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DTpropeller - Carbon and wooden propellers for paramotor and paramotor trikes
I have not tried one yet, they seem to offer some fancy tipped props.

Gyrocopters are pretty noisy if my memory serves

6 hours ago, Bob27 said:

All 40 units of batch one where pre-ordered months ago

Lets hope we get an unaffiliated Youtuber giving us some real world feedback on what its like to live with one.
 

6 hours ago, Bob27 said:

and they haven't even shipped yet.

 That would make me a little nervous but fingers crossed.
 

6 hours ago, Bob27 said:

Meant to say 1,000 hours.

100hrs was a bit low, 1000hrs is quite optimistic without any practical evidence.
The potential is there especially for motors... Not sure yet about controllers though.
 

6 hours ago, Bob27 said:

As mentioned before the biggest reason it's not done is that in increases torque (by about 30%)

All torque reaction is wasted energy so that 30% increase in induced blade drag is a dead loss, like i said, higher pitch seems to come at a cost.
Its not something I would do on a machine that needs every efficiency gain it can get its hands on.
Frames diameters can be made to fit larger props.
Airconception have a 160 frame
 

7 hours ago, Bob27 said:

and other than noise there isn't really any advantage for an ICE paramotor.

You have lost me here... Prop diameter has the single largest effect on efficiency... Electic, IC or even human powered

 

 

7 hours ago, Bob27 said:

The only way to reduce this noise is to reduce the speed that the tip of the prop is traveling.

At the expense of efficiency.
I don't know the fundamental's in detail... It seems there is a "sweet spot" between diameter, pitch, chord, profile and speed, its not trivial.... Think best L/D on a sailplane wing.
With the diameter being the strongest influence (all thing being equal) Think aspect ratio on a sailplane.
I suspect you may be focusing on one item (noise) and neglecting the effects of doing so.

Electric has some problems to solve and noise is NOT one of them :) 

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5 hours ago, Blackburn Mark said:

All torque reaction is wasted energy so that 30% increase in induced blade drag is a dead loss, like i said, higher pitch seems to come at a cost.

It is not wasted energy. The output of an engine comes in torque and rpm. So if the gear reduction changes by 30% to drop the propeller rpm from 3000 rpm to 2300 rpm the torque output will be increased by about 30%. The prop size pitch, blade count, and width can then be increased to provide the same thrust at a lower rpm, but but at a higher torque. Your still putting the exact same power into the propeller and getting nearly identical thrust out and with nearly identical efficiency, but one side effect is the paramotor will torque more.

5 hours ago, Blackburn Mark said:

You have lost me here... Prop diameter has the single largest effect on efficiency... Electic, IC or even human powered

I was referring more to the lower rpm combined with increased pitch and blade "thickness". Of course going to a large prop will definitely help with efficiency, but most people will tell you that it gets pretty hard to foot launch with anything much bigger than a 140cm prop. Naturally this would be mostly based on height of the pilot, but a 160cm prop foot launch PPG would not work well for most people.

 

5 hours ago, Blackburn Mark said:
13 hours ago, Bob27 said:

The only way to reduce this noise is to reduce the speed that the tip of the prop is traveling.

At the expense of efficiency.

Not true. Go look at any well developed paramotor and do some research into their power systems. If you look, you will notice with few exceptions that they all have motors wound to peak at almost exactly 2300 rpm, and they almost all have 140 or 150cm 3 or 4-blade, high pitch props. This is the recipe that has been discovered for creating the most efficient E-ppgs. I am personally most familiar with the motor side of this as that is what I work on. From a motor standpoint, that 700 rpm drop give us an extra ~4% efficiency. So even if the propeller might be 4% less efficient (which it's not) and cancel out the efficiency gained from the motor, most electric manufactures would still opt for the lower rpm system.😉

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2 hours ago, Bob27 said:

So if the gear reduction changes by 30% to drop the propeller rpm from 3000 rpm to 2300 rpm the torque

You have us speaking to different points.
I did ask you if you meant output torque or frame torque, you fail to clarify so i assumed you meant 30% more frame torque.

 

2 hours ago, Bob27 said:

Your still putting the exact same power into the propeller and getting nearly identical thrust out and with nearly identical efficiency, but one side effect is the paramotor will torque more.

.....?
Now you "are" talking about frame torque...An increase in frame torque = a decrease in efficiency, it cant be anything other than an "increase" in blade drag (THATS the "dead loss" I was talking about)
What do you think causes frame torque?

