# Electric Autogyro - Sanity Check



## duncgyro (Aug 22, 2017)

Most modern autogyros (or gyroplanes) are powered by 100 HP Rotax 912 engines. So 100 hp = 74.6 kW, and a full power take-off and 5 minute climb uses 22.4 MJ. A common 75% power cruise setting = 75 hp (56 kw/h) = 201.4 MJ PER HOUR, thus 22.4+201.4=223.8 MJ for the first hr. If electric used, then same MJ energy usage - neglecting efficiency losses.

If modern 18650 cells are used, then the 3.7 V at 12.6 Watt-hr specs yields 45 KJ, thus 5000 cells are needed to provide the ~225 MJ per above. At .1 lb/cell, this means 500 lbs in batteries. For the first hour?

However the Sun Flyer just announced a four place, all electric aircraft, that has a four hour endurance, and uses 500 lbs of batteries.

So something is wrong with either my initial assumptions or calculations by a factor of four. Can anybody find my error?


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## Duncan (Dec 8, 2008)

Hi
I suspect that "Sun Flyer" is stretching the truth a bit!

Your numbers sound OK 

Just do you really need 100% power to take off and 75% for cruise?

And how efficient is the prop on the Rotax?

Even if they can significantly improve the prop efficiency I still don't see four hours endurance


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## duncgyro (Aug 22, 2017)

Background check: I am a 6,000+ hour commercial licensed pilot and CFI-ASMEL, with hundreds of hours in Boeing B-17, Grumman TBM Avenger, AT-6/SNJ, and flown numerous other WWII aircraft besides 35 types of the usual "spam cans" made by Cessna, Piper, Beech, etc. since 1972. I am also a well-experienced licensed A&P, and designed/own a couple STC and numerous FAA Form 337 approved airframe and engine modification. 

I am NOT a gyroplane pilot, but want to build one. I teach mountain flying in Colorado where there are 56 peaks above 14,000 ft, and live at an airpark located at 6500 MSL. A 90 degree summer day here creates a 10,000 ft density altitude at field elevation. That is the reason why I am investigating the possibility of electric propulsion.

Besides my personal high altitude needs, pilots are taught to use full throttle for all takeoffs for two reasons:

1. One wants to fully stress the engine while upon or near the runway. If the engine is going to fail, do it early.

2. One wants to gain as much altitude as possible immediately after takeoff in order increase the possibility of a safe emergency landing. 

Most GA aircraft are rated and flown at 75% rated power settings while in cruise. Aircraft range, given in hours or miles, always use this setting. Of course the pilots can always elect to reduce power and thereby increase range. There are "Lean Of Peak", "over-square", and other operating modes that increase efficiency, but these are rarely given in specifications.

The autogyro creates a lot of drag simply due to the high angle of attack of the main rotor blades. Therefore most are flown with higher throttle settings than conventional aircraft, although they can be safely flown at 20 MPH or lower. With top speeds in the 80-100 MPH range, one does not buy an autogyro if they want to go fast. They are quite popular in Europe, so most are manufactured there.

Any propeller that can be attached to a Rotax can be used with an electric motor. So the efficiency is awash at this point, but variable pitch prop may be used. I might also look into ducted fan methods of increasing thrust, efficiency, and reducing noise, but that is after the basic sanity check and costs are evaluated. 

The 100 kw (continuous) motor used by the Sun Flyer weighs 44 lbs and costs about $5,000 new. Compared to the popular 172 lb Rotax 914 engine at $27,000.

With all that said, what is the energy density (MJ/lb) of typical 18650 batteries? That might be a more appropriate method of determining the sanity of this effort! I am hoping this forum can provide the answers.


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## duncgyro (Aug 22, 2017)

Duncan said:


> Hi
> Even if they can significantly improve the prop efficiency I still don't see four hours endurance


Prop efficiency has no effect on endurance. It would only affect airspeed at any given power setting, and thus range.


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## duncgyro (Aug 22, 2017)

Sun Flyer just released an in-depth article seen here:
http://spectrum.ieee.org/aerospace/aviation/cheaper-lighter-quieter-the-electrification-of-flight-is-at-hand

I thought this might be of general interest to readers.


