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- Thread starter OwenN
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The airframe is not radically unstable, as with some modern full-size aircraft, but has not a lot of self-correction. It will keep going where you point it, and dives on banked turns, as do quads. The motor power-up on landing saves a massive amount of throttle fiddling as you try to balance speed, drag, angle of attack, and downthrust, when landing. If you do it by hand, you are likely to thump the aircraft down rather heavily, or drop the nose too quickly.

I was looking at the new wing platform, and it could now do with an additional front spar. I was thinking of cross-laminated 1/16 fuselage rib rings, epoxy coated, and tied via 2.5 ply tab strips to doubled 1/16 webs, extending straight out and intersecting the leading edge,

This takes twisting loads off the main spar area, and reduces flex loads on the fuselage. The skinned wing design should distribute wing loading well.

Sounds like you have it all worked out good luck with it let me know how it goes.

View attachment 184504

Looks like the 9x5 triple blade prop is a good option with 60 amp ESC's you should have more than enough thrust there with two motors ?

and 4s lipo's would be ample.

1) How do you find these bench test charts?

I can't find any on Google.

2) what was the 8 inch prop mentioned? = the chart is scrambled. It seems to give more thrust and less amps.

3) what effect does varying number of blades have on thrust and amps?

Um- I have a chart for my new motor on 6s,(2808, 1900 KV) and I want to produce a motor plot that will predict performance on 4S , for different propellers. I can't find any online.

It has rpms on the table, so this should be possible.

I see that the 1500 KV version makes more thrust with less amps than the 1900 KV, with a 6 inch prop, so The 1900 KV one is underpropped with a 6 x 3.5 x 3-blade.

I worked out that the previous 2806.5 1800 KV was loaded down to 8000 rpm with a 9x5 3-blader, less than half of its potential loaded rpm.

The motor should easily spin an 8 inch on 4S, and probably do it more efficiently.

Anything I need to know? is torque directly proportional to amps?, and volts at a particular rpm?

Can I use load-rpm-thrust curves for propellers to assist with this?

Will it get over 2000 gms thrust on an 8 x 5 inch prop, 4S??

Any extra efficiency , output torque calcs I need to do?

It has rpms on the table, so this should be possible.

I see that the 1500 KV version makes more thrust with less amps than the 1900 KV, with a 6 inch prop, so The 1900 KV one is underpropped with a 6 x 3.5 x 3-blade.

I worked out that the previous 2806.5 1800 KV was loaded down to 8000 rpm with a 9x5 3-blader, less than half of its potential loaded rpm.

The motor should easily spin an 8 inch on 4S, and probably do it more efficiently.

Anything I need to know? is torque directly proportional to amps?, and volts at a particular rpm?

Can I use load-rpm-thrust curves for propellers to assist with this?

Will it get over 2000 gms thrust on an 8 x 5 inch prop, 4S??

Any extra efficiency , output torque calcs I need to do?

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The 4S battery will limit the motor to 22,800 max, or top end around 15,000 rpm

Amps, torque shouldn't be affected much in the intermediate rpm ranges.

I would expect maximum power to not be much different.

A plot of given load vs rpm doesn't say much, except that torque at the top end drops to 0.31 N-m

at 25326 rpm

Free speed, no resistance would be 38,000 rpm, so it is 66% of the way there.

I assume 78% efficiency, but part load will be much better.

on 6s, 11189 rpm A = 5, W = 100., torque = 0.6 N-m

Now, this motor may be able to draw much more amps at this speed: 50 amps corresponds to

6N!

50 amps at 15815 rpm would be 4.6 N-m

0.75 N-m is reasonable to spin a 9 x 5 prop at 8000 rpm, 875W, 54 A

at 78%, this is 0.81 N-m

I will look at some prop tables.

I rally need a maximum torque-rpm graph.

i will watch the discussion you-tube video as well.

Amps, torque shouldn't be affected much in the intermediate rpm ranges.

I would expect maximum power to not be much different.

