Prop wash as prerotator?

[MikeBoyette]

All good points being disused here, I thought my Mac helped spin the blades a little more than my 582, but could never prove it.
I am not sure what blades are in the video, but they seem to be spinning the other direction than any blade I have seen, or is it just an optical illusion?

Dave,
I would say you are correct. The Mac has much more low end torque. Just from being larger displacement. I know Gary’s Super Mac would pull those white SkyWheels up fast. He could blast off in less than 20’ even with no wind.

 

[Jean Claude]

I get the concept of running into the aero/drag wall prematurely if the gear ratio is too high, but the graph is throwing me off because it doesn't seem to line up to what you are explaining.

I think you've not understood my graphics.: The horizontal scale is the time (graduated in seconds) from “Full throttle”.
In this way, you can follow the evolution at any moment of the rpm, forward speed and length travelled that my calculations have found.
For example, on these curves corresponding to 8 ft/rev you can see that after 9 seconds the rpm would be 380 rpm, the speed would be 55 km/h, and the distance covered would be 90 metres.

My AC only gets about 125Rrpm from the Wunderlich prerotator that weighs over 30 lbs.

30 lbs for 125 rpm? I understand you're looking for another device

 

[Aerofoam]

I think you've not understood my graphics.: The horizontal scale is the time (graduated in seconds) from “Full throttle”.
In this way, you can follow the evolution at any moment of the rpm, forward speed and length travelled that my calculations have found.
For example, on these curves corresponding to 8 ft/rev you can see that after 9 seconds the rpm would be 380 rpm, the speed would be 55 km/h, and the distance covered would be 90 metres.
View attachment 1162074

30 lbs for 125 rpm? I understand you're looking for another device

Yeah, you should have seen the 10lb. trim spring assembly I removed!!!!
Ok, that makes sense now. (The graph.)
So, 380 Rrpm amd 55k/hr is not needed, I think it would be great to get over 200Rrpm in 60ft. with a speed of at least 20k/hr.
That combination would make a healthy launch (Better than a Wunderlich) and you could carry extra line and a stake with very little weight penalty.....

I do think an electric prerotator could weigh less than 6lbs. so it is probably a better way to go. I still want to try this though....

 

[rcflier]

Starbee used to sell some kind of hand driven prerotator...

 

[MikeBoyette]

Yes they did. I think I saw an example of it one time. I didn’t see it used though. Was it good enough to use on the DW’s they provided with their kit?

 

[Jean Claude]

I think it would be great to get over 200Rrpm in 60ft. with a speed of at least 20k/hr.

200 rpm wouldn't be enough, because it requires you to manage the throttle after the end of the rope to avoid rotor flapping.
250 rpm with 6 ft/rev. would keep the full throttle.
92 ft of rope gives 250 rrpm and 17 mph with 250 lbs prop. thrust
So, Take-off would can occur in about 350 ft with no wind
Good luck!

 

[dabkb2]

Dave,
I would say you are correct. The Mac has much more low end torque. Just from being larger displacement. I know Gary’s Super Mac would pull those white SkyWheels up fast. He could blast off in less than 20’ even with no wind.

I never got that much out of the prop blast, but it did help on a no wind day.

 

[Aerofoam]

Starbee used to sell some kind of hand driven prerotator...

"Jim" I think... at KBFFI had one of these on a gorgeous Bensen type he built.
he pulled it repeatedly (Downward from the rotor head) like starting a lawn mower to get about 60 Rrpms which was enough to get over the "Hump" and continue bringing them up to speed.

 

[EI-GYRO]

"Jim" I think... at KBFFI had one of these on a gorgeous Bensen type he built.
he pulled it repeatedly (Downward from the rotor head) like starting a lawn mower to get about 60 Rrpms which was enough to get over the "Hump" and continue bringing them up to speed.

I tried a bicycle pedal-operated arrangement on my gyroglider. Got it to 60 rrpm on wood Bensen blades, but not really worth the effort.
Reverted to standard armstrong method.

 

[Aerofoam]

You can check that at 300 rpm the rotor absorbs about 3.5 kW and that the propeller pushing 250 lbs at 7 mph delivers a power of about 3.5 kW. So there's no extra power available to go faster.

Jean Claude, this is very encouraging, I was wondering if it would include mechanical losses in efficiency.
I was under the impression it would take about 7hp. to effectively bring Rrpm to flight speed, but your calculation seems to support about 5hp.
What do you think the minimum power should be for a high powered prerotator?
I already have a couple of hobby motors that are above 4hp, and possibly almost 6hp. One is a 4.5"Dia. outrunner and produces a lot of torque at lower RPM., it only weighs about 1lb.
This is no longer a big deal in the RC world, large brushless 3 phase DC motors and their speed controllers are readily available operating at up to 60vdc and over 150A, so the small powerful system is accessible.
Do you think more than 4hp is needed for a 24' rotor?

