rpm, AoA disc 10.5 degrees.

I find the rotor drag is 112 lbs, and the parasitic drag is 58 lbs
And total drag of 170 lbs requires 25 hp from the propeller, ie 35 hp from the engine.

Now, if you want to halve the rotor drag, then you must reduce disc A.o.A by half . ie 5.25 degrees.
In this conditions, I find you need 413 rpm for the same lift at the same forward speed, and 12.7 hp is required on the rotor shaft.
So, the total drag is no longer 114 lbs wich requires 16.7 hp from the propeller, ie 24 hp on his shaft . Added to the 12.7 hp for the rotor, it is 36.7 hp despite a supposedly perfect mechanics.

Why? This due to the pitch setting of autorotation is no longer adapted with a partially powered rotor.

Also, if positioning of the RTV wrong by a bit, the result is an airframe line wrong in the relative wind, and an other stick position is required. So caution his the stops.
 

rtfm

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Hi Vance,
I'm not sure I follow. I thought that a "partially powered rotor" meant powering the rotor up to a point, and then allowing the airstream to complete the task. Is my understanding incorrect? I refer to the initial post in this thread by C. Beatty.

Regards,
Duncan

Vance;n1120656 said:
Hello Duncan,

Your idea of a partially powered rotor is divergent from mine.

What you describe sounds more like a powerful pre-rotator.
 

rtfm

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Smack;n1120673 said:
Somewhere I read about someone who planned to be able to move the mast fore/aft to trim the machine's balance.
Brian
Hi,
I have been working on just such a system. Basically, the mast is anchored to the keel by a second "teeter bolt", so that the mast can pivot fore/aft. All that is then required is a lever to adjust the angle.

Perhaps either you or Vance could explain what you understand by "partially powered"? It seems my understanding if flawed.

(On a side-note) I've realised why most gyros are pushers. It's mostly because (I think) designing a side-by-side tractor gyro requires black arts. The pilot/passenger are just too far from the CG. And even if you get the craft balanced for a single occupant, adding an extra 200lbs on a quite large moment arm is going to throw it off whack considerably. Hence the large number of tandem gyros, with the passenger seated on the CG. Gyros which do have side-by-side seating always have the engine in the rear, with the occupants as close to the engine as possible. In this arrangement, the mast now conveniently sits between the engine and the cabin, and any change to cabin occupants affects the CG far less.

However, I only have eyes for tractors, so I think Tervamaki has the right idea. A tractor with a rear mounted engine. Sure, one has to contend with torsional vibration in the shaft which passes between the pilot and the passenger, but we've known about this issue for decades now, and have solved that problem in a number of ways. The underlying structure would still be the tried and trusted aluminum tubes - just like a pusher. In fact, what I'm suggesting is a pusher with a shaft to the nose for the prop. And a pretty fuselage to go with it.

Something like the third drawing (nowhere near as good as Tervamaki's...), with stub wings for the fuel (and possibly about 100lbs of lift at cruise).

So - back on topic - how do you see partially powered rotors working?

Regards,
Duncan
 

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Vance

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rtfm;n1120682 said:
Hi Vance,
I'm not sure I follow. I thought that a "partially powered rotor" meant powering the rotor up to a point, and then allowing the airstream to complete the task. Is my understanding incorrect? I refer to the initial post in this thread by C. Beatty.

Regards,
Duncan
In my opinion for a patricianly powered rotor to have value some percentage of power needs to be applied to the rotor in flight allowing a shallower angle for the disk requiring less horsepower to go a given speed.

In my opinion having the rotor outrun the drive negates the value.

Air is not very efficient way to drive the rotor and doing it mechanically has the potential to do it more efficiently.

The Predator will fly level with a rotor rpm anywhere between 315 and 400 depending on conditions and load.

The only time The Predator’s rotor is powered is during pre-rotation.
 