A plot of given load vs rpm doesn't say much, except that torque at the top end drops to 0.31 N-m

at 25326 rpm

Free speed, no resistance would be 38,000 rpm, so it is 66% of the way there.

I assume 78% efficiency, but part load will be much better.

on 6s, 11189 rpm A = 5, W = 100., torque = 0.6 N-m

Now, this motor may be able to draw much more amps at this speed: 50 amps corresponds to

6N!

50 amps at 15815 rpm would be 4.6 N-m

0.75 N-m is reasonable to spin a 9 x 5 prop at 8000 rpm, 875W, 54 A

at 78%, this is 0.81 N-m

I will look at some prop tables.

I rally need a maximum torque-rpm graph.

i will watch the discussion you-tube video as well.

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Is this similar motor?

What can we find out from this graph? At 22,000 rpm, 875w - is Pi input power, Po output power?, 52.5 A, calc torque

is 0.38 N. however, if this goes to 28,000 rpm on 4S and still makes 1250w power, this is probably a higher KV at 1800 KV, max should be under 22,000 rpm. I suspect this is really 3100 KV, and a big frame number, too.

<edit.> I see that it is over-volted to 16 volts.

effy is around 69%

torque at 22,000 rpm is 0.14 N-m.

I wonder how that compares with the 2806.4 1800KV motor?

At 10,000 rpm, torque = 0.35 N-m.

This leads me to think that my calculations for the 9 inch propeller were way out, and it was using less torque, more rpms,

less drag, less flow efficiency. This could happen if dead area on the disk was bigger.

probably the inner inch and outer half inch could be considered dead? Recalc for 10,000 rpm.

F theory = 31 N actual = 21 N = 68%

W theory = 651, torq theory = 0.62, so 0.35 is 56%.-I will look at some prop charts.

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Charts- for 9x5, APC is totally uo the wop for torque-11.8 N-m at 10,000 1045 gf

Master airscrew looks better.

9x 4.5 MAS 1200 gf (or 1050, depending on graph,) 0.17 N-m, 200 w, 17 A

to get to 2100 gf, need 1.75 x thrust, or 1.32 times rpm (sqrt 1.75)

Check-calc. this give 1.88 x thrust, 13200 rpm

The torque-rpm curve is rising as a square curve, so I will add another .06, then .06 + 06, etc , for each 1000 rpm

This gives 0.17 + .06 + .12 + .18 = 0.53 N-m- looks fairly OK - my estimate might be slightly high.

From the plot above, 13000 rpm = 13/18 = 72% = 7/3 +7 = 9.3 N-cm or 0.093 N-m

Thus this prop won't spin over 10,000 rpm on this motor, and the motor only produces a fraction

of the torque of the 2806.5 1800 KV at 13,000 rpm.

So, this graph isn't much help in benchmarking the 2808 1900KV motor.

Master airscrew looks better.

9x 4.5 MAS 1200 gf (or 1050, depending on graph,) 0.17 N-m, 200 w, 17 A

to get to 2100 gf, need 1.75 x thrust, or 1.32 times rpm (sqrt 1.75)

Check-calc. this give 1.88 x thrust, 13200 rpm

The torque-rpm curve is rising as a square curve, so I will add another .06, then .06 + 06, etc , for each 1000 rpm

This gives 0.17 + .06 + .12 + .18 = 0.53 N-m- looks fairly OK - my estimate might be slightly high.

From the plot above, 13000 rpm = 13/18 = 72% = 7/3 +7 = 9.3 N-cm or 0.093 N-m

Thus this prop won't spin over 10,000 rpm on this motor, and the motor only produces a fraction

of the torque of the 2806.5 1800 KV at 13,000 rpm.

So, this graph isn't much help in benchmarking the 2808 1900KV motor.

