# How far are we from the perfect electric "homebuilt" ?

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#### Aerowerx

##### Well-Known Member
The benefit ought to be more than the loss for the PRSU gearing.
Huh?

Can you give an example explaining this, with equations?

So, you are saying that a motor "optimized" to run at, for example, 3000 RPM, going through a PSRU to turn a prop at, for example, 2000 RPM is more efficient than a motor "optimized" to run directly at 2000 RPM???? And weighs less (motor and PSRU)? And is more reliable? (Remember Pops signature: If its not there, it cost nothing, weighs nothing, and is 100% reliable.)

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#### pictsidhe

##### Well-Known Member
Electric motors produce torque in direct proportion to the active magnetic area. Take a square whatever of iron surface area, run 1 tesla of flux through it and a number of ampere-conductors. That gives you a force. It is the same if you have 1000 conductors and 1 amp, or 1 conductor and 1000 amps. Rewinding does not change torque potential. The flux density is limited by magnetic core properties and maybe your magnets. The ampere-conductors is limited by space in the core and resistive heating. Power is force times speed. The faster you spin at that force, the more power you produce. A motor that produce 1Nm will need a lot less iron, copper and magnets than one producing 10Nm. yes, it'll need a 10:1 gearbox, but the gearboxes tend be much lighter than DD motors.

For an example with a low desired output speed and a given power, have a look at e-bike motors. Compare the weight of the direct drive and geared motors...

#### Speedboat100

##### Well-Known Member
Huh?

Can you give an example explaining this, with equations?

So, you are saying that a motor "optimized" to run at, for example, 3000 RPM, going through a PSRU to turn a prop at, for example, 2000 RPM is more efficient than a motor "optimized" to run directly at 2000 RPM???? And weighs less (motor and PSRU)? And is more reliable? (Remember Pops signature: If its not there, it cost nothing, weighs nothing, and is 100% reliable.)
No cannot give calculations...air resistance comes to play too.

#### Speedboat100

##### Well-Known Member
Lazair has made no such claim, you have not substantiated this any time you've mentioned it. Their endurance for the e-Lazair on their website is about an hour, their endurance doesn't agree with your claim.
On the video there is a claim for 2+ hrs endurance.

#### stanislavz

##### Well-Known Member
HBA Supporter
For an example with a low desired output speed and a given power, have a look at e-bike motors. Compare the weight of the direct drive and geared motors...
Yes and no... Just to certain degree. Hub motor are way to slow.

But on standart thick of silicon iron - you ho up to 100hz of frequency for ok losses. For 0.1mmlamination - this can go up to 300-400hz.

And how much rpm you will get from 400hz - depends on your poles number. But you have maximum possible force per each coil in any case.

But.... With e motor, you can make in place of one big prop, 4 smaller. Or 8.. And have much lover prop weight and inertia.

#### stanislavz

##### Well-Known Member
HBA Supporter
Was thinking, while driving to work - I do like more and more option with 4 small motors and 4 small propeller in place of one big. Or you have one 1.5 m diameter of prop, or 4 with 0.75m - same area, but with 0.75m - you can turn it 2 times more rpm with same efficiency ?

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#### Speedboat100

##### Well-Known Member
Okay...lets keep them simple.

#### stanislavz

##### Well-Known Member
HBA Supporter

So, 2-3 kw for cruise should be the target for longer distance with clean air frame..

Plus https://en.wikipedia.org/wiki/Distributed_propulsion

Recent analytic and experimental distributed propulsion studies suggest several improvements in aircraft performance.[2] They include fuel consumption efficiency, noise abatement, steep climbing for short take off and landing (STOL), novel control approaches (in particular eliminating control surfaces for roll, pitch and yaw moments), and high bypass ratios. It has also been suggested that smaller propulsors will be cheaper to manufacture and easier to handle during assembly and maintenance.[3]

#### Speedboat100

##### Well-Known Member
Yes..these are only 23:99 USD a piece..have 4 of these to fly at 7.2 KW.

I bet the propeller are sold at 5-10 USD a piece.

Props are cheap too: https://www.amazon.com/Sail-Propeller-USA-Beechwood-Propellers/dp/B01HCBMPI4

Sorry they use 17x10 props: https://www.amazon.com/Sail-Propeller-USA-Airplane-Beechwood/dp/B07RBZSGC6

That would make almost 150 usd to engines and props.

Simplicate and add lightness has to remebered in the designs.

http://www.houseoffrog.co.uk/gliders.htm

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#### Aerowerx

##### Well-Known Member
.... place batteries in wing tips....
Masses away from the CG like this does bad things to the stability.

It is better to have masses as close to the CG as possible.

#### Aerowerx

##### Well-Known Member
No cannot give calculations...air resistance comes to play too.
Ok, then. Can you give an authoritative source for this?

Or an actual working example?

#### stanislavz

##### Well-Known Member
HBA Supporter
Masses away from the CG like this does bad things to the stability.

It is better to have masses as close to the CG as possible.
Why ? I do only found for addition flutter possibility if wrongly designed. From wing spar it is life much easier, especially on long wing with high aspect ratio.

On normal situation with 100% mass at the center of wing of 200 kg and with 25% on sides and 50% at the center - you have half load at center section ? Or think as two airplanes joined by wing tips, with half of wing aspect ratio.

#### stanislavz

##### Well-Known Member
HBA Supporter
authoritative source for this?
Windage losess

Windage losses refers to the losses sustained by a machine due to the resistance offered by air to the rotation of the shaft. Windage Losses occurs in electric rotating machines such as motors and generators.

HBA Supporter

#### Speedboat100

##### Well-Known Member
Ok, then. Can you give an authoritative source for this?

Or an actual working example?

I don't think I can. But I have one in my wind turbine..that works the opposite..it rews higher than the system rotates to increase the rpms of the generator.

Planet gear in question looks like this:

Another:

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#### Speedboat100

##### Well-Known Member
In german electric flight explained:

IC engine produces only 30% of its energy to movement...whereas electric engine 95%.

#### 12notes

##### Well-Known Member
Log Member
On the video there is a claim for 2+ hrs endurance.
You mean at 3:40 or so of that video, where the interviewer says "you were up for over two hours" and the pilot responds "my timer said I was up for about 50 minutes"? The interviewer's mistaken opinion doesn't count as evidence.

#### pictsidhe

##### Well-Known Member
Masses away from the CG like this does bad things to the stability.

It is better to have masses as close to the CG as possible.
Spin recovery;
1: jetison batteries.

2