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Power Systems Talk about motors, ESC speed controllers, gear drives, propellers, power system simulators and all power system related topics

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Old 05-08-2014, 02:54 AM   #1
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Default Full size gyrocopter prerotater electric motors

I need a very light prerotater for the rotor on a full-size single-place gyrocopter.

Do current R/C power systems have these capabilities?
-- max weight of 7 pounds (motors, batteries, controllers, props)
-- duration of 15 minutes
-- reversible rotation so the motors can be used for prerotation AND for rotor braking after landing
-- 10 lbs of thrust per motor
-- wireless control for on, off, reverse
-- propeller brake so that went the motor is off the prop won't rotate

The attached pic approximates what I am talking about, but it was done 15 years ago and didn't work very well. I am hoping the vastly improved motor/battery/controller technology of today will make enough of a difference to pursue this idea.

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Old 05-08-2014, 03:48 AM   #2
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All of those requirements can be met by choosing the correct motor, speed controller and battery... with the possible exception of total weight while getting 15 min duration.

Braking function of the speed controller would work for slowing the rotor and minimizing rotation, but it won't lock the rotor.
Maximum braking is by shorting the 3 wires of a brushless motor. That will resist rotation very effectively, but strong wind will still windmill the rotor and could burn out the motor windings. You can do nearly as well with a resistor bridge (carefully chosen resistors) and the resistance will protect the motor.
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Old 05-08-2014, 02:17 PM   #3
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Originally Posted by fhhuber View Post
All of those requirements can be met by choosing the correct motor, speed controller and battery... with the possible exception of total weight while getting 15 min duration.

Braking function of the speed controller would work for slowing the rotor and minimizing rotation, but it won't lock the rotor.
Maximum braking is by shorting the 3 wires of a brushless motor. That will resist rotation very effectively, but strong wind will still windmill the rotor and could burn out the motor windings. You can do nearly as well with a resistor bridge (carefully chosen resistors) and the resistance will protect the motor.
The 15 minutes is not a concrete number. It would take a couple minutes of full power to get the rotor up to about 100 RRPM, I would guess,.. based on what I have seen older systems like this do. It would take a similar amount of time after landing to slow the rotor down to a safe RRPM for taxiing to the parking spot. I'd like to have a few prerotations and a few mainrotor slowdowns for each charge of the batteries. I was thinking about not hooking the battery(s) up to the charging system to save weight,.. but it could be done with commutators I would think.

I don't know what you mean by "but it won't lock the rotor." All I care about is that the main rotor (a 24' 42lb stick) slows down more rapidly than just letting it spin down. Just sitting there and waiting for it to slow down to 10 RRPM takes a few minutes at least.

Maybe you were talking about the electric motor when you refer to "rotor.".

What would be the wear and tear on a shutdown motor/prop being subjected to a changing wind going from 0 knots to 120 knots six times every second? These motor/props would be mounted on the main rotor bar, about two or three feet from the center of rotation. In forward flight of about 60 knots, as the rotor turned at about 360 RRPM, each motor/prop would see 120 knots as the blade advanced, and then 0 knots when on the retreating blade side.

Are there folding props for these motors?

Thanks for helping me out with this.
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Old 05-08-2014, 02:18 PM   #4
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What would be the pros and cons of using electric ducted fans for this purpose?

Would reversing the direction of rotation of the fan to create reverse thrust for main rotor slowdown would be very effective? Or,.. would you need to actually have the ducted fan propulsion unit rotated by a servo 180 degrees so the thrust would retard the rotation of the main rotor?
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Old 05-08-2014, 02:39 PM   #5
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Are you looking for blade tip motors to try to accelerate your blades or a motor at the main shaft for spinning the main rotor?

I doubt you want to play with using folding props in an attempt to avoid the cyclic issues of mounting motors at the blade tips.

I don't know of an appropriate speed controller for the power needed if you want to reverse thrust of an electric motor at the tips. I haven't seen RC car type controllers rated high enough to give the power you are looking for. But there is a way to set up a double pole double throw switch between ESC and motor to reverse the motor.

Some gyrocopters use a gear drive from the engine to spin up the main rotor then disengage the clutch and proceed with takeoff. You could do a similar power start-up of the main rotor with an electric motor. THEN the motor's ability to brake would work. This is what I thought you were looking for.
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Old 05-08-2014, 03:11 PM   #6
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Originally Posted by fhhuber View Post
Are you looking for blade tip motors to try to accelerate your blades or a motor at the main shaft for spinning the main rotor?