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By my math 1900kv * 4.2v * 4cells= 31,920

You also mention 16 Volts as being over-volted. Lipo cells fully charged are 4.2 Volts per cell, meaning a 4s would put out 16.8 Volts. The absolute lowest you would run a lipo cell to is 3V per cell or 12V. I don't like to discharge mine much past 3.6V. Most people will use either 3.7V or 4.0V for their data because it splits the difference between fully charged and the average discharge level.

The rest of your posts are pretty much number-soup to me.

I think my sources were using 3.7 v, 3S = 11.1 V, 4s = 14.8 V , 6S = 22.2 V - I may be mixing up 3S and 4S and 6S test figures in some places.

I also saw figures of 10V and 14V mentioned in tests, so it is a bit of a number soup.

I think the 38,000 is for the 6S tests on the other motor??There are 3 motor versions in play, here,

and excessive guestimate cell voltages.

38000 6S would be 20 V at 1900 KV. That is mentioned on the 2808 chart (the orange one).

I have established that the motor with 4S lipo written on it is not the same as the 2806.5 1800 KV motor tested elsewhere.

Here it is! It uses 16v for 4S as well.

I would love to find out what the bottom prop here actually is!!

I also saw figures of 10V and 14V mentioned in tests, so it is a bit of a number soup.

I think the 38,000 is for the 6S tests on the other motor??There are 3 motor versions in play, here,

and excessive guestimate cell voltages.

38000 6S would be 20 V at 1900 KV. That is mentioned on the 2808 chart (the orange one).

I have established that the motor with 4S lipo written on it is not the same as the 2806.5 1800 KV motor tested elsewhere.

Here it is! It uses 16v for 4S as well.

I would love to find out what the bottom prop here actually is!!

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The 2808 1900 KV motor appears to give less thrust overall than the 2806.5 1800KV motor?

Is this just an illusion created by the different propellers and voltages?

Should they be similar in rpm-torque?? they both use a lot of amps.

I see there is a video of one being used in a big stunt plane-I will watch that.

Maybe I should bench test the motor before making the duct/shroud.

What gear would I need? just various props and a balance scale?

Is this just an illusion created by the different propellers and voltages?

Should they be similar in rpm-torque?? they both use a lot of amps.

I see there is a video of one being used in a big stunt plane-I will watch that.

Maybe I should bench test the motor before making the duct/shroud.

What gear would I need? just various props and a balance scale?

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this gives : rpm-17000

pwr 734 w 0.966 hp

torque 0.42 N-m 3.691 in-lbf

thrust - 2435 gf 4.783 lbf

This well outperforms the 8 x 5- it doesn't seem to work well at 17,000 rpm +

and also the 9 x 5 at 13,999 rpm, which is:

695 w

0.47 N-m

2145 gf

The motor is also quite inefficient at this load and revs. (2806.5 1800 KV), showing over 870 W in (79%)

4.5 pitch gives me 90 % of my pitch speed.

now I need to recreate the torque-rpm curves for the 2808 1900 KV from 6S to 4S,

using my sample curves for the 875 w 1800 KV 4S motor,

and see if it will spin these props.

I assume it has the same maximum torque, but the torque dropoff is steeper, lower at higher rpms.

Here is a reasonable estimate of torque vs rpm for the BH 2808 1900 KV motor on 4S

I didn't have the exact prop specs for the 6 x 3.5 x 3 blades originally used, but I got one close.

This shows both 9x5 and 8x4.5 can be used, and thrust will be over 2000 gf.

The real line would not be exactly straight, but close. Theoretical power goes from 624w at the fast end to 795 w at the slow end.

Actual power seems to be about 1.22 times this, or 822 to 970 w.

It can be greater at the low rpm end, where efficiency is lower.<edit> - and really bad at the top end-see the other graph with the red line!

(4S Lipo- 875w, 1800 KV motor).

The "Bad" zone shows combinations of torque and rpm which cannot be generated.

Where the prop lines cross the torque-rpm line is the rpm of the prop at full power.

I didn't have the exact prop specs for the 6 x 3.5 x 3 blades originally used, but I got one close.