 

[Jean Claude]

I already have a couple of hobby motors that are above 4hp, and possibly almost 6hp. One is a 4.5"Dia. outrunner and produces a lot of torque at lower RPM., it only weighs about 1lb.
This is no longer a big deal in the RC world, large brushless 3 phase DC motors and their speed controllers are readily available operating at up to 60vdc and over 150A, so the small powerful system is accessible.
Do you think more than 4hp is needed for a 24' rotor?

My calculation of the power required for pre-rotation, in the international system of units, is
P = 0.005* ρ* chord* R^4* Ω^3
Taking ρ = 1.2 kg/m3, Chord = 0.18 m, R = 3.65 m (24' rotor) and Ω = 24 rd/s i,e 230 rpm (70% of the flight Rrpm with 24' rotor) then we find
P = 2650 W Say P = 3 kW i,e 4 hp for reach this rpm in in a reasonable time.
Increasing this power is useless because it does not shorten the take-off run

Can you tell me the part numbers of the brushless motor you are talking about? One lb seems very light.

 

[rcflier]

This is no longer a big deal in the RC world, large brushless 3 phase DC motors and their speed controllers are readily available operating at up to 60vdc and over 150A, so the small powerful system is accessible.

My 600 class R/C heli has a 4 HP brushless motor, so the power is there. But as I see it, the clutch is the problem for a prerotator.
You don't have that much in the way of torque, just a lot of RPM's. So we need a large diameter motor with a low RPM - and still a need for a clutch, so we don't bur out the motor right away. We have discussed this before. We need an elegant "neat" solution like a conax clutch.

 

[Aerofoam]

My 600 class R/C heli has a 4 HP brushless motor, so the power is there. But as I see it, the clutch is the problem for a prerotator.
You don't have that much in the way of torque, just a lot of RPM's. So we need a large diameter motor with a low RPM - and still a need for a clutch, so we don't bur out the motor right away. We have discussed this before. We need an elegant "neat" solution like a conax clutch.

The speed controller should eliminate the need for a clutch.

 

[rcflier]

The speed controller should eliminate the need for a clutch.

Ok - just try to get a heavy rotor with a lot of inertia running from 0 RPM - I guess the motor will burn out quickly.
It's just to get it started - then the clutch will lock.

 

[Aerofoam]

My calculation of the power required for pre-rotation, in the international system of units, is
P = 0.005* ρ* chord* R^4* Ω^3
Taking ρ = 1.2 kg/m3, Chord = 0.18 m, R = 3.65 m (24' rotor) and Ω = 24 rd/s i,e 230 rpm (70% of the flight Rrpm with 24' rotor) then we find
P = 2650 W Say P = 3 kW i,e 4 hp for reach this rpm in in a reasonable time.
Increasing this power is useless because it does not shorten the take-off run

Can you tell me the part numbers of the brushless motor you are talking about? One lb seems very light.

I have a Hacker B-75XL with 6.7 to 1 planetary gearing that is over 4hp and just over 1lb., but it appears they don't make it anymore.
They do make a B50 series, but that seems to peak at about 2700W and is very expensive.

A quick search turned up this outrunner at just under 4hp. and 1.58lbs. and less than $100 US.

Turnigy Aerodrive SK3 - 6364-245KV Brushless Outrunner Motor

Turnigy Aerodrive SK3 - 6364-245kv Brushless Outrunner Motor

hobbyking.com

This one is significantly more powerful (over 5hp.) but also just over 4lbs.

Turnigy CA80 160kv Brushless Outrunner (50~80cc Eq)

Turnigy CA80 160kv Brushless Outrunner (50~80cc Eq)

hobbyking.com

This one is a monster, but heavy..(.it seems the rotomax 1.4 is discontinued, it is very close to 4hp and less than 2lbs....)

Turnigy RotoMax 100cc Size Brushless Outrunner Motor 167kv

Turnigy RotoMax 100cc Size Brushless Outrunner Motor 167kv

hobbyking.com

I need to re-research motors, Axi motors are better than the Chinese stuff and they would custom wind for specific purposes, but I need to dig
in to what they currently have.
They would probably be my first choice....

 

[Jean Claude]

I suspected that the 1 lb weight for a 2800 watt electric motor was very optimistic.
The reason is that the value quoted by these sellers is the electrical power consumed, not the mechanical power produced, as any motor is supposed to do.
The motor's thermal losses must be deducted.
In addition, the thermal losses of the controller must be deducted.
Finally, these motors have a very high rpm, incompatible with the direct drive of an 11-tooth pinion on a 100-tooth or nearly 100-tooth ring gear.
For example, if Rpm /v = 245 then you'll need to add an axial gearbox with a ratio of around 1/4 or 1/5, and deduct the inevitable power losses.
And don't forget to add the weight of this axial gear, ring gear, the power cables, the contrôler and the LiPo battery.
My forecasts had led me to predict more than 10 lbs (included Bendix)

 

[Aerofoam]

