Size & Pitch of Propeller


#1

@Pdwhite, have you launched from higher altitudes like 4,500 above sea level? Do you see any need to have a higher pitch prop for that altitude? Will that be available in the kit?

My understanding is that with thinner air (at higher altitudes) the craft not only needs to fly faster but it’s also easier to fly faster to achieve the same amount of lift. This can be done with a higher pitch prop and won’t draw more amps than the same craft at sea level with a lower pitch prop.

Of course the inverse is also true and perhaps more critical to consider… if a craft was designed and tested at a higher altitude then flown at sea level without adjusting the prop pitch, it would draw more amps and perhaps damage the equipment.


#2

The vast majority of paramotors today don’t have or come with an adjustable prop. You are correct in that the air is thinner with higher elevations, but you have to be quite high, like 10000 ft. to have any significant effect on thrust. The props that are shipping with the kits are quite versatile and can definitely handle 4500 ft.

I think most of the reason you asked about altitude is because with a gas unit you do have to adjust the air-fuel mixture or you could damage your gas motor. Which is another benefit of electric - you don’t have to change that no matter how high or low you are. As far as speed in higher elevations that is easy to change to whatever speed range you want by getting a slightly larger wing. Hope that helps clear up your question.


#3

Yes, I love electric for many reasons including not having to worry about air mixtures as the altitude changes.

My experience with this topic is with electric RC. The amp draw for a given setup can be tuned by selecting different prop sizes. The idea being to draw more amps for more power without going over what the system can handle. When this is optimized at a lower altitude the power suffers when at a higher altitude but the amp draw is lower as well. After changing to a higher pitch prop the power is restored and the amp draw is back to what it was. Perhaps this is more significant with smaller scale RC.

I am very excited to order your kit and think it is very nicely designed! I have been dreaming of owning an electric paramotor for many years now! Thank you so much for being so happy to share your passion with us!


#4

@Pdwhite, I just noticed you listed 2 different prop pitches in the specs (10 and 14) with 2 different thrust estimates (150 and 165) but the ones you are including in the kit are the lower pitch ones. Is that because it’s a safer amp draw? Or is there another reason behind that like noise or efficiency?

Would it be possible to get the higher pitch props in our kits upon request?


#5

AMP draw is slightly higher but so is thrust, I can add them to your kit if you would like. We also will have both prop size available on the store if you want to try the other.


#6

+1 for the higher pitch if it doesn’t have too much negative effect.


#7

@Pdwhite, please do ship my kit with the higher pitch props. I ordered a batch 2 kit. Will this message suffice or is there a preferred method for specifying this detail?


#8

Assuming 19kg thrust per prop, each prop will require around 4.3kW of mechanical power. With the drive as is, the efficiency of inverter and motor will not exceed 80%, it will need around 5.4kW of electrical power, so the ESCs will each need to handle 121A on a good day, and the batteries 484A.


#9

Ill get that updated for you.


#10

I have tested the A draw and it is not 121 but closer to 110A. The ESC can handle that current with adequate cooling (which it has and its mounted on the arms and have plenty of airflow during flight). The batteries also can easily handle that current also. Even with just two Bonka 22000mah packs, they have a continuous discharge of 550A and a burst discharge of 1100A. http://www.bonkapower.com/a/Multicopter/20150825/11.html


Lukas Build Log
#11

Hello Paul
You are suggesting 22x10 CW and CCW wood pairs
What is the limit in diameter ?
Is a 23x08 (CW or CCW) passing physically in rotation (free tip airgap) ?
Louis


#12

@Pdwhite Please update mine also…thank you. (order #1663)


#13

A 23in will just barely fit. There about a 1.5 gap between props now.


#14

Is it worth specifying a 100A ESC for the higher thrust prop? It would then put the current within the 10 second rating of the ESC.


#15

I have been investigating prop sizes and a 22 x 10 prop can typically develop 35lbs thrust @ 7450 RPM which gives a total of 140lbs of thrust. As yet I haven’t found any motors that can deliver that rpm for a 22 x 10 prop even the best American motors by KDE can’t deliver that power. Is the 155 lbs max thrust a judgement call or has this been measured?


#16

There has been a change to show an optional 22x14 prop, which should deliver the originally specified thrust, albeit with a higher current draw. Paul has suggested that this was just over 110A per motor with the 22x14 prop (I get closer to 120A, but I am assuming a lower efficiency from the Block Commutating ESC drive). Getting 19kg of thrust from a 22x10 propeller was ‘optimistic’ - It confused me for quite a while (I thought that I had missed something in my efficiency calcs, as it put them at over 100% at some points). There are quite a lot of incorrect manufacturer quoted figures in the battery, ESC, motor and propeller specs that you get for RC stuff (always optimistic), but that’s because they don’t normally get used on something that is connected to a human. The paramotor is a prototype, so test it very hard on the ground with your setup before strapping it to your back (particularly battery, ESC and motor). Take a typical flight of full thrust for 2 minutes (900ft climb), then 40% for how long you would want to altitude hold, and play that profile in to the paramotor. Personally, I would put a bigger ESC on the motors, and one which is phase and bridge current limiting. Normal ESCs don’t understand inertia, so they will often voltage control only, and exceed both battery and phase current limits during throttle transients.


#17

What motor can drive a 22x14 prop to produce a thrust of 18kg?


#18

There are a number of >5kW machines around. Ideally a 7+kW machine would be best, just make sure that the bearings are arranged such that they can react push force (most are set up for pull only)


#19

I have only found ones made in China and reliability looks questionable can you suggest some that might be suitable.


#20

I can’t imagine that you will find anything that isn’t made in China, even if it has a different badge! The original (non-custom) machines were these I believe http://www.dualsky.com/motor/GA_Series.shtml GA6000.8 They claim 6.8kW with 180rpm/v gain. No idea if the custom machines have the same continuous power.