This shows both 9x5 and 8x4.5 can be used, and thrust will be over 2000 gf.

The real line would not be exactly straight, but close. Theoretical power goes from 624w at the fast end to 795 w at the slow end.

Actual power seems to be about 1.22 times this, or 822 to 970 w.

It can be greater at the low rpm end, where efficiency is lower.<edit> - and really bad at the top end-see the other graph with the red line!

(4S Lipo- 875w, 1800 KV motor).

The "Bad" zone shows combinations of torque and rpm which cannot be generated.

Where the prop lines cross the torque-rpm line is the rpm of the prop at full power.

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Related to static-thrust is stall-thrust. If the prop has a pitch steep enough relative to the rpm, the prop blades can stall and will generate a lot of drag (high amp draw) and significantly less thrust than a lower-pitched or slower turning prop. Once the aircraft gets to a certain airspeed the blades will un-stall and generate better thrust numbers and draw less amperage. This may have some relation to why the 1900kv motor produced less thrust than the 1800kv in testing.

I don't know the formulae for determining the speed at which a prop un-stalls, but eCalc and various other prop calculators will generate those numbers for a given setup; so there is a mathematical way of determining it.

Related to static-thrust is stall-thrust. If the prop has a pitch steep enough relative to the rpm, the prop blades can stall and will generate a lot of drag (high amp draw) and significantly less thrust than a lower-pitched or slower turning prop. Once the aircraft gets to a certain airspeed the blades will un-stall and generate better thrust numbers and draw less amperage. This may have some relation to why the 1900kv motor produced less thrust than the 1800kv in testing.

I don't know the formulae for determining the speed at which a prop un-stalls, but eCalc and various other prop calculators will generate those numbers for a given setup; so there is a mathematical way of determining it.

I think the reason for the poor-looking 2808 motor performance is that the small, high speed prop produces less static thrust for a given power( P=TV, + friction, drag) and that it is probably less efficient than a larger prop spinning slower. The 65% rpm range on the motor seems to be quite efficient.

Regarding using high-pitch props to get the pitch speed to go 200 + mph, the only cure for that is a variable pitch prop.

Otherwise, you just have to put up with poor low-speed thrust.

<edit>

A sign of a prop stall? no obvious sign-loss of thrust compared with less pitch setting-failure to take off, with a full sized aircraft with pitch control.-been doing some google lookups!

A pylon racer at 30,000 rpm with 6 inch pitch doesn't seem to stall the prop.

I see you can get up to 14 inches pitch, on APC.

I suppose this is handy for a large scale model warbird, which would be limited to under 10,000 rpm with a 250cc single motor?

In theory those could do 18,000 rpm, too, but that would be a racy 2-stroke with a pipe- not a model airplane motor.

If they use a 24 inch propeller, they want to keep it well under about 11,000 rpm.

(speed of sound at the tips).

To get to 200 mph they wold need 3x the pitch of the pylon racer, or 18 inches?

(pitch speed of 230 mph)

Do model aircraft props even come in that pitch? - No-one trying to make those big models fly fast?

The larger pitch numbers APC show are probably mixed units-inches diameter, and mm pitch.

Those WW2 fighters with 13 foot constant speed props that go over 450 mph must get up to some huge pitches at speed?

Maximum prop speed = 1500 rpm.

Prop plane speed seems to be ultimately limited to 550 mph-the extreme pitch needed must be inefficient, plus

prop spinning speed adds to aircraft speed, and hits the speed of sound, then prop efficiency really drops off.

Supersonic props don't seem to work well, even if they are thin and curved. The sonic transition zone on the prop probably causes huge drag.... One guy was building a speed plane with motor direct drive, but he gave up.-possibly couldn't get flight approval??

Also his mate that he wanted to fly it got killed in an air crash. They do seem to have a few horrible crashes with unlimited pylon racing!

https://www.popularmechanics.com/flight/how-to/g631/4-amazing-diy-planes-and-how-to-build-your-own/

David Rose:

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