 

2 hours ago, Bob27 said:

So even if the propeller might be 4% less efficient (which it's not)

 Is that an educated guess or have you something to link to that shows it to be true?
I would much appreciate a look at that evidence as it contradicts my "An increase in frame torque = a decrease in efficiency" claim above.


 

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2 hours ago, Blackburn Mark said:

Now you "are" talking about frame torque...An increase in frame torque = a decrease in efficiency, it cant be anything other than an "increase" in blade drag (THATS the "dead loss" I was talking about)
What do you think causes frame torque?

For everything action there is an equal and opposite reaction (basic physics). To generate thrust a propeller pushes air backwards. In turn the air pushes back on the propeller trying to slow it down. The engine then uses it's power to spin the propeller. This means that you could have a 100% efficient paramotor propeller and as long as it is pushing air there would still be torque at the frame. If this was not the case then propellers would be perpetual motion devices.

 

2 hours ago, Blackburn Mark said:
5 hours ago, Bob27 said:

So even if the propeller might be 4% less efficient (which it's not)

 Is that an educated guess or have you something to link to that shows it to be true?

I just sent a message to the man who currently holds the world record for foot launch E-PPG on this topic. He often builds his own propellers and tests everything, so I Will be interested to hear actual numbers.

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6 minutes ago, Bob27 said:

This means that you could have a 100% efficient paramotor propeller and as long as it is pushing air there would still be torque at the frame.

That makes no sense to me.
A 100% efficient propeller would have zero frame torque besides the momentary "spin up" torque.

All things being equal, 100% of frame torque is caused by blade drag, mainly "induced" seconded by "form drag" followed by "skin drag" (Think this is true of subsonic)
Increasing pitch increases the former and reduces the latter two as the speed falls, all of them acting in opposition to the blades travel (Action)
What other possible action in "basic physics" could invisibly cause this particular frame "reaction"?

 

14 minutes ago, Bob27 said:

I just sent a message to the man

Cool, maybe he can throw some light :) 

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3 hours ago, Blackburn Mark said:

That makes no sense to me.
A 100% efficient propeller would have zero frame torque besides the momentary "spin up" torque.

I am not great at articulating my words as you have seen, but let me try to explain. I think that I could best tell why the torque is required rather than the physics behind how it works.

I hope we can agree that if you put 10 foot pounds of torque into the propeller, the cage will have 10 foot pounds torque in the opposite direction(back to every action has an equal and opposite reaction). 

I hope that you also understand that torque x rpm= power

So to begin, an typically paramotor with a large and efficient wing and a medium-light weight pilot requires about 2 kW (2.68 horsepower) constant energy to stay in the air. A paramotor propeller is typically about 50% efficient so we will go with that number. So because of the prop efficiency we will need to put 4kW power into the propeller. At 4kW engine power the average paramotor propeller will spin at about 1800 rpm. If we run the math we get that the propeller is receiving 5.36 horsepower (4kW) at 1800rpm and 15.6 foot pounds torque. This also means that the torque at the frame is 15.6 foot pounds. If we then go back to our 50% propeller efficiency we can see that of the 15.6 ft lbs torque, 7.8 ft lbs torque is used to fight drag, but the other half is used to move the air to create thrust. This also means that even if the propeller is 100% efficient we would still need 2kW and 7.8 ft lbs torque to fly. If it required no torque to fly with a 100% efficient propeller, then you would be getting 2kw energy out of a propeller that you are putting no energy into which is impossible.

This also means that if you swap propellers to create the same thrust at say half the rpm with equal efficiency, you will need double the torque (5.36 horsepower @ 900 rpm = 31.2 foot pounds torque vs 5.36 horsepower @ 1800 rpm = 15.6 foot pounds torque. It doesn't meant that it's any less efficient, it just means that you are exchanging rpm for torque.

(By the way, I never told you this, but i think your profile picture is fascinating👌)

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6 hours ago, Bob27 said:

If we then go back to our 50% propeller efficiency we can see that of the 15.6 ft lbs torque, 7.8 ft lbs torque is used to fight drag,

If that "7.8ft lbs" is to fight drag, that would be 7.8ft lbs of frame torque... At 100% efficiency, that 7.8ft lbs would not be lost in drag and would be added to our thrust instead leaving us with no frame torque.
 

