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May 15, 2018, 06:15 AM
Jack
jackerbes's Avatar
One way to compare thrust for different props is to hold the plane with the nose up and (for safety purposes) above the level of your head and eyes. Then slowly open the throttle until the thrust causes it to become weightless in your hand or even tries to lift you hand higher. The prop that does that at the lowest throttle setting is the prop that will deliver the most thrust in flight.

For theoretical performance data you can look at the APC performance data here: https://www.apcprop.com/technical-in...formance-data/

If you can't find the exact same diameter, pitch, and type, look at the one that is most similar to what you have. Here is an example of two prop at 6,000 RPM:

8 x 4.5 SF = PER3_8x4.5SF.dat = 6,000 RPM = 0.720 Lbf

6 x 5 Thin Electric = PER3_6x55E.dat = 6.000 RPM = 0.178 Lbf

You'll see that the 6 x 5 TE prop does not deliver the same thrust as the SF prop does at 6,000 RPM until it get up to around 12,000 RPM.

So then you can consider all the other factors (Kv, pack voltage, no load RPM, prop RPM limits, etc.) and see if you have a power system that will get the motor up to that higher RPM if you want to use that smaller, non-slow-fly type, prop.

RPM limits are important!

APC Thin/Folding Electric = 145,000 RPM / Diameter (inches) - 6" prop = 24,166 RPM max
APC Multi-Rotor Props = 105,000 RPM / Diameter (inches) - 6" prop =17,500 RPM, 8" prop = 13.125 RPM
APC Slow Fly = 65,000 RPM / Diameter (inches) - 8" prop = 8,125 RPM max

For the most part, if you look at the testing data at flybrushless.com you'll see we only use the APC props at about 75% of their rated max RPM. And that is because they become insanely power hungry and inefficient at those higher speeds and in static testing.

Generally speaking, when you have a motor that has a Kv that is well matched to the pack voltage you will find that the maximum safe throttle setting (the throttle setting that give you a moderately high but safe and no longer rising temperature in the windings) that is about 75% of the no load RPM. And with that same setup, if you go to full throttle you will get about 90% of the no load RPM but the temperature in the windings will start rising so you only want to use full throttle in short bursts, not continuously, or it may burn up the motor.

Jack
Last edited by jackerbes; May 15, 2018 at 12:19 PM.
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May 15, 2018, 11:19 AM
flyin' fool
goldguy's Avatar
and ........................ buy a wattmeter.
May 15, 2018, 11:32 AM
It seemed like a good idea!!!!
Hookit's Avatar
Quote:
Originally Posted by goldguy
and ........................ buy a wattmeter.
Very big +1 to that.

It's all very well going by the calcs and manufacturers specs.

But I have found power setups not conforming to what the labels have said before.

It is always a good idea to test the power system on the bench, using what you intend to install in the aircraft.

I used to have a test rig setup but found inconsistencies between that and the intended kit was enough to make me swap from that approach.

Although limited by being a static test I know I should have a system that can cope (I tend to work to an 80% of max). I also run the test mounted on a thrust meter but it's the amps part of the test that is most important.
May 17, 2018, 10:11 AM
Registered User
yes I will get getting a wattmeter, I'm more going by other people's experiences rather than manufacturers specs. last time I trusted a motor spec (emax clone motor that had a label on it saying 9-10" prop 11.1v) the motor ended up going up in smoke that was with a 10x4.5 slowfly on 3 cell.

ok so I got this thing flying very well! don't use a 6x5 prop on 3 cell. Not enough thrust or speed. it will fly but you really need to baby it on launch until it starts flying, even with a good chuck and almost full throttle it's just not got enough thrust.

I put the 7x6 prop I had on it back on, used hotglue to take up the slack in the shaft adapter and used an undersized 2nd o ring. this held on the prop super tight! Vibration was gone and all of a sudden it actually had some power. Full throttle and it was going pretty damb quick close to the original videos fast but not super fast. climbs vertical.

With a 7x6 prop held on properly it will give plenty of fun. unfortunately I broke it again on landing. probably cut the throttle too early,ran out of up and it nosed into the ground. broke at the last repair (easily fixed in about 2 minutes) after looking at the wind sock realised I had landed downwind which didn't help.I may move the battery back a bit more and see what it does but it was flying beautifully both upright and inverted. in a dive it would maintain the dive and inverted it needed just a touch of down elevator to fly easy confortableinverted circuits which I struggle to do on my wot trainer and jumper 25.

defiantly getting somewhere.

I'm actually thinking of taking my 2 old remaining 3cell 850 batteries and making a 4 cell, borrow someone's watt meter and test it with a 6x5 or a 5x5.5 prop I have lying around. only issue no 4 cell balance connectors.
Quote:
Originally Posted by Hookit
Very big +1 to that.

It's all very well going by the calcs and manufacturers specs.

But I have found power setups not conforming to what the labels have said before.

It is always a good idea to test the power system on the bench, using what you intend to install in the aircraft.

I used to have a test rig setup but found inconsistencies between that and the intended kit was enough to make me swap from that approach.

Although limited by being a static test I know I should have a system that can cope (I tend to work to an 80% of max). I also run the test mounted on a thrust meter but it's the amps part of the test that is most important.
May 17, 2018, 11:30 AM
It seemed like a good idea!!!!
Hookit's Avatar
Quote:
Originally Posted by bazsound
last time I trusted a motor spec (emax clone motor that had a label on it saying 9-10" prop 11.1v) the motor ended up going up in smoke that was with a 10x4.5 slowfly on 3 cell.
Really glad to hear that you have this flying nicely at last.

