Prop wash as prerotator?

[Jazzenjohn]

I have a short tail, but the tail isn't the issue. The issue is the motor turning the blades imparts a twisting force on the gyro itself.
I would not use the 10s because i use 12s and there are many charging options for (2) 6s batteries in series but few for 10s batteries.
You might be able to figure out onboard charging, but I wasn't able to. I don't find it much of a problem because I only charge about once every 2 weeks or so. I use less power than JC suggests, more like 600 to 800 Milliamp hours.
My motor is smaller and lighter and less powerful than the Rotomax 100cc brushless you posted.
The motor i use is closer to the axi 80 motor you mentioned earlier, but it was discontinued years ago. I bought a few of them in case I ever burned one up. Never did, I've only lost one from dropping it and bending the shaft.

 

[Aerofoam]

I have a short tail, but the tail isn't the issue. The issue is the motor turning the blades imparts a twisting force on the gyro itself.
I would not use the 10s because i use 12s and there are many charging options for (2) 6s batteries in series but few for 10s batteries.
You might be able to figure out onboard charging, but I wasn't able to. I don't find it much of a problem because I only charge about once every 2 weeks or so. I use less power than JC suggests, more like 600 to 800 Milliamp hours.
My motor is smaller and lighter and less powerful than the Rotomax 100cc brushless you posted.
The motor i use is closer to the axi 80 motor you mentioned earlier, but it was discontinued years ago. I bought a few of them in case I ever burned one up. Never did, I've only lost one from dropping it and bending the shaft.

Understood the torque issue, but was wondering if P factor added, or took away from it.
What is the motor you are using?
Most motors are easy to replace shafts, so your's may not be dead yet....

 

[Jazzenjohn]

I appreciate the repair idea, but i have enough spares for the forseeable future. I use a multistar drone motor discontinued years ago. The ideal motors for prerotators are on the fringe on usefulness in other areas, so the motors i have picked in the past have all been cancelled. There are quite a few outrunner pancake drone motors available now compared to 12 years ago when i started working on this stuff. I don't want to recommend a motor that i have never tested and don't actually know anything about.

 

[Jean Claude]

In the case like Johns even at only around 170rrpm, it is perfect for what can be a very short take off with correct technique.
A pre rotater separate from the main engine is a great way to go. Pre spin close to the max and leave spinner engaged all through the take off roll will accelerate the rotor very fast. With that relatively low power spinner you could also leave that on until airborne. ( Not as a beginner though).

It is often thought that a launcher keeping operated during the run can shorten the take-off distance. But this is based on the implicit assumption that the launch torque is maintained up to take-off rpm (in this case 300 rpm instead of previous 230 rpm . The power delivered by the launcher at maximum current (and therefore maximum torque) is therefore increased by 30%, distorting the comparison.
While at identical maximum power, the pre-launch Rrpm would inevitably be reduced by 15%, due to the 30% reduction in torque. Therefore 195 rpm instead of the previous 230. In the end, this results in only a slight improvement in the take-off run, despite the increase in rpm acceleration.

 

[wolfy]

It is often thought that a launcher keeping operated during the run can shorten the take-off distance. But this is based on the implicit assumption that the launch torque is maintained up to take-off rpm (in this case 300 rpm instead of previous 230 rpm . The power delivered by the launcher at maximum current (and therefore maximum torque) is therefore increased by 30%, distorting the comparison.
While at identical maximum power, the pre-launch Rrpm would inevitably be reduced by 15%, due to the 30% reduction in torque. Therefore 195 rpm instead of the previous 230. In the end, this results in only a slight improvement in the take-off run, despite the increase in rpm acceleration.

I think it means one can accelerate the gyro much faster and feed more air into the rotor quicker, to get a bigger gain than the numbers suggest.

 

[Jean Claude]

Yes Wolfy, it's possible when the gear ratio wasn't optimized for launcher maximum power reached at standstill.
It is also true be that the gear ratio optimized for the stanstill, and then maintained only until autorotation, is more efficient. The yaw effect remains, however.
I can simulate this solution and will give the result as soon as possible.
(In all these cases, my hypothesis is always the standard run with the stick at the rear stop)

 

[Jazzenjohn]

I would be interested to see the different electric prerotators you have built JC.

 

[Jean Claude]

As I said before, it is just a theoretical analyses.