6 hours ago, Bob27 said:

even if the propeller is 100% efficient we would still need 2kW and 7.8 ft lbs torque to fly. If it required no torque to fly with a 100% efficient propeller, then you would be getting 2kw energy out of a propeller that you are putting no energy into which is impossible.

A 100% efficient propeller would ONLY transfers 100% of the input torque, nothing more.
Furthermore, a 100% efficient propeller would have zero drag, turning 100% of that torque into perpendicular thrust.
Where would the frame torque reaction be coming from?

A 100% efficient prop would act more like a 90 degree bevel gear with a road wheel on it pushing us forward, all the torque actions and reactions would be contained within the assemblies frame of reference putting 100% of the output torque into forward thrust only... No frame torque.

Airfoil_lift_and_drag.thumb.jpg.23ad949eea4e2b411e73b69b7e225190.jpg

Removing the drag would place all the force at 90 degrees to the output shaft... At 100% efficiency, there is nothing there to cause frame torque... Nothing there acting in opposition to the blades travel (Newtons third law :) )

I must admit that I am still scratching my head looking for stray lift vectors... Cant see any at the moment.

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I got a reply back from the foot launch E-PPG record holder.

Here was my question: "I have noticed that most E-ppgs tend to run around 2300rpm at the prop compared to the ~3000rpm at the prop of most ICE paramotors. I know this helps with both noise and motor efficiency, but I was curious if their is any efficiency gained or lost from running a higher pitch, “thicker” 3 or 4-blade prop at lower rpm compared to running a “skinnier”, lower pitch 2-blade at higher rpm."

His response(his native language is german): "in europe, the highly efficient eppg with 2200-2300 rpm are only operated briefly for start-up. (2300-2400 for heavy trike). in an efficient climb it is around 1900-2000 rpm. in the level around 1500 -1600 rpm. Depending on the trim speed of the wing, 3 blades are usually best with a circular area coverage of around 5.5% and a pitch speed of approx. 55 -6o km / h at full throttle. in the level, the pitch speed is the ideal value for maximum efficiency. petrol ppg have never really been interested in noise reduction or efficiency. with a few exceptions. At the moment, however, a very clear trend can be seen with some manufacturers to limit the speed to a maximum of 2200 rpm and to increase the prop diameter to 140 cm. the first are already flying and are extremely quiet and very efficient. it’s going in the right direction with some. I think it’s very good."

1 hour ago, Blackburn Mark said:

Airfoil_lift_and_drag.thumb.jpg.23ad949eea4e2b411e73b69b7e225190.jpg

Removing the drag would place all the force at 90 degrees to the output shaft... At 100% efficiency, there is nothing there to cause frame torque... Nothing there acting in opposition to the blades travel (Newtons third law :) )

In your diagram the drag vector is for both the drag caused by the boundary layer, tip vortexes, etc and the other part is drag caused by the lift. Even if the propeller is 100% efficient most of the drag will still be present when thrust is created.

Imagine it like a glider climbing at an angle. If it doesn't have any power input, it will eventually run out of airspeed and stall. That glider angle is the pitch of the propeller and the gravity pulling it down is the drag on the paramotor that the propeller is creating thrust to push against.

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7 minutes ago, Bob27 said:

if their is any efficiency gained or lost from running a higher pitch

That's a shame, he doesn't seem to answer the question you asked.
Instead he seems to be speaking to "relative" efficiency in regards to the given diameter and attainable thrust: EG: three blades can deliver more thrust at 140cm diameter than a two blade... He says nothing about the cost in requiring extra torque to do that.
Two blade props are more efficient than three in regards to the "torque to thrust" efficiency that we are grappling with.

 

23 minutes ago, Bob27 said:

petrol ppg have never really been interested in noise reduction or efficiency.

Suddenly I find myself distrusting the man, he is over-egging his sales pitch to the point of giving himself away with this statement!
An absurd thing to say.
 

 

27 minutes ago, Bob27 said:

Even if the propeller is 100% efficient most of the drag will still be present when thrust is created.

....?
It wouldn't be 100% efficient if there is any drag at all!
Induced drag (lift drag) increases with pitch.
Induced drag reduces with aspect ratio.
Three blades have one more tip to shed an induced drag vortex.

 

 

1 hour ago, Bob27 said:

At the moment, however, a very clear trend can be seen with some manufacturers to limit the speed to a maximum of 2200 rpm

For all I know, that could be to do with the inverse torque curve of electric (in comparison to IC) in the attempt to avoid the need for a reduction drive... It certainly sounds like it.
Pretending that its a deliberate attempt to be quiet would be a posh way of hiding that higher pitch = less efficiency and a redrive (or broader motor) + less pitch would give more flight time... On a heavily priced paramotor, I would expect a little more focus on the "real" limitations and a little less on the ancillary concerns.