I doubt you want to play with using folding props in an attempt to avoid the cyclic issues of mounting motors at the blade tips.

I don't know of an appropriate speed controller for the power needed if you want to reverse thrust of an electric motor at the tips. I haven't seen RC car type controllers rated high enough to give the power you are looking for. But there is a way to set up a double pole double throw switch between ESC and motor to reverse the motor.

Some gyrocopters use a gear drive from the engine to spin up the main rotor then disengage the clutch and proceed with takeoff. You could do a similar power start-up of the main rotor with an electric motor. THEN the motor's ability to brake would work. This is what I thought you were looking for.
Mounting the motor/props at the blade tips will not work. The tips of the rotors in forward flight when the rotor is spinning 385 RRPM is about .7 mach.

I am looking for a set-up as shown in the pic in the opening post: Electric motors spinning props that will pre-rotate the rotor to something above 100 RRPM, not blade tip rotors. These units would be mounted about three feet from the main rotor axis, as shown in that pic.

There are a few systems available to gear an electric motor to a large gear on the main rotor head. Those work well but they add maybe 15-25 lbs to weight of the gyro and reduce the weight of my wallet a few pounds. Too heavy,... that is why I am investigating this option.

There have been a few gyros that used this small electric motor/prop option and it did work,.. but not all that well. I am hoping that since over the past decade or two the startling improvements in motor/controller/battery technology will improve the performance of this idea well enough to try it again.
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Old 05-08-2014, 05:06 PM   #7
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Originally Posted by Gyrobob View Post
Mounting the motor/props at the blade tips will not work. The tips of the rotors in forward flight when the rotor is spinning 385 RRPM is about .7 mach.

I am looking for a set-up as shown in the pic in the opening post: Electric motors spinning props that will pre-rotate the rotor to something above 100 RRPM, not blade tip rotors. These units would be mounted about three feet from the main rotor axis, as shown in that pic.

There are a few systems available to gear an electric motor to a large gear on the main rotor head. Those work well but they add maybe 15-25 lbs to weight of the gyro and reduce the weight of my wallet a few pounds. Too heavy,... that is why I am investigating this option.

There have been a few gyros that used this small electric motor/prop option and it did work,.. but not all that well. I am hoping that since over the past decade or two the startling improvements in motor/controller/battery technology will improve the performance of this idea well enough to try it again.
H'mmm
Putting a battery/esc/motor/prop at the three foot point from the main rotor axis would result in the airspeed at that location of about something around 35 MPH. To get decent efficient thrust, you'd need a fairly big diameter propeller on the motor.

As Fhhuber points out, there are a number of electric motors capable of many horsepower for the job. The $$$$ Hacker line is one of them. As an example, a Hacker A60 series motor is rated for around 3 Horsepower or so. They normally turn at around 7000 RPM at full load.

One main concern is the security of the mount for this type of power system. There would be a fair amount of centrifugal (Centripetal?) force on the battery pack, and associated wiring between that pack, the ESC, Radio receiver, and motor. Just checked, it would be around 80 G's. If any of it ever came loose in flight, it could be catastrophic for the pilot. If a folding propeller were used, if it ever threw a blade, again, bad news. That off balance prop with a missing blade will rip the motor completely out of its mount. No matter how strongly the motor is mounted. That happened on one of my models years ago.

http://www.calctool.org/CALC/phys/newtonian/centrifugal

Here is some good information on propellers, horsepower input, and thrust.
http://www.apcprop.com/v/downloads/P...B/datalist.asp

And, attached is some data for a 20X8 prop turning at 7000 RPM.

I don't want to speak for mr fhhuber, but I think both of us suspect that you could build up a very nice looking system that cost a big pile of money for something that may not work out very well.

fhhuber, any ideas on what a static load of 80 G's would to to a big LiPo battery I absolutely have no clue.


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Old 05-08-2014, 06:23 PM   #8
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Here's a few links that a friend of mine suggested...he and his father have one of these parked at the Redlands Municpal Airfield......it may a be a bit bigger than the OP's desire, but same principal:

http://www.aviomania.com/booster-prerotator.htm

Here's a link to the web forum they belong to:

http://www.rotaryforum.com/forum/showthread.php?t=25580

Probably get more "informed" ideas and practicle solutions there rather than from a bunch of "helpful" RC hobby guys.......