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## brian_ (Feb 7, 2017)

duncgyro said:


> However the Sun Flyer just announced a four place, all electric aircraft, that has a four hour endurance, and uses 500 lbs of batteries.


Initial marketing claims are almost always nonsense.

In this case, the article explains that the aircraft is targeted at training (just like the Pipistrel Electro) because it has such short endurance... they say less than three hours. Maybe by the time it is in production, it will be down to two hours... 

From the linked article:


> Our 330-kg battery pack easily allows normal flight, putting out a steady 18 to 25 kW and up to 80 kW during takeoff. The total energy storage capacity of the battery pack is 83 kWh.


That's 727 pounds, not 500... because the capacity is up from 225 MJ (62 kW-h) to 300 MJ (83 kW-h). And cruise power is 23% to 31%, not 75%, explaining an endurance of much more than an hour.

Overall, the article seems a little strange because it is primarily a sales job for the idea of electric flight, which would have been more appropriate a couple of years ago. By now, an electric aircraft manufacturer should be describing a proven aircraft, not a development project.



Duncan said:


> Just do you really need 100% power to take off and 75% for cruise?
> 
> And how efficient is the prop on the Rotax?


As duncgyro explained, the issue isn't the prop, it's the rotor. An autogyro is essentially - depending on how you want to think about it - either

a helicopter stuck forever in autorotation (coasting down like it has an engine failure) but kept going by an engine driving a prop, or
an airplane with an inefficient rotor instead of fixed wings.

Either way, efficiency is not good, because rotors are not efficient wings. This is why helicopters use so much fuel, why a helicopter autorotating descends so steeply compared to a fixed-wing plane gliding, and why an autogyro is slower at the same power level as a fixed wing aircraft of the same size with the same engine. Autogyros do take off in a very short distance and land almost vertically, which I think is the primary reason that they exist.

So yeah, to keep moving at decent speed you need that much power, unfortunately. This makes electric operation less than ideal, but as duncgyro explained, you get the same endurance (time flying) as with a fixed wing aircraft... you just don't go as far.


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## brian_ (Feb 7, 2017)

duncgyro said:


> Any propeller that can be attached to a Rotax can be used with an electric motor.


An alternative is to use multiple motors and props (still direct-drive) with a smaller prop diameter and a corresponding higher shaft speed. This makes more readily available motors usable, and provides redundancy which could be a good idea without any redundancy within the motor or controller, and no aviation-grade components.

I note that Siemens is building an aircraft motor with two separate sets of windings, driven by separate controller/inverters, for redundancy. For those who haven't already read it: 350hp Electric Aircraft Motor (it takes a while to get to meaningful technical content).



duncgyro said:


> So the efficiency is awash at this point, but variable pitch prop may be used.


Permanent magnet AC motors have a relatively wide speed range of efficient operation (compared to other electric motors) and a wide speed range for peak power (compared to an engine) so I don't think a variable pitch prop would have much advantage over fixed pitch in an electric application.


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## Duncan (Dec 8, 2008)

You guys don't get it!
The prop is used to convert motor power into thrust

You can get massively different efficiency - which will result in a LOT more power being needed for the same thrust

Just as a first look the Rotax will need to run at a specific speed - that speed will then determine the prop dimensions
Which may not be ideal for the speed of the aircraft

A larger prop may well be able to produce the same thrust for less power

Remember the Thrust = Mass flow x Exhaust velocity - But the power required = 1/2 Mass flow x Exhaust velocity SQUARED

The prop is all important in the efficiency - especially of a comparatively low speed machine like an AutoGyro


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## kennybobby (Aug 10, 2012)

duncgyro said:


> Sun Flyer just released an in-depth article seen here:
> http://spectrum.ieee.org/aerospace/...eter-the-electrification-of-flight-is-at-hand
> 
> I thought this might be of general interest to readers.


That is quite interesting, thanks for sharing.

What about a different cell package than the 18650--are you willing to look at other options, or want to stick with the tesla approach.

i like the gyro concept and hope you will share your thoughts as you build it.


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## brian_ (Feb 7, 2017)

Duncan said:


> You guys don't get it!
> The prop is used to convert motor power into thrust
> 
> You can get massively different efficiency - which will result in a LOT more power being needed for the same thrust...