All_In

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I'm not sure but I think Vance is agreeing with you.
I encourage you to keep on keep'n on!! Cannot wait to hear about a test flight. I'm not an expert but I feel it should be able to obtain a faster cursing speed once you get the ratios figured out.
 

Jean - Claude

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Ducan, As I explained,
1) without device of blades pitch setting in flight, the rotor partially powered will gives no fuel economy.
2) powered or no, the drag rotor is Total weight * Tan(disc A.o.A ).
So, in my example I reduced the rotor drag by halve, due to partially powered. This reduced of 5 degrees the rotor thrust angle.
It is non neglectible on the pitch balance.
3) If the mast is settible in flight, then ask yourself what effort it requires.
 

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rtfm

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Hi,
I am not clear in my mind by what you mean by "partially powered". How do you achieve this? How much power do you give the rotor? What RRPM is possible with no help from the wind at all?
Duncan
 

Jean - Claude

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Duncan, As I said in my post #584, the engine add a torque ie 12.7 Hp on the described rotor shaft. Wich reduces the rotor drag of halve. This torque would give about 335 rpm with no wind but during the cruise the rrpm will be 413 rpm instead 353 rpm without torque added
 
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Vance

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A friend of mine (Dick) has found a way to send a percentage (18% if I remember correctly) of the engine power directly to the rotor in flight and in his opinion the gyroplane uses less power for a given cruise speed.

It looks a little like a differential from an automobile and splits power according to demand.

This is what I would refer to as a partially powered rotor.

In my opinion the rotor on a gyroplane operates at some specific rotor rpm in unaccelerated flight to maintain one g and running engine power directly to the rotor doesn't change the rotor rpm required to maintain one g significantly. I feel a partially powered rotor does allow the disc to be operated at less angle for a specific speed and creates less drag. In my opinion this may save as much as 10% on fuel consumption particularly at minimum power required speed.

I suspect it would be possible with enough power to spin a gyroplane rotor to destruction without any assistance from the wind.
 
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Jean - Claude

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More precisely with the assumptions post #584:
With free rotor I find the rotor drag is 112 lbs, and the parasitic drag is 58 lbs
And total drag of 170 lbs requires 25 hp from the propeller, ie 35 hp from the engine.

Now, if you want to halve the rotor drag, then you must reduce disc A.o.A by half . ie 5.25 degrees.
In this conditions, I find you need 413 rpm for the same lift at the same forward speed, and 12.7 hp is required on the rotor shaft.
So, the total drag is no longer 114 lbs wich requires 16.7 hp from the propeller, ie 22.6 hp on his shaft (efficiency is a bit better with less thrust) . Added to the 12.7 hp for the rotor, it is 35.3 hp despite a supposedly perfect mechanics.

with powered rotor 18%, ie total drag only 149.6 N or rotor drag =149.6 - 58 = 91.6 lbs, this requires disc A.o.A = 8.59 degrees.
So, for the same weight at the same forward speed, I find 372 rrpm and a required torque of 82 Nm on the rotor shaft, ie 4.28 hp
On the other hand
149.6 lbs requires 22 hp from the prop. or 30.8 hp from the engine. Total power gives 4.28 hp + 30.8 = 35.1 hp
What else , except a placebo effect?

Now, keep the same rrpm as free rotor by a extra blades pitch I find 2.9 hp on the rotor shaft, ie a total 33.7 hp and therefore -4% on fuel consommation.
 
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Jean - Claude

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Vance;n1120723 said:
In my opinion the rotor on a gyroplane operates at some specific rotor rpm in unaccelerated flight to maintain one g and running engine power directly to the rotor doesn't change the rotor rpm required to maintain one g significantly
The tilt back of the disc at the same rpm increases the A.o.A of the blades and consequently increases their lift
So, less tilt back requires more rpm for the same lift.
 