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Old 05-08-2014, 07:02 PM   #9
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Wouldn't it be far better to do it with a reduction spur gear driven pre-rotator, like this commercially available unit: http://www.aviomania.com/booster-prerotator.htm



While this might cost more and require more engineering it would at least have the advantage of working as intended, which to be brutally honest i dont think the prop idea ever would with any degree of reliability or efficiency.
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Old 05-08-2014, 07:13 PM   #10
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Originally Posted by JetPlaneFlyer View Post
Wouldn't it be far better to do it with a reduction spur gear driven pre-rotator, like this commercially available unit: http://www.aviomania.com/booster-prerotator.htm



While this might cost more and require more engineering it would at least have the advantage of working as intended, which to be brutally honest i dont think the prop idea ever would with any degree of reliability or efficiency.

Been there done that.

I have built a few gyros, each of which had a prerotater. All the commercially available systems work fairly well. Some them use hydraulics and can get the rotor up to 250 RRPM!!

What I seek here is something very very light weight that can get the rotor up to maybe 100 RRPM. The system I have been describing with the two R/C electric motors has been done a few times already, and it does work.

What I seek is a similar system using much newer (meaning lighter and more powerful) components that will only weigh maybe 5 lbs instead of 15-25 lbs.

It is really challenging to get a gyro to weigh less than 254 lbs, the limit for an ultralight. Every lb counts, so if I can save a few lbs with the prerotater, I want to do that.
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Old 05-08-2014, 07:29 PM   #11
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Ok, but as well as the weight you would need to factor in the drag on the rotors caused by the props. My guess is that would be worth at least 15-25lb in lost efficiency.

There is nothing stopping the use of new lightweight technology on the gear driven system. Gears when compared to props will always end up with far higher efficiency in terms of torque applied to the rotor for watt of input power (prop efficiency probably 50-60% gears 95%+) plus you don't have the parasitic drag when the rotor is autorotating..
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Old 05-08-2014, 07:41 PM   #12
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You will only be able to save significant weight in the batteries if anywhere.

The motors simply have to weigh a certain amount in order to be able to develop the power desired. Better magnets can get you the same power at a slight weight savings but we have had that for a long time. If you were using a brushless motor you already had the best power:weight for the motor.
Maybe choosing a better motor for the task or a better propellor... Optimizing the system will be difficult because of the changing "airspeed" the prop has to deal with. But if you can find the best prop for the task then you will get the best results possible for the motor chosen.

Electronic speed controls have gotten cheaper but not really better in the past several years.

By replacing a big heavy lead acid battery with a LiPo you can save a lot of weight. But then you have to modify your charging system to prevent overcharging the LiPo.

Going to the minimum size lead acid practical for starting the gasoline engine and then a LiPo charger and LiPos for the pre-rotator motors you might get somewhere.

The motors are available in a wide range of sizes and by carefully shopping you can find some very powerful ones at a low price.

The only real hope I see for improving the motors placed as in the first post's picture is that cost should be lower and battery weight might be saved, allowing bigger motors.

Folding props might help by reducing drag when the motors are off... All ESCs I know of have a "brake mode" to aid in getting the prop to fold. The high quality folding props you'd need to be able to rely on the blades staying on the motors would bring costs back up and limit your choices of prop size.

If they ever start producing a true 3-phase ESC we could get significantly more power from the same brushless motors, but we'd be back to needing to use a sensor pack on the motor to give the timing signal to the ESC, adding weight. (still better power:weight than we get now)
This would be a rather expensive ESC...
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Old 05-08-2014, 07:52 PM   #13
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Originally Posted by kyleservicetech View Post
fhhuber, any ideas on what a static load of 80 G's would to to a big LiPo battery I absolutely have no clue.
You don't put the battery on the main rotor... you use wipers (similar to commutator but no make/break) and pass the current up to the motors. That's been done and works.