I certainly get that. I'm just assuming that the prop is already appropriate for the engine and airspeed, and that a similarly suitable prop would be used with an electric motor. There's no big improvement opportunity by changing the prop, because if there were then gas-engined autogyros would already have made that improvement. Conversely, there no big increase in power requirement, because the electric version would not use an inappropriate prop.

While some people build vehicles of all types with nearly random bits of inappropriate hardware, it seems reasonable to assume that viable production aircraft do not fit in that category. Changing the shaft power source from a gasoline engine to an electric motor doesn't change the aerodynamics, so I don't see any reason to expect an improvement in the efficiency of thrust generation from shaft power. It seems sensible to me to start with the working premise that an electric autogyro would need the same shaft power as the original gasoline-engined autogyro from which it was derived... or more due to greater powertrain weight.


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## duncgyro (Aug 22, 2017)

Lots of wanderings within this thread, and most are simply opinions of aeronautical engineering aspects with no math or cited facts. I am looking for your experienced thoughts pertaining to energy and weight calculations. Propeller and/or rotor efficiency are not the issue, but I will gladly discuss these on another thread. Be prepared to discuss airfoil lift/drag ratio relative to the angle of incidence. 

As a multi-engine CF I, I also caution that the use of multiple engines and propellers is a dangerous handful when one of them stops working - electric or not. I am not sure the designer of this https://youtu.be/CFZeUO4wZII autogyro has considered this fact. 

At this year's annual autogyro convention in Mentone, IN a unique twin propeller autogyro attended, that used bevel gears to drive both props from the same IC Rotax engine. It could also do a vertical "Jump Takeoff" as the main rotor received partial power from the engine. See photo










Back on target subject of "sanity check" - reading the recently cited article revealed they are using some regeneration during descent for a 13% recovery. The article also implies a cruise power use of 18-25 kw (24-32 hp) - this seems low to me although "at what airspeed" airspeed is not given. 

I had initially recorded the battery weight wrong. They are using 727 lbs of batteries to achieve a four hour endurance. So my anticipated two hour "sight-seeing, not cross-country" mission would be one half, or approximately 365 lbs. Add the 45 lb, 80 kw (continuous rated) motor for approximate full powerplant weight of 410 lb, neglecting controller weight. I assume this is negligible? No reduction gearing needed.

A 172 lb, 115 HP turbo'ed Rotax 914 IC engine with full fuel - 25 gallons (150 lbs) weighs 322 lb total. Thus electric power is only about 88 lb heavier once accepting a more limiting two hour endurance. Read the article concerning energy use for take-off, cruise, and regeneration during descent. 

So still possible in my mind, although I'd like you "experts" to confirm with your own calculations and observations. But stay away from those pesky perceived aeronautical engineering issues unless truly qualified.


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## brian_ (Feb 7, 2017)

Duncan said:


> Just as a first look the Rotax will need to run at a specific speed - that speed will then determine the prop dimensions
> Which may not be ideal for the speed of the aircraft...


It actually works the other way around: given a workable propeller diameter and aircraft speed, the shaft speed results. Aircraft engines are designed to run at that prop speed, using a reduction drive (normally a gearbox, but some homebuilts use a belt) if required. This is why conventional light aircraft piston engines are huge and slow (e.g. 180 hp from a 360 cubic inch or 5.9 litre 4-cylinder at 2700 rpm).

The Rotax 912 and 914 incorporate a reduction gearbox, so that they can drive the prop at the conventional prop speed, but with a higher engine speed allowing for the target performance with less engine displacement (for a 912, 80 to 100 hp from 74 c.i. or 1.2L at 580 rpm).

An electric motor to drive the same prop without a reduction box will be running quite slowly, and so will be heavier than a motor without this speed constraint; the motor has the same speed/size/gearing issue as the engine.


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## Karter2 (Nov 17, 2011)

duncgyro said:


> So something is wrong with either my initial assumptions or calculations by a factor of four. Can anybody find my error?


 Err,?....can you realisticly compare the power consumption/efficiency of an auto gyro craft... to a fixed wing design ?