Here are the promised results established for a 7.3m x 0.18 m rotor and a pitch setting of 3 degrees. inertia 85 kg.m2 Loaded with 200 kg . Static propeller thrust of 1200 N

Fig 1: With prerotator of 2500 W at 230 Rrpm, released just before the run full throttle (Gear ratio adapté to best launching Rrpm)
Fig 2: With prerotator of 2500 W at 230 Rrpm , kept engaged during the run to maintain this rrpm until autorotation takes the load off it completely
Fig 3: With pre-rotator of 2500 W kept engaged until take-off holding its maximum torque (The gear ratio adapted to this high rrpm allows only 200 rpm at standstill).

It shows:
- no difference in distance between 2 and 1
- a moderate shortening between 3 and 1

 

[Jazzenjohn]

Using your calculations, what would you expect the takeoff distance to be on my gyro? 23' Dragon wings, 3000 watt prerotator. 560 pounds all up takeoff weight. anything else you need for the calculation?

 

[Jean Claude]

The most influential data on take-off rolling distance are
- Total weight
- Diameter
- Rotor inertia (or blade mass)
- Chord
- Aerodynamic blade pitch setting or in-flight Rrpm at indicated weight and sea level
- Engine power
- Initial propeller thrust
- Angle of attack of the hub when the control stick is on the rear stop.
- Prerotation speed before full throttle
- Rolling drag at the indicated weight
- Headwind speed (this goes without saying, but it's better said)

 

[Jazzenjohn]

DW's are about 40 pounds, pitch is twist (est 6 degrees). Engine thrust is ~310 pounds engine power is about 50 HP. AOA of rotorhead is back 18 degrees. RRPN is about 185 or 190 but adds to ~240 or so. Drag is small, concrete runway. no headwind.

 

[Jean Claude]

Chord ? rpm in steady flight ?

If I've understood correctly, your pre-rotation is 185-190 rpm, and then kept engaged until 240 rpm.

Is the current maintained constant (max) from 0 to 240 rpm (i,e. pmax at 240) or is the current decreasing beyond 190 (i.e. pmax at 190)?

 

[F5 Tornado]

I recently saw a photo of a chrome moly gyro with a Rotax 503. It was around 210 pounds, well under part 103 range. It would still be under with a 582.
View attachment 1162031

I wish I could find more about the diameter and wall thickness of the chrome moly tubing required.

Looks like 1.25" or 1.5" and I'd guess maybe .090" wall. If he welded any of it, I hope he used a TIG and not a MIG.

 

[Jazzenjohn]

Chord is 7" inflight RRPM is around 330 inflight.

 

[Jean Claude]

My results: Distance 78 m with 3000 W launcher and prerotation to 185 Rrpm then full throttle , in your mass and prop. thrust conditions

 

[Jazzenjohn]

I believe my takeoff distance is closer to 45 meters under the stated conditions. I also think my speed is closer to 50 KPH than 65 at lift off. some of the figures i've given are a bit questionable like back angle, 18 degrees to the stop, but what about rock back? I sometimes rock back from the prerotator and often take off from the rear wheel at a fairly high angle of attack such that i must push the nose down after takeoff. This may partly be because my engine isn't very powerful.

 

[Jean Claude]

My result is based on your indication of a hub inclined backwards by 18 degrees. If this angle differs due to the rear wheel being brought into contact with the ground, then this obviously changes the data.
Don't forget, too, that a light wind of, say, 5 km/h, i.e. 1.4 m/s, already shortens the stroke by 1.4 m/s x 7 s = 10 m.

 

[Jazzenjohn]

Aerofoam posted: You can run multiple motors on the same speed controller, so using 2- 2700w motors gets you into the 6hp+ range. but now you have a weight penalty.


What would the output of me using 2 motors either driving the same chain or 2 separate chains as compared to using only 1 motor of double size? Would there be a large efficiency loss? It would be comparatively easy to add a second motor than to try and find a bigger suitable motor I think.... I can easily find a bigger speed controller.

 

[Jazzenjohn]

As far as wind aiding, It might be better to generally assume a 5 knot wind as absolute zero wind is less common, at least near me.

 

[Jerry_Forest]

Prop wash as prerotator?​

Back to the topic. I used to use the propwash from the 5 or 6 FW's lined up at the bottom end of the runway at the Brampton A/P. Worked best on a no-wind day.
Jerry.