If you do put the battery on the rotor you put it right at the blade "grips" at as close to 0 G as possible. That also has been done for RC heli lighting systems. even using the rotation forces to actuate the switch turning on the lights.
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Old 05-08-2014, 09:14 PM   #14
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For braking you can use a simple disc brake type set up. It would be a disc with a pad on a lever and a handle to pull down to push the pad up to the disc. Alot of wind mills use this system. The other thing you could do is get an old 10si alternator and install a v belt pulley on the shaft and when you want to stop the rotor you could push a button and use the battery to short the 3 phases of the stator which will slow the rotor down. It does not need a regulator, brush holder and trio. all that is needed is the rectifier and stator and rotor. They can be bridged together and use the case as ground so theoretically the only wire going the the alt is the shorting wire if the case and battery are grounded to the frame. Some permanent magnet windmills use this system too. Slows the blade down really fast.
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Old 05-08-2014, 09:25 PM   #15
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Originally Posted by fhhuber View Post
You don't put the battery on the main rotor... you use wipers (similar to commutator but no make/break) and pass the current up to the motors. That's been done and works.

If you do put the battery on the rotor you put it right at the blade "grips" at as close to 0 G as possible. That also has been done for RC heli lighting systems. even using the rotation forces to actuate the switch turning on the lights.
Never thought of that!

On the other hand changing stuff in a MODEL airplane is not the same as when your butt is also flying

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Old 05-08-2014, 09:45 PM   #16
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Originally Posted by kyleservicetech View Post
Never thought of that!

On the other hand changing stuff in a MODEL airplane is not the same as when your butt is also flying

Yep......that's probably why those with only RC skills and no true aviation equipment design and build experience....should butt-out on this one.....

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Old 05-08-2014, 09:48 PM   #17
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Originally Posted by propnut48 View Post
For braking you can use a simple disc brake type set up. It would be a disc with a pad on a lever and a handle to pull down to push the pad up to the disc. Alot of wind mills use this system. The other thing you could do is get an old 10si alternator and install a v belt pulley on the shaft and when you want to stop the rotor you could push a button and use the battery to short the 3 phases of the stator which will slow the rotor down. It does not need a regulator, brush holder and trio. all that is needed is the rectifier and stator and rotor. They can be bridged together and use the case as ground so theoretically the only wire going the the alt is the shorting wire if the case and battery are grounded to the frame. Some permanent magnet windmills use this system too. Slows the blade down really fast.
Along the same line here, you could use a 12v Brush Starter / Generator and a v belt, use the Starter Phase of the motor to turn the rotor blades, switch to off so it free wheels, and switch to generate to stop the blades,seperate circuit with a big one way diode, you can use the same battery for the engine, just run cables to the motor / generator assy. the motor below is 1/3 HP, but with reduction of the pullies, lets say 20 to 1 that would give you about 7 hp

http://www.ebay.com/itm/DC-Motor-Gen...-/191125030933


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Old 05-08-2014, 10:09 PM   #18
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12v brush motor

Product Features

  • Measures 3.1 inches diameter, (78 mm) by 4 inches long, (102 mm)
  • No-load RPM: 6900 at 24 Volts; Can be used at voltages ranging from 12V to 36VDC.
  • Maximum power: 1740 Watts, (2.3 horsepower); Maximum stall torque: 10 Nm, (1370 oz-in.)
  • Peak efficiency at 24V: 82% at 28 Amps and 6300 RPM.
  • Power and torque at peak efficiency: 560 Watts, (0.75 horsepower), and 840 mNm, (120 oz-in.)


http://www.amazon.com/AmpFlow-F30-15...3QMEZRY89D7SKA




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Old 05-08-2014, 11:07 PM   #19
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Originally Posted by pizzano View Post
Yep......that's probably why those with only RC skills and no true aviation equipment design and build experience....should butt-out on this one.....
LOL

Agreed!

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Old 05-08-2014, 11:12 PM   #20
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Originally Posted by JetPlaneFlyer View Post
Ok, but as well as the weight you would need to factor in the drag on the rotors caused by the props. My guess is that would be worth at least 15-25lb in lost efficiency.

There is nothing stopping the use of new lightweight technology on the gear driven system. Gears when compared to props will always end up with far higher efficiency in terms of torque applied to the rotor for watt of input power (prop efficiency probably 50-60% gears 95%+) plus you don't have the parasitic drag when the rotor is autorotating..
"you would need to factor in the drag on the rotors caused by the props."

With the props stopped, the drag will be minimal. They will be only three feet or so from the axis of rotation, so the drag on the rotation of the rotor would be small. The drag caused by stopped props as the gyro flew at 60 knots might need 1/2 hp to overcome.