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## duncgyro (Aug 22, 2017)

brian_ said:


> It actually works the other way around: given a workable propeller diameter and aircraft speed, the shaft speed results. Aircraft engines are designed to run at that prop speed, using a reduction drive (normally a gearbox, but some homebuilts use a belt) if required. This is why conventional light aircraft piston engines are huge and slow (e.g. 180 hp from a 360 cubic inch or 5.9 litre 4-cylinder at 2700 rpm).
> 
> The Rotax 912 and 914 incorporate a reduction gearbox, so that they can drive the prop at the conventional prop speed, but with a higher engine speed allowing for the target performance with less engine displacement (for a 912, 80 to 100 hp from 74 c.i. or 1.2L at 580 rpm).
> 
> An electric motor to drive the same prop without a reduction box will be running quite slowly, and so will be heavier than a motor without this speed constraint; the motor has the same speed/size/gearing issue as the engine.


As an licensed A&P mechanic for 25 years, I am well aware of what you posted, but also offer that most radial engines also use reduction gearing for the propeller. However the cited Sun Flyer article says they are not using any gear reduction, and the EMRAX 268 motor they use is just 20 kg (44 lbs).


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## duncgyro (Aug 22, 2017)

Karter2 said:


> Err,?....can you realisticly compare the power consumption/efficiency of an auto gyro craft... to a fixed wing design ?


The best comparison would be to use the relative airspeeds with a given power. There are streamlined, retractable landing gear, 100 hp aircraft that cruise in excess of 150 mph, whereas 100 mph is the fastest 100 hp autogyro I have found. BTW - No autogyros have retractable landing gear, however the fastest are nicely streamlined as seen below. But this is not the style I will be building.










I never said that autogyros were efficient. But that is not the point. I want to fly lower and slower, and have an aircraft that I can easily tow around the country within a short trailer. No standard aircraft can fly at 20 MPH. All autogyros can. Few aircraft can land within 10 ft. All autogyros can. Short and slow is what I am looking for like this ...


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## brian_ (Feb 7, 2017)

Karter2 said:


> Err,?....can you realisticly compare the power consumption/efficiency of an auto gyro craft... to a fixed wing design ?


Yes and no... 

*Yes*
The original assumption was that, although the aircraft fly differently, they are driven by the same engine at the same power level, so the power output/consumption are the same. The autogyro just flies more slowly than a fixed-wing aircraft at the same power. The intent was to match flying time, not distance, so this is valid.

*No*
The _efficiency_ of the lift system (wings versus rotor) is quite different; however, that doesn't matter because the expectation was not to travel the same distance or at the same speed, just to run at the same level of power consumption.
Although they take off at the same power, the assumption of 75% _cruise power_ for both (which is reasonable for both fixed-wing and autogyro) turned out to be invalid in this case (the fixed-wing plane in this case is operated at unusually low power); this turned out to be part of the reason for the endurance mismatch.

Half of the Sun Flyer's battery thus will yield less than half of the Sun Flyer's 4-hour endurance, because the cruise power is higher for the autogyro (despite using the same engine as the gas version of the Sun Flyer).


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## brian_ (Feb 7, 2017)

duncgyro said:


> ... I am well aware of what you posted, but also offer that most radial engines also use reduction gearing for the propeller.


I realize that - the comment was mostly background for Duncan and others who might be interested.

It wasn't just radials: reduction gearing became common by the end of the golden era of piston engines. With their necessarily large propellers, even the modest speed of the big radials (2300 rpm at takeoff for the B-17's R-1690 Hornet 9-cylinder 28-litre) was too fast (the R-1690 had a 3:2 reduction); later, other engines used higher speed and more reduction (e.g. 3000 rpm and 2.5:1 for a typical Merlin).

After turbine engines took over large aircraft, essentially only the general aviation industry continued to build new aircraft with piston engines. The engines of their light planes are low-powered compared to those of larger aircraft, and they nearly universally settled on the use of the direct-drive configuration, which works in the sizes needed and simplifies the system. Reduction drives were nearly forgotten until homebuilders wanted to use small engines, and they came back, both as add-ons and as an integral component of the Rotax 912/914 and others.



duncgyro said:


> ... However the cited Sun Flyer article says they are not using any gear reduction, and the EMRAX 268 motor they use is just 20 kg (44 lbs).