You will have to excuse my skepticism, keeps me warm at nigh :) 

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33 minutes ago, Blackburn Mark said:

You will have to excuse my skepticism, keeps me warm at nigh :) 

I can totally understand that.

 

Can we agree that to get 10 horsepower worth of energy out of a 100% efficient propeller we need to put 10 horsepower of energy into it?(that's the definition of 100% efficient). As you may know the equation for horsepower is foot pounds torque x rpm x 5252 = horsepower. This means that if a 100% efficient proper needed no torque it would be creating infinitely more energy than you are putting into it. (1000rpm x 0 ft-lbs x 5252 = does not 10hp (10hp @ 10,000rpm = 5.25 ft pounds torque)) So mathematically you can see that to not break the laws of physics, a 100% efficient propeller would need to convert torque to thrust by some means. Infact the best modern airplane propellers are exceeding 90% efficiency which means that scientifically speaking 90% of their input torque needs to be converted into thrust.(10 hp input equals 10,000rpm x 5.25 ft-lbs torque x 0.9 efficiency x 5252 constant = 9 hp energy output.) I hope this can help you see that from a mathematical and physics standpoint that torque needs to be converted into thrust to have a net power output. 

If this was not the case explain how a windmill produces torque to drive a generator. The way that a windmill takes energy from moving air to generate torque is the same process, but in reverse that a propeller uses torque to make air move.

1 hour ago, Blackburn Mark said:
2 hours ago, Bob27 said:

At the moment, however, a very clear trend can be seen with some manufacturers to limit the speed to a maximum of 2200 rpm

For all I know, that could be to do with the inverse torque curve of electric (in comparison to IC) in the attempt to avoid the need for a reduction drive... It certainly sounds like it

If you read carefully what he said, you will see that he was specifically referring to ICE paramotors for the quoted section.

 

1 hour ago, Blackburn Mark said:
2 hours ago, Bob27 said:

petrol ppg have never really been interested in noise reduction or efficiency.

Suddenly I find myself distrusting the man, he is over-egging his sales pitch to the point of giving himself away with this statement!
An absurd thing to say

Think about it, you yourself said larger propeller is more efficient, so why have almost no ppg manufactures gone to a 140+ cm props? He isn't wrong, most paramotor manufactures in the past have been more concerned about the torque and size than noise and efficiency. It's just one trade off they have decided to make.

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Just to confuse the issue a bit (g), consider two possibilities, both conceivable with electrics, neither feasible with shaky ICE engines: either counter-rotating props or ducted fan. Neither would produce any steady state torque (well, ducted might need laminating vanes behind the prop). Ducted would also have a benefit in that what noise is emitted would be primarily back from the motor, rather than radiating out to the sides.

Current torque compensation strategies rely on airfoil struts (scout/nirvana), lamels (draggy little things, but easily retrofitted) or various flavors of weight shifting, which distorts the wing, effectively moving the wasteful drag up to the wing and away from the motor.

Both noise and torque compensation options would open up if we got well balanced, smoothly running electrics.

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5 hours ago, Bob27 said:

This means that if a 100% efficient proper needed no torque it would be creating infinitely more energy than you are putting into it.

I have absolutely no idea where you are getting the idea that i think I am claiming to get more out than I put in... Can you quote where I have said this please.

 You have wasted quite some ink on debunking and argument I have not made.
 

5 hours ago, Bob27 said:

If you read carefully what he said, you will see that he was specifically referring to ICE paramotors for the quoted section.

That's open to interpretation and considering that he says "petrol ppg have never really been interested in noise reduction or efficiency"  its reasonable of me to infer that he meant EPPG 
 

5 hours ago, Bob27 said:

so why have almost no ppg manufactures gone to a 140+ cm props?

Now you are contradicting what you think your man with the plan is saying.
Which one is it?

This would be a whole lot easer if you just explained where you think the "frame" torque originates.... Or more precisely, what is acting in opposition to the propeller travel in order to cause the paramotor frame to react in an equal and opposite fashion? 
I'm saying it is all drag which means the additional pitch and frame torque on the SP140 you mention is an indicator that the designer may have got his priorities wrong for a motor that already has a very short flight time.