"you don't have the parasitic drag when the rotor is autorotating"

Gyrocopter rotors are always autorotating. Except for prerotation, they are unpowered.
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Old 05-08-2014, 11:17 PM   #21
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Originally Posted by fhhuber View Post
You don't put the battery on the main rotor... you use wipers (similar to commutator but no make/break) and pass the current up to the motors. That's been done and works.

If you do put the battery on the rotor you put it right at the blade "grips" at as close to 0 G as possible. That also has been done for RC heli lighting systems. even using the rotation forces to actuate the switch turning on the lights.
The few systems using R/C electric motors to drive small props mounted on the main rotor have used both systems,... small batteries mounted close to the center of rotation of the main rotor,.. and commutators that allowed a stationary battery.
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Old 05-08-2014, 11:19 PM   #22
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Originally Posted by propnut48 View Post
For braking you can use a simple disc brake type set up. It would be a disc with a pad on a lever and a handle to pull down to push the pad up to the disc. Alot of wind mills use this system. The other thing you could do is get an old 10si alternator and install a v belt pulley on the shaft and when you want to stop the rotor you could push a button and use the battery to short the 3 phases of the stator which will slow the rotor down. It does not need a regulator, brush holder and trio. all that is needed is the rectifier and stator and rotor. They can be bridged together and use the case as ground so theoretically the only wire going the the alt is the shorting wire if the case and battery are grounded to the frame. Some permanent magnet windmills use this system too. Slows the blade down really fast.
A "disc" brake on the main rotor is quite common. Usually it is nothing more than a pad pushing on the side of the ring gear around the rotor head. It works okay, but it weighs a few lbs.
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Old 05-08-2014, 11:23 PM   #23
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Originally Posted by CHELLIE View Post
Along the same line here, you could use a 12v Brush Starter / Generator and a v belt, use the Starter Phase of the motor to turn the rotor blades, switch to off so it free wheels, and switch to generate to stop the blades,seperate circuit with a big one way diode, you can use the same battery for the engine, just run cables to the motor / generator assy. the motor below is 1/3 HP, but with reduction of the pullies, lets say 20 to 1 that would give you about 7 hp

http://www.ebay.com/itm/DC-Motor-Gen...-/191125030933

This thing probably weighs 25 lbs. What diameter prop would it swing?
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Old 05-08-2014, 11:25 PM   #24
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Originally Posted by CHELLIE View Post
12v brush motor

Product Features

  • Measures 3.1 inches diameter, (78 mm) by 4 inches long, (102 mm)
  • No-load RPM: 6900 at 24 Volts; Can be used at voltages ranging from 12V to 36VDC.
  • Maximum power: 1740 Watts, (2.3 horsepower); Maximum stall torque: 10 Nm, (1370 oz-in.)
  • Peak efficiency at 24V: 82% at 28 Amps and 6300 RPM.
  • Power and torque at peak efficiency: 560 Watts, (0.75 horsepower), and 840 mNm, (120 oz-in.)
http://www.amazon.com/AmpFlow-F30-15...3QMEZRY89D7SKA



This weighs about 5 lbs. Two of them would weigh 10 pounds. Even without all the peripheral stuff and batteries, it is too heavy.

Assuming it were going to be used, how large a prop would it swing, and how many pounds of thrust would it produce?
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Old 05-08-2014, 11:37 PM   #25
fhhuber
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Gearing does not alter HP... it trades RPM for torque. You always lose a little HP in a gear train due to friction. (often a very small % of HP is lost... but its always a loss)

Lets say you can get 10 ft-lb torque from a motor without gearing... gear it 2:1 (2 revs of motor for 1 of final shaft) and you should measure 20 ft-lb torque.

**********

Until VERY recently practically all full scale aircraft development involved making models and trying them first. All of the mechanical principals and physics are the same for the model and the full scale.
They even did RC models for the Space Shuttle.

Ultralights (Part 103 man carrying aircraft) are really oversize model aircraft in many cases.
Paying attention to the structure and power needed for the larger aircraft, just about anyone who builds RC models could build a successful ultralight.

The realm of the autogyro needs some knowledge that isn't as common... I had never heard of putting model aircraft motors mid-span of an autogyro before. I've seen the little propane ram jets on the tips though.

You never know what someone on here actually has experience with... such as restoration of a full scale aircraft in the house. A wing on the grand piano, one in the front hall and the fuselage in the enclosed back porch.
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