No argument from me. The same power could be produced by an even lighter motor at higher speed, but choosing direct drive is logical and this motor is presumably matched to the prop normally used on the aircraft. Despite what a quote in the article suggests, this motor is not a special version for the Sun Flyer: 2000 rpm is the bottom end of the normal peak continuous power range for the EMRAX 268, and all nine stock versions could meet the power specs for the application.

A higher-speed motor would also be smaller in diameter than the EMRAX 268's 268 mm (I see a correlation there...), which could be valuable in a nacelle mounting, such as an electric version of Dick DeGraw's homebuilt "Rhino 2" (the yellow one). Of course this is irrelevant in the typical single-prop pusher autogyro installation.


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## brian_ (Feb 7, 2017)

duncgyro said:


> Back on target subject of "sanity check" - reading the recently cited article revealed they are using some regeneration during descent for a 13% recovery.


The flying scenario in this case is repeated patterns for flight training (climb, pattern, approach, repeat), which is the ideal case for regeneration. This has come up in other discussions in this forum, and has little relevance for recreational flying (even in a fixed-wing aircraft)... in which there is only one descent. I think for the proposed type of use, it would be wise to plan on no energy recovery from regeneration. Even if you get some, it doesn't help your endurance, because you don't have that energy until the flight is complete; it just saves a few cents worth of electricity and some time during recharging on the ground.


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## brian_ (Feb 7, 2017)

duncgyro said:


> ... I also caution that the use of multiple engines and propellers is a dangerous handful when one of them stops working - electric or not.


Multiple engines and props well off the centreline are certainly a problem; I wouldn't suggest that.
Multiple engines which interact can be a problem when one has an issue; I certainly didn't suggest that.
Driving two props from one engine with a complex mechanical system is questionable; doing that with an electric motor would be pointless and just too problematic to even think about.


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## brian_ (Feb 7, 2017)

duncgyro said:


> It could also do a vertical "Jump Takeoff" as the main rotor received partial power from the engine.


I'm sure you're aware that pre-spinning the rotor up to flying speed for a "jump" takeoff is a long-established autogyro feature. This would be easiest to do with an electric motor (one dedicated to the main rotor, not some Rube Goldberg contraption using the thrust motor), if you're interested... but that motor would be just ballast once off the ground. Production pre-rotators can be electric or pneumatic (and who know what else). That seems like a possible future enhancement, rather than anything one would consider initially.

For an amusing electric discussion point: if the pre-rotator is electric, the energy it puts into the kinetic energy of the spinning rotor (a big flywheel) is available for regenerative recovery after touchdown. Normally, the pilot just waits for it to spin down due to drag. Again, this doesn't help flight power or endurance, but saves a bit of energy from whatever the charging source might be.


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## brian_ (Feb 7, 2017)

duncgyro said:


> ... Add the 45 lb, 80 kw (continuous rated) motor for approximate full powerplant weight of 410 lb, neglecting controller weight. I assume this is negligible?
> ...
> Thus electric power is only about 88 lb heavier...


On the scale of a typical car, I suppose the controller mass is negligible. In a small autogyro, I'm not so sure of that. Also remember to add in wiring, contactors, battery housing...
Does the 45 pound motor weight include a cooling system? A 80 kW continuous power output in a compact motor without liquid cooling seems unlikely, although EMRAX allows air, air/liquid, or liquid cooling for this motor (with varying resulting power and time restrictions).

My guess is that a workable electric propulsion system will turn a two-seat tandem light gyroplane (such as that AutoGyro) into a fun single-seater. The AutoGyro specs for performance include an 80 kg pilot, no passenger, and only 40 L (~30 kg or 65 pounds) of fuel. With liquid fuel, endurance (fuel load) can be traded for payload (passenger or cargo); with battery power, unless you have readily removable modules and can leave some behind, you're at full-fuel weight all the time.


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## duncgyro (Aug 22, 2017)

brian_ said:


> I'm sure you're aware that pre-spinning the rotor up to flying speed for a "jump" takeoff is a long-established autogyro feature. This would be easiest to do with an electric motor (one dedicated to the main rotor, not some Rube Goldberg contraption using the thrust motor), if you're interested... but that motor would be just ballast once off the ground. Production pre-rotators can be electric or pneumatic (and who know what else). That seems like a possible future enhancement, rather than anything one would consider initially.


