Effect of prerotation on takeoff distance

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John I think the one Nick makes for the Aviomania is a great pre spinner but I think it has a blade length and wide limit. I like how it runs off the engine not the battery. I think something like that that could handle bigger blades would be great.

Sincerely SWilliams
Yes you right Aviomania's is for our and other light weight gyro's using light weight Dragon Wings.

However PRA does have a project that really Marc is doing by himself for himself and PRA. It's using the same state-of-the-art motors only more HP and different gearing. His version you can add a second motor in parallel for even more HP so it could give large blades almost jump take-off speeds. We will not know until Marc tests it.
Once finished PRA will buy the parts in volume and sell them at cost as another PRA member benefit.
 

Jean Claude

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Chris,
Here are my results matching your data.

 

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That sounds great John. Thank Mark and the PRA. You to.

Sincerely SWilliams
 

ckurz7000

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Chris,
Here are my results matching your data.

Hi Jean Claude,

thank you for running the numbers. If I interpret your results correctly, the red line is the rotor rpm, the blue aund yellow lines show forward speeds during the take off. I guess the blue line is a normal take off (with the stick full back) and the yellow line is the speed with an initially flat rotor disk. Right?

It appears that in your computer simulation the optimal time to take the stick back is after 2 seconds, when reaching 32 km/h. This seems too soon in my experience. Maybe some of the input numbers are not quite right.

I will try to conduct a series of tests aimed at determining the equilibrium rotor speed for a given forward speed with the stick fully back. I will also try to establish acceleration as a function of time with the stick forward as well as the stick back. This should give you some better handle on the behavior. Unfortunately I can only acquire data once per second. This is a little bit too coarse. Ideally I would like a 10 Hz GPS logger but I don't have one available...

Greetings, -- Chris.
 

Jean Claude

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Chris, My diagram is that gives me the shortest takeoff distance under prescribed conditions.
Yes, optimum is stick back when reaching 32 km/h.
Red is the rrpm,
Blue is the forward speed
Yellow is distance traveled
Takeoff at 8.6 s, rolling 110 meters

Note Rrpm accelerates. Therefore not stabilized relative to the forward speed at the same instant .
 
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ckurz7000

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Thanks for your clarification, Jean Claude.

I guess you have a rotor model programmed in your spreadsheet which allows you to model the takeoff behaviour. I have two suggestions to make, which may help in improving the accuracy of the simulation:

1) Would knowing the equilibrium rotor rpm at several forward speeds (with stick fully back) help you improve the accuracy of input prarameters to your simulation?

2) Would knowing the forward speed of the gyro as a function of time -- with the disk flat as well as with the stick fully back -- help you in your computer simulation?

In about 2 weeks I should be able to have a record of speed taken in 100 ms intervals.

Greetings, -- Chris.

P.S.: 64 km/h spead (blue line) is a bit too slow for the ArrowCopter. Vx is 85 km/h.
 

Jean Claude

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Chris,
Of course, the comparison with your measurements is always interesting for checking the entered datas. Thank you send them to me when you can.

64 km/h just is the result when the angle of the plate is 25 degrees backward during the rolling phase. The constant forward speed at this instant, means maximum prop. thrust dedicated to accelerate the rrpm, until lift. becomes sufficient. So, the rolling distance is the best, not the best distance for 50 ft.
If you want takeoff to 85 km/h, then we must reduce the plate angle, but then the rolling increases.
 

ckurz7000

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I can accelerate well past 90 km/h with the disk tilted all the way back. So some parameters must be off. That's why I think it would be good if I measured equilibrium rotor rpm for several speeds. And also the aceleration as a function of time as the gyro starts the takeoff roll with the disk tilted back.

-- Chris.
 

Jean Claude

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Yes, Chris. Possible if the stick tardily pulled. Or if the prelaunch has not reaches 240 rpm.
Anyway, your mesurements will allow to check the ground effect with the rrpm acceleration. It is a sensitive point here.
Meanwhile for to check the blade pitch, what is the rrrpm in level flight to 560 kg and 90 km/h Out Ground Effect ?
and for to check rotor inertia, what is weight of one blade? Ballast or no to your blade tip?
Thank you.
 
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Jean Claude

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I guess you have a rotor model programmed in your spreadsheet which allows you to model the takeoff behaviour. I have two suggestions to make, which may help in improving the accuracy of the simulation:

1) Would knowing the equilibrium rotor rpm at several forward speeds (with stick fully back) help you improve the accuracy of input prarameters to your simulation?

2) Would knowing the forward speed of the gyro as a function of time -- with the disk flat as well as with the stick fully back -- help you in your computer simulation?

In about 2 weeks I should be able to have a record of speed taken in 100 ms intervals.
I'm still interested in your rrpm and forward speed recordings during the rolling, for comparisons with my computer simulation, Chris.
Thanks in advance for posting.
 

eddie

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vance stated that when his rotor gets bite his forward speed is effected,when I get to

200 rotor rpm and go to full power forward speed is achieved long before takeoff rotor

rpm,in fact I have to adjust the rotor angle to increase rotor rpm in relationship to forward

speed achieved, this is helped by having my main gear moved aft 6",I can achieve a more

efficient angle of the rotor blades to increase rotor rpm in a shorter distance.