I suspect its more to do with deleting the need of a redrive (or higher diameter motor) and attempting to butter over that by pretending it is a deliberate design feature for lowering noise (Sales spin)
If I am correct (you haven't demonstrated my error yet) , $7320 with such a small parts count and they cut corners at the expense of efficiency, its a bad look for a machine that needs every inch

I had a look at Open PPG... They also sell a painfully inefficient multi propeller design unit.
At least they are moving in the right direction :) 
 

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2 minutes ago, Ganbatte said:

counter-rotating props

Less efficient than a simple two blade
Four wig tips.
 

3 minutes ago, Ganbatte said:

ducted fan.

less efficient unless you go for the full 140cm ducted fan... That would make it quite bulky and hard/expensive to make :) 

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1 hour ago, Blackburn Mark said:

I'm saying it is all drag which means the additional pitch and frame torque on the SP140 you mention is an indicator that the designer may have got his priorities wrong for a motor that already has a very short flight time.

I suspect its more to do with deleting the need of a redrive (or higher diameter motor) and attempting to butter over that by pretending it is a deliberate design feature for lowering noise (Sales spin)
If I am correct (you haven't demonstrated my error yet) , $7320 with such a small parts count and they cut corners at the expense of efficiency, its a bad look for a machine that needs every inch

I would disagree with all of this. I am helping a different company design an electric paramotor that will be a direct competitor with the SP140 and Thomas Brandstetter who I previously mentioned his reply to my question (holds the E-ppg world record) is not affiliated with them either. So neither of us have a reason to blindly defend them, but I will say they did a great job at designing the SP140.

I know you still don't understand it and I can't make you, but us electric paramotor designers understand that lower rpm systems are inherently quieter and more efficient with the sacrifice of torque felt at the frame which can be counteracted by weight shift and a variety of other methods. This was not a mistake on the part of OpenPPG. 

1 hour ago, Blackburn Mark said:

I'm saying it is all drag which means the additional pitch and frame torque on the SP140 you mention is an indicator that the designer may have got his priorities wrong for a motor that already has a very short flight time.

Not sure where you are getting this, there are only two other E-ppg currently on the market capable of 4kw cruise. This means the SP140 is incredibly efficient.

1 hour ago, Blackburn Mark said:

I suspect its more to do with deleting the need of a redrive (or higher diameter motor) and attempting to butter over that by pretending it is a deliberate design feature for lowering noise (Sales spin)
If I am correct (you haven't demonstrated my error yet)

Also makes no sense. A reduction drive reduces the rpm and allows for bigger props, so If it was about getting rid of a reduction drive they would be using a smaller, less efficient, louder, and higher rpm prop. Instead they are using a very large and high pitch 3 blade just to improve efficiency and in turn they are spending a ton more money on one of the highest power motors available (mad M50 40kv motor to be exact).

 

1 hour ago, Blackburn Mark said:

$7320 with such a small parts count and they cut corners at the expense of efficiency, its a bad look for a machine that needs every inch

OpenPPG has done an incredible job at getting such a tight profit margin to allow this to be by far the best bang-for-your-buck E-ppg on the market. They are using a good well designed motor, high quality motor controller(apd 300a esc), molicel P42A li-ion battery packs(one of the best li-ion cells out there), a good frame, one of the best throttle systems on any paramotor in the world. It may not be to the quality level of a rolls-royce, but it is still an E-ppg that will last a long time and leave the buyers happy.

 

1 hour ago, Blackburn Mark said:

I had a look at Open PPG... They also sell a painfully inefficient multi propeller design unit.

You got that right.

 

1 hour ago, Ganbatte said:

counter-rotating props

Primarily difficult as it increases weight and rotational mass which leads to a bad throttle response

1 hour ago, Ganbatte said:

ducted fan

Looked into this, but there are legitimate safety reasons why it is not done and as mentioned previously, it would lead to an increase in weight and cost.

 

1 hour ago, Ganbatte said:

effectively moving the wasteful drag up to the wing and away from the motor.

For some reason people can't get their head around this, the torque you are feeling in the frame is not a waist of energy. It is equivalent to pushing a heavy shopping cart. You feel it "pushing back on you" because you are putting energy into it, but just because you feel it doesn't mean that it's wasteful. Anyway, finding a way to cancel that torque (and gyroscopic precession) would be very nice.