Not quite Brian. A "jump" take-off requires that the main rotor be variable pitch and controlled by a collective control - the very same as for regular helicopters. It also requires special training and precise timing as one needs to gain forward speed and reposition the blade pitch immediately to have the main rotor powered by relative wind - not by the pre-rotating mechanism. A flexible drive shaft is a common pre-rotating mechanism as it weighs less than most other motors (electric, hydraulic, etc.) The shaft often uses a drive belt off the engine, which is only engaged prior to the takeoff roll. 

The common pre-rotation system spins the rotor up to about 200 RPM, then the wheel brakes are released and forward air speed is used to maintain then increase rotor rotation speed until enough lift is generated to fly.

The big green autogyro actually uses about 20% of the main engine power for continual supplementary rotation energy. This is part of the experiment to reduce the main rotor drag and increase airspeed.



brian_ said:


> For an amusing electric discussion point: if the pre-rotator is electric, the energy it puts into the kinetic energy of the spinning rotor (a big flywheel) is available for regenerative recovery after touchdown. Normally, the pilot just waits for it to spin down due to drag. Again, this doesn't help flight power or endurance, but saves a bit of energy from whatever the charging source might be.


This is a good idea. I have considered this approach, but want to KISS from the start. Most modern gyrocopters are equipped with a rotor braking system used cautiously after landing due to the rotor torque effect on directional control.


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## duncgyro (Aug 22, 2017)

Can we please get back on the subject? Does anybody have controller, battery, or other useful suggestions about the power plant design? Books or websites?


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## duncgyro (Aug 22, 2017)

brian_ said:


> The flying scenario in this case is repeated patterns for flight training (climb, pattern, approach, repeat), which is the ideal case for regeneration.


The four place, four hour endurance is the subject aircraft. Not the two place trainer. FWIW there is nothing that says the autogyro can not be approved to to use another 200 lbs of batteries. There is nothing sacrosanct about the targeted weight of any experimental aircraft. The rotor and landing gear systems just need to support it, and a 15% gross weight increase would require very little effort. In fact, in Alaska, it is perfectly legal to fly any aircraft at 15% above its gross weight with no modifications at all.


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## kennybobby (Aug 10, 2012)

Well you never answered my question in post #9 about cell format, i.e. tesla 18650 vs other, polymer flat packs, so are you serious or not?

The RC polymer flat packs have energy density on the order of .7 MJ/kg.

What is the density you need to determine if concept is viable?


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## brian_ (Feb 7, 2017)

duncgyro said:


> A "jump" take-off requires that the main rotor be variable pitch and controlled by a collective control - the very same as for regular helicopters. It also requires special training and precise timing as one needs to gain forward speed and reposition the blade pitch immediately to have the main rotor powered by relative wind - not by the pre-rotating mechanism. A flexible drive shaft is a common pre-rotating mechanism as it weighs less than most other motors (electric, hydraulic, etc.) The shaft often uses a drive belt off the engine, which is only engaged prior to the takeoff roll.
> 
> The common pre-rotation system spins the rotor up to about 200 RPM, then the wheel brakes are released and forward air speed is used to maintain then increase rotor rotation speed until enough lift is generated to fly.


Nice explanation of the two systems 
Either way, in an aircraft with an electric motor driving the prop, a separate electric motor seems like the only sane way to drive the rotor, whether for simple pre-rotation or the whole "jump" scheme.



duncgyro said:


> The big green autogyro actually uses about 20% of the main engine power for continual supplementary rotation energy. This is part of the experiment to reduce the main rotor drag and increase airspeed.


So it needs continual rudder effect when doing this. This sounds like the long and complicated path to building a compound helicopter, but just not as effective. Oh well, driving the rotor in flight is not part of the subject of electrically powering a low-speed autogyro.


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## brian_ (Feb 7, 2017)

*Regeneration from rotor braking after landing*



duncgyro said:


> I have considered this approach, but want to KISS from the start. Most modern gyrocopters are equipped with a rotor braking system used cautiously after landing due to the rotor torque effect on directional control.


I can just picture a pilot yanking the brake on hard and twisting the craft around!

I agree, both pre-rotation and the complementary regenerative braking belong on the "later" list.


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