I suspected that the 1 lb weight for a 2800 watt electric motor was very optimistic.
The reason is that the value quoted by these sellers is the electrical power consumed, not the mechanical power produced, as any motor is supposed to do.
The motor's thermal losses must be deducted.
In addition, the thermal losses of the controller must be deducted.
Finally, these motors have a very high rpm, incompatible with the direct drive of an 11-tooth pinion on a 100-tooth or nearly 100-tooth ring gear.
For example, if Rpm /v = 245 then you'll need to add an axial gearbox with a ratio of around 1/4 or 1/5, and deduct the inevitable power losses.
And don't forget to add the weight of this axial gear, ring gear, the power cables, the contrôler and the LiPo battery.
My forecasts had led me to predict more than 10 lbs (included Bendix)

I am thinking about a small profile cog belt. The bendix units I have seen are all cut off of automotive starters and are fairly heavy.
The cog pulleys can be AL. and fairly light. Some of the out runners are 105 to 160rpm/v and with the soft start of the speed controller they should be able to handle it. I have used a lot of brushless motors beyond there stated purpose and capacity. it usually boils down to heat management and most of them can handle an over amp situation for pre set amount of time. In our case this will not be very long because we don't have good air flow though.
I was also considering using the same motor as an alternator to recharge the Lipo in flight and thus needing a smaller lipo...
I think 10lbs is an easy goal no matter what you do, but I think it could be shaved down to 5, or under for a reasonable prerotator.

 

[Jean Claude]

Some of the out runners are 105 to 160rpm/v and with the soft start of the speed controller they should be able to handle it.

Torque is given by current, rpm is given by voltage. Power requires current AND voltage
When the rpm of the electric motor is not respected, the voltage delivered by the speed controller is automatically reduced and the power produced buy the motor is therefore also reduced.

The bendix units I have seen are all cut off of automotive starters and are fairly heavy.

My weight assessment has already taken into account the fact that the ring gear is half as light as that of a car , thanks to the absence of overtorque relatively to car starter.

I am thinking about a small profile cog belt. The cog pulleys can be AL. and fairly light.
I was also considering using the same motor as an alternator to recharge the Lipo in flight and thus needing a smaller lipo...

For safety reasons, the pinion must be released from the crown during flight. That's why I've considered a bendix system (without the solenoid) and not a belt.

So Yes, 10 lbs is possible, but not easy

 

[Aerofoam]

Torque is given by current, rpm is given by voltage. Power requires current AND voltage
When the rpm of the electric motor is not respected, the voltage delivered by the speed controller is automatically reduced and the power produced buy the motor is therefore also reduced.

The speed controllers will take care of the soft start which is usually the stumbling block to the high amp/inertial forces at start up.
They are not just regulating voltage, they are applying a PWM sort of frequency control. I have never had a starting problem using them for over rated loads. But, I have not done this type of application since 2018 and the products do seem to have changed a bit. I wouldn't think they would change their entire control system.... If anything they should be getting better because the large Acro guys are hovering airplanes that weigh over 30lbs. and applying ridiculous power for the 3D flying contests. our application will be gentle in comparison.
Another possible motor source is the bicycle conversions. They are using some impressive motors on the high end stuff.
It seems like a lot of these plateau at around 3000 to 3500w
The next jump in the UAV world is 8 to 15hp. which is probably over kill.

My weight assessment has already taken into account the fact that the ring gear is half as light as that of a car , thanks to the absence of overtorque relatively to car starter.

It's not the ring gear, they are fairly light weight. I have not seen a light weight bendix. Most of the cut off auto versions I have seen weigh more than the brushless motor. If you know of a light weight one, I would probably like to get it!!!

For safety reasons, the pinion must be released from the crown during flight. That's why I've considered a bendix system (without the solenoid) and not a belt.

So Yes, 10 lbs is possible, but not easy

I think the belt could be sized to be strong enough handle the application of power for prerotation, but in the event of a seizure, or rapid obstruction, it would just break off, or strip the teeth, then melt. I have also considered a fail safe in the tensioner, or using a non cog belt with a tensioner that can be released.
I really don't think those small motor belt combinations could over power a rotor that was already at flight speed and in 25 years of working with brushless DC motors I have never seen, or heard of one seizing up. they just fry the windings.
We used them as alternators for larger 2stk. motors and tested them to failure multiple times because heat dissipation was the limiting factor.
We ended up with a motor about the size of a single shot demitasse espresso cup that was providing up to 1200w on the bench without airflow.
In flight it could produce 2000w. it was a custom wind operating at higher than normal voltage..
I have seen bearings blow out on some of the really high Rpm types, but they were still able to spin too.
I am willing to experiment with this realm until I see evidence to the contrary....

 

[Jean Claude]

Why did you keep your 30 lb prerotator for so long?
Now, you just need to find a reduction of 7000 at 230 rpm and an output torque of 125 mN.
and then farewell to rope drums