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4 minutes ago, Bob27 said:

I know you still don't understand it and I can't make you, but us electric paramotor designers understand that lower rpm systems are inherently quieter and more efficient with the sacrifice of torque felt at the frame which can be counteracted by weight shift and a variety of other methods. This was not a mistake on the part of OpenPPG. 

Are you sure I don't understand?
When you say "more efficient" in this particular context, are you sure you are not conflating two entirely different regimes?
More frame torque = more lost energy, there is no escape from that... You cant get something for nothing and frame torque is not nothing!

 

15 minutes ago, Bob27 said:

Not sure where you are getting this, there are only two other E-ppg currently on the market capable of 4kw cruise. This means the SP140 is incredibly efficient

What has that got to do with nub of our disagreement?

 

 

20 minutes ago, Bob27 said:

Also makes no sense. A reduction drive reduces the rpm and allows for bigger props, so If it was about getting rid of a reduction drive they would be using a smaller, less efficient, louder, and higher rpm prop. Instead they are using a very large and high pitch 3 blade just to improve efficiency and in turn they are spending a ton more money on one of the highest power motors available (mad M50 40kv motor to be exact).

A redrive can be fitted to allow for any ratio in synchronizing propeller efficiency and motor efficiency.

Typical paramotor engines = £2500
mad M50 40kv = £1100 retail,
£800 of molicel P42A li-ion batteries =  4000kwh (ish)
That's £600 to cover the ESC
I will tentatively concede that the price point "may" be comparable ish with IC

 

 

1 hour ago, Bob27 said:

For some reason people can't get their head around this, the torque you are feeling in the frame is not a waist of energy.

That depends on how you look at it... More frame torque is indicative of a loss in efficiency just like an increase in resistance when pushing a trolley if a bearing sticks... its lost some efficiency.
For some reason, people cant get their head around that :)  
 

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38 minutes ago, Blackburn Mark said:

More frame torque = more lost energy

You could put a rock on one side of your cage to create the same force of torque, but your not loosing energy to it.

 

38 minutes ago, Blackburn Mark said:

That's £600 to cover the ESC

Actually the standard APD 24s 300a esc is $1500usd and on top of this they had APD upgrade the heatsink and and a few other components for better performance. So I would say at least $1600 or about £1100. That still doesn't cover the BMS(£150), aluminum battery pack case(£150), battery pack assembly cost(£200), other electric systems such as throttle, display,flight controller, etc(£100), and the motor/battery mounts(£100). At this point we are easily up to £3650 just for the power system to replace a £2500 engine. Trust me, I have built two E-ppgs already (actually just finishing up the second) and I know how amazing it is that they have reached the price point that they have.

 

38 minutes ago, Blackburn Mark said:

More frame torque is indicative of a loss in efficiency just like an increase in resistance when pushing a trolley if a bearing sticks... its lost some efficiency.

Except it's like pushing a trolley up a ramp, over 50%  of the resistance is due to pushing it up the hill, the rest is due to metaphorical bearings resistance.

 

38 minutes ago, Blackburn Mark said:

For some reason, people cant get their head around that :)  

😝

Edited by Bob27
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5 hours ago, Bob27 said:

You could put a rock on one side of your cage to create the same force of torque, but your not loosing energy to it.

That doesn't change the facts.
Energy is being burned to sustain that frame torque... The propeller is like a slipping clutch holding a rock off the floor.
 

5 hours ago, Bob27 said:

Actually the standard APD 24s 300a esc is $1500usd

They must be hiding gold in there!

 

5 hours ago, Bob27 said:

Except it's like pushing a trolley up a ramp, over 50%  of the resistance is due to pushing it up the hill, the rest is due to metaphorical bearings resistance.

........?


If you do build another, and you want to attempt to out perform the "current trend", it might be worth you getting to the bottom of the conundrum as you seem to be either dismissing or trivializing an aria of great importance… That pesky flight time :) 

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FULLTEXT01.pdf (diva-portal.org)

This seems to somewhat corelate with your basic premise... Not sure if you have read it or the person giving you information has read it (I suspect so)
It is dry, complex and boring so I have yet to parse (if I ever do) the efficiency statements that at first glance, "seem" to contradict contemporary thoughts and theories.

So in inference, I suspect, to our untrained eyes, that (for example) the statement that a "three blade propeller" is more "efficient" is in reference to a confined scope (EG: combined static and cruise) 
We are pretty much a single speed (very slow) machine so the gap between static and cruse is small in comparison to the Pipistrel in this paper.

 

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