The cloned motor was probably rated to up to 10" prop but would have been on its amp limit with a 1060 standard electric prop. A 1045 SF prop will pull considerably more amps and deliver lots more thrust.

To put it in perspective I am running a 1300KV 2830 motor that will easily pull a 9060 standard electric prop. But I get more thrust from an 0845 SF prop (but at more amps).

Another reason I bench test first with what I plan to fit to the aircraft.
May 18, 2018, 03:17 PM
Registered User
Right....

For those that say this thing doesn't wake up til you out it on 4s (dst 1200 motor) I don't think your running the right prop or the props used are badly balanced.

I had same experience it flew but wasn't that fast. It sounded noisy. My prop would not sit on the prop saver properly. I ended up hotglueing it to take up any play but it still vibrated.

I then put orings on that were too small and Bam it's like a rocket unlimited vertical and pretty fast.

I got a new prop though aswell one of those bone coloured props and that's even better. It fits nicely aswell with just 2 o rings. 7x6 accelerates fast and has enough thrust to get you out of trouble. You can turn stall whack on the pour and pull out. Turn almost on a dime
May 18, 2018, 04:23 PM
Jack
jackerbes's Avatar
Quote:
Originally Posted by bazsound
..<snip>...
I then put orings on that were too small and Bam it's like a rocket unlimited vertical and pretty fast.
O-ring have very poor elasticity so if the span up and across the hub and back down is not close to the O-ring's size or just a little more you have very poor holding power there. And that was why the prop was not working well.

Use Thera-Band, rubber bands, any size of surgical tubing that will stretch far enough without breaking (and multiple bands to get more holding strength) and you'll find the use of prop savers to be easy and an effective way to use props.

I am a strong proponent of using the Thera-Band tubing, it give you bands for less than a penny apiece and multiple bands are easily added when needed. I always use two or more and have never lost a prop in flight.


Prop Savers - Fitting, Using, Testing, & Making Thera-Band Bands - http://www.rcgroups.com/forums/showthread.php?t=1419378

Jack
May 18, 2018, 04:39 PM
Registered User
Quote:
Originally Posted by jackerbes
O-ring have very poor elasticity so if the span up and across the hub and back down is not close to the O-ring's size or just a little more you have very poor holding power there. And that was why the prop was not working well.

Use Thera-Band, rubber bands, any size of surgical tubing that will stretch far enough without breaking (and multiple bands to get more holding strength) and you'll find the use of prop savers to be easy and an effective way to use props.

I am a strong proponent of using the Thera-Band tubing, it give you bands for less than a penny apiece and multiple bands are easily added when needed. I always use two or more and have never lost a prop in flight.


Prop Savers - Fitting, Using, Testing, & Making Thera-Band Bands - http://www.rcgroups.com/forums/showthread.php?t=1419378

Jack
Tbh I think I'm at the stage where prop savers are more a pain than breaking props. I never land under power which is what breaks props. Any plane I have risks breaking props has the brake activated
May 18, 2018, 07:37 PM
Ken's CAD Models
dz1sfb's Avatar
Jack,
I use the Therabands as well on non contact models, but in combat, if you use prop savers, you will lose props in flight. Caused by the inevitable mid air contacts.

KEn
Latest blog entry: Mini H Quad Project
May 18, 2018, 07:41 PM
flyin' fool
goldguy's Avatar
'O' rings are meant to be compresses, not stretched. I slice up surgical/high start tubing and use two rings to hold the prop captive.
May 19, 2018, 01:42 AM
Registered User
The last bump pushed the prop saver all the way onto the shaft. Leaving it like that. Essentially rendering the prop saver useless. This prop is nice plenty of thrust and speed on 3s a d seems to get good battery life too.

May 19, 2018, 02:30 AM
flyin' fool
goldguy's Avatar
The prop saver should be mounted tight against the motor. If required, cut off the excessive shaft.

This helps to stop the shaft from bending from an impact. Also, file a flat on the shaft where the screws seat.
May 19, 2018, 03:04 AM
Registered User
Quote:
Originally Posted by goldguy
The prop saver should be mounted tight against the motor. If required, cut off the excessive shaft.

This helps to stop the shaft from bending from an impact. Also, file a flat on the shaft where the screws seat.
How to you cut the shaft with it getting shrapnel in the motor
May 19, 2018, 06:45 AM
Jack
jackerbes's Avatar
Quote:
Originally Posted by bazsound
The last bump pushed the prop saver all the way onto the shaft. Leaving it like that. Essentially rendering the prop saver useless. This prop is nice plenty of thrust and speed on 3s a d seems to get good battery life too.
<snip image>
Leaving extra shaft length behind the prop saver invites a bent shaft. When I mount them I seat the saver against the top of the dome and trim the shaft length until the prop can tip to the side. And I grind small flats on the shaft for the two screws that lock it in place. And I put blue Loctite on the screws.

I never have any issues with the save moving on the shaft...

Jack
May 19, 2018, 06:59 AM
💩
ooOoo's Avatar
Quote:
Originally Posted by bazsound
How to you cut the shaft with it getting shrapnel in the motor
I use a piece of plastic from a blister pack. Make a small hole for the shaft to push through. The plastic shields the motor. Some people use a plastic baggie also.


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