Vance

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Originally posted by Vance View Post
In my opinion the rotor on a gyroplane operates at some specific rotor rpm in unaccelerated flight to maintain one g and running engine power directly to the rotor doesn't change the rotor rpm required to maintain one g significantly

Jean - Claude;n1120747 said:
The tilt back of the disc at the same rpm increases the A.o.A of the blades and consequently increases their lift
So, less tilt back requires more rpm for the same lift.
Thank you for your participation in this thread Jean-Claude.

I have expressed my opinions based on my observations Jean-Claude and have not done extensive testing.I have not flown a gyroplane with a partially powered rotor.

The Predator weighs around 1,100 pounds solo with ten gallons of fuel on board.She has thirty foot eight and a half inch chord Sport Copter blades.

It is my observation flying The Predator that the rotor rpm goes from around 320 rrpm at 30kts indicated air speed to around 360 rrpm at 90kts indicated air speed.It has been a long time since I checked it so my numbers may be a little off.

I have not tried to measure the rotor disk angle in flight.A rough estimate of the rotor head angle would be around 20 degrees at 30kts and around 5 degrees at 90kts. She is quite nose high at 30kts indicated air speed and a little nose low at 90kts indicated air speed.

I don’t know if this observed performance fits your calculations.Thirteen percent rotor rpm change fits my use of the phrase “not significantly” for twenty five percent of the rotor head angle Jean-Claude.

I feel comfortable postulating based on observed performance that the rotor system has a third of the drag at 90kts as it does at 30kts.

In my opinion the wind is not a very efficient way to drive the rotor and finding a more efficient way would lead to lower power required to fly at a given air speed.

A partially powered rotor is sufficiently complex to be something I would not try.

My friend Dick feels a partially powered rotor has value.
 
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Jean - Claude

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Vance;n1120757 said:
]I don’t know if this observed performance fits your calculations.Thirteen percent rotor rpm change fits my use of the phrase “not significantly” for twenty five percent of the rotor head angle Jean-Claude
I even suspect that your rrpm would change much less if your blades were rigorously rigid in torsion.
It appears from your numbers that the pitch of the blades to tip decreases almost 1 degree between 30 and 90 kts
However, the correlation you are making here is irrelevant for know the rpm increase due to the add of torque on the shaft, in forward flight. And you can be sure that adding torque on the shaft increases the rrpm, when the lift is not changed. Unfortunately the benefit disappears, because the losses by blade friction in air incresases as rrpm^3,
improve appears only if you increase the pitch setting of blade for keep the same rrpm despite the torque added .
Note that it may be sometimes by the elastic torsion of the blades without the designer is aware of this.

My theorical results fits with the best documented measurements I read: Naca report 475 and 515
So, rpm, drag, longitudinal and lateral flapping, calculated from only blades shape, local pitch setting at rest, and elastic twisting, gives at 45 km/h
141.4 rpm instead 141.1rpm measured
27.1° disc A.o.A instead 26° measured
1.3° longitudinal flapping instead 1.1° measured
2.6° lateral flapping instead 2.6° measured
 
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C. Beaty

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Gyroplane rotor RPM stays nearly constant with partial power.

As mechanical drive is applied to the rotor, its disc angle of attack decreases so as keep total rotor power constant. A reduced rotor disc angle of attack extracts less power from the slipstream. Dick DeGraw’'s gyros fly with nearly a flat rotor disc in the upper speed ranges.

That said, partial power to the rotor is not simple. It must be via a soft coupling to the engine; a hard coupling would render a gyro nearly unflyable as a result of the throttle/yaw coupling.
 

Jean - Claude

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C. Beaty;n1120817 said:
Gyroplane rotor RPM stays nearly constant with partial power.
Certainly. But whatever the partially power chosen, this little difference of rpm produced by the torque mecanically applyed on the rotor shaft absorbs all this extra-power.
Improving is true only if rpm keep unchanged, by correcting on the blades pitch setting. As probably makes by Dick DeGraw's gyros.
 
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Good post!!!
Thank you all for sharing
 
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