Forward speed is not effected by rotor rpm when you have 230 HP





Best regards,
 

Resasi

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With the thread title very much in mind I followed this thread when it first started with interest, and would like to offer a perspective from one, who for most of my gyro time has not had a prerotator, but has always been interested in decreasing TO distance. A prerotator obviously has huge influence hoever I have also found that there is a definite correlation between feeding in the angle of attack of the rotor disc and the increase of power during the take off roll in achieving minimal ground roll.

Obviously the TO ground roll distance varies with the headwind component experienced, but when experimenting on a still/windless day the ground roll distance is still definitely influenced by the simultaneous feeding in of both power to the engine and rate of back stick/angle of attack of the disc.

Doing it too fast is to incur the onset of flap, do it too slowly sees an increase of ground roll. An optimum rate on both throttle increase and backstick pressure achieves the shortest ground roll but as this is a rather imprecise variable it can only be achieved by practice.

One of the things I enjoyed about the Bensen was with no prerotator, minimal instrumentation, no rotor rpm guage it was all done by feel. Close to the ground, with minimal airframe to block vision and only peripheral vision on the rotor blades and runway to judge the ‘blur’and forward motion, you accelerated both the gyro forward and the blade RPM, lifted the nose... and then the gyro into the air less by numbers than by the feel of the machine and sight of the blades.

With a multitude of machines, weights, props, rotors, engine HP and prerotators...or not, there will be probably be a variety of techniques that people will use. But with a good appreciation of what our rotors are doing, and how it can go badly wrong very quickly, hopefully we can all keep it safe.
 

Resasi

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If anyone has had an awful lot of practice Ernie, with your business, it is you.
 

Jean Claude

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Looking at the theory more closely, we must consider two distinct phases during the take-off run:

The first is the anti-rotative phase, ie when the rrpm decreases due the disc crossed from the upper side to the lower side, despite the forward speed.
During this phase, if you keep the stick full forward then rrpm less decreases, and the rotor thrust less brakes the forward acceleration.

The second is the auto-rotative phase, ie when the rrpm increases due to the disc crossed from the lower side to the upper side, despite the induced speed, and the stick in full backward.
During this phase, the rrpm is better accelerating when the speed ratio μ is just below which the blades hits the flapping stops, say μ = 0.15 (1)

Thus, we should start the run with the stick full forward, and at μ = 0.15 put the disc full backward (2) and try to hold this values (probably the full throttle is required, but it is sometimes too).


For example: If you launched a 23 feets rotor to 250 rpm, then go full throttle and stick full forward. Then, at 30 mph airspeed (μ = 0.15) put the stick full backward and continue full throttle unless μ increases.

Other example: If you launched by hand to 60 rpm, then put the stick full backward at 7 mph (μ= 0.15). Due to the low rotor drag, ajust the throttle to not exceed this μ ie progressively 9 mph at 80 rpm, 11.5 mph at 100 rpm and so on.

(1) μ is the ratio Forward speed / tip blades speed. The selected value depends on the pitch setting of blades, and the disc angle.
(2) If prop thrust - parasitic drag < weight * Sin(disc angle), then it will be necessary to reduce A.o.A until equality.
 

Vance

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My opinion of beginning the takeoff roll with the disk flat.

My opinion of beginning the takeoff roll with the disk flat.

In my opinion one of the worst things to do in The Predator is to accelerate with the cyclic forward.

Without the load of the rotor she accelerates very quickly. She has a weak pre-rotator and it is easy to flap the blades with too much airspeed and not enough rotor rpm.

Chris has a powerful pre-rotator on his aircraft and is a very experienced and skilled pilot.

I recommend not getting yourself in a position where you need to clear a 50 foot obstacle with a short take off roll. I feel there are too many things that can go wrong that will adversely affect the flight.

I recommend following the Pilot’s Operating Handbook takeoff procedure.
 

ckurz7000

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I agree with Vance. Don't try to adopt a take-off procedure where you apply full throttle with the rotor disk flat. It can get you into troubles very quickly.

I did this with a fully instrumented gyro that takes readings of all variables at least once a second. And I made darn well sure to keep a 10 rrpm margin from flapping the blades. And I approached this threshold very carefully over the course of many data runs.

I am a pilot with a low fear threshold, as Vance would say. It is precisely this what has so far kept me out of harm's way.

-- Chris.
 

Jean Claude

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Without the load of the rotor she accelerates very quickly. She has a weak pre-rotator and it is easy to flap the blades with too much airspeed and not enough rotor rpm.
This is precisely what I said, Vance: Shorten the distance by more acceleration in the beginning, and monitor μ to avoid too much airspeed and not enough rotor rpm.
I said that my remarks are theoretical. A difficulty is introduced due to the landing gear of the Bensen clones, which tilts the gyro inadvertently during the run. Also, the fear threshold is not entered in my simulation. The practice would require appropriate instrumentation.
However the principle is not new. This was the procedure recommended for Cierva C30, as you can read in this A.R.C report 1859

 
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XXavier

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Three seconds... according to 'The Autogiro and how to fly it'.

 

Jean Claude

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Yes, Xavier.
3.4 seconds is the required duration for mu = 0.15 after launch at 200 rpm with the full forward stick for a Magni M16 of 500 kg. And during this time the rpm decreased to 175 rpm.
When the shaft is now put to 23 degrees, then still 6 more seconds before leaving the ground, ie 120 meter of run.
This simulation take in account of the increasing parasitic drag , and the decreasing of the roll drag as the lift increases.
 
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