Espritmodel.com Telemetry Radio
Reply
Thread Tools
Old Apr 27, 2012, 12:28 PM
Team Hillbilly (UK)
Space Bat's Avatar
United Kingdom, England, Cambs
Joined Oct 2011
1,396 Posts
P factor is the performance of the propeller at increased angles of attack and how it behaves when cutting its path through the air....a lot to do with the design of the tips and washout

This is a good guide
http://www.free-online-private-pilot...odynamics.html
Space Bat is offline Find More Posts by Space Bat
Reply With Quote
Sign up now
to remove ads between posts
Old Apr 27, 2012, 12:58 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Aberdeen
Joined Mar 2006
11,402 Posts
Me bad.. Yep I know what p-factor is in relation to asymmetric disk loading on props, that's not what I meant. What I should have said in the previous post was 'P-constant'.

P-constant is the figure you use in prop power calculations. You will find it in programs like Motocalc and the like.

Anyway the prop is balanced but I need to leave it overnight before I do any testing to give the varnish a chance to harden properly.

Steve
JetPlaneFlyer is online now Find More Posts by JetPlaneFlyer
Reply With Quote
Old Apr 27, 2012, 12:59 PM
Not as Good as The Kid
Aeroplayin's Avatar
South Pasadena, FL
Joined Sep 2009
6,391 Posts
Quote:
Originally Posted by JetPlaneFlyer View Post
I've been testing 15x6 props. I tried a Xor and APC on the Motrolfly DM2820-750 and both pulled far too many amps for comfort, the Xoar was slightly the lower in amps but still too high:
Xoar 15x6: 1129W, 75.7A, 8190RPM, 3.90Kg Thrust

So I got to wondering if it was possible to reduce the 'p' factor on one of these props by thinning the blades down. In theory it may then be possible to get some of the thrust advantage that the big prop gives but with less of the over-amping.

So I've reduced the width of the prop blades by about 3mm and sanded down the whole blade to restore the original airfoil, but thinned slightly. See attached photo with one blade trimmed and one yet to do.The varnish is currently drying. I'll post some numbers once it's done.
Along the same lines, I finally mounted the Q80-7M on my 27% Extra 300 and will be using a Xoar electric 24x10 for starters. The prop is a clear e-version of the Xoar 24x8 that was on the R-50, which pulls much better than the DA-50 because it is 52cc and rated to 5.7 HP, but lighter. The DA was swinging a 23x8 about the same RPMs, maybe a little less.

I should be testing it this weekend, but the lighter, thinner prop (both blade and hub) should give me the length and pitch I need to generate about 4800 Watts-out with a 123A reading, which is about 5200 Watts-in. This is what I'm going for anyway.

The question is thrust. Theoretically, I should get almost 40 pounds of thrust and this will determine if all the numbers and calculations extrapolate to a useful relationship with actual performance. We'll see how well these values hold up as we get bigger.
Aeroplayin is online now Find More Posts by Aeroplayin
RCG Plus Member
Reply With Quote
Old Apr 27, 2012, 01:12 PM
Team Hillbilly (UK)
Space Bat's Avatar
United Kingdom, England, Cambs
Joined Oct 2011
1,396 Posts
Props have bending moments and these are calculated specifically against the material used and overall shape and prop geometry - removing that much material may make the prop unstable stress wise...and cause a catastrophic failure....

Suggest testing this carefully...

Am interested in the fact that biela have introduced a variable pitch propeller - which must utilise the prop and induce flexing to product various pitch profiles under load....they only do it for the bigger props - but guess may introduce smaller versions
Space Bat is offline Find More Posts by Space Bat
Reply With Quote
Old Apr 27, 2012, 01:25 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Aberdeen
Joined Mar 2006
11,402 Posts
Yeah, I'll be careful with it. I really don't think I'll have any problems, the blade shape and thickness is pretty much now identical to the 14x7 prop, just a 12" 'extension' at each end.

But better safe than sorry, that's for sure!.. I'll stay well behind the prop.

Steve
JetPlaneFlyer is online now Find More Posts by JetPlaneFlyer
Reply With Quote
Old Apr 27, 2012, 01:48 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Aberdeen
Joined Mar 2006
11,402 Posts
I ran out of patience and tested it without waiting for the varnish to fully harden

Good news:
Prop didnt break and thrust was pretty much as before

Less good news:
Watts and amps were also almost identical to before

oh well, it was worth a try
JetPlaneFlyer is online now Find More Posts by JetPlaneFlyer
Reply With Quote
Old Apr 27, 2012, 02:01 PM
Not as Good as The Kid
Aeroplayin's Avatar
South Pasadena, FL
Joined Sep 2009
6,391 Posts
The difference between the gas Xoar 24x8 and the PJN 24x10 is a reduction in weight of about 30g as published. But when I used a broken gas prop, and cut it down to equal the PJN weight so I could get some preliminary CG data, I had one blade cut completely off from about 1.5 inches off the hub. Seems more than 20% to me, but that's the kind of difference in weight, and pitch, the manufactures feel is needed to produce similar thrust with less prop weight at similar RPMs.

We'll know more after the weekend, I hope.
Aeroplayin is online now Find More Posts by Aeroplayin
RCG Plus Member
Reply With Quote
Old Apr 28, 2012, 02:42 AM
VOLTS > AMPS
stgdz's Avatar
United States, MN, Buffalo
Joined Jul 2011
3,865 Posts
you island people can enjoy this but I am off to bed, after pulling the the 32gage servo wires out of both my edge and mxs, and then swapping the esc's and bec's for both (the edge had a 50 and the mxs had a 60) I pulled the following on the sunnysky 800-2820.

13x6.5 xoar
8200rpm
523watts
36amps

Recalibrated my esc and everything, way less than what gary was getting for amps and watts (the reason why I swapped escs as i was expecting about 12amps higher).


God I am tired though, you guys can crunch the numbers tomorrow but I am ready to pass out. The numbers look good, I may try a 14x7 on it tomorrow.
stgdz is offline Find More Posts by stgdz
Reply With Quote
Old Apr 28, 2012, 04:22 AM
bryansifsof44's Avatar
United States, AK, Anchorage
Joined Oct 2011
4,554 Posts
I think Gary was running the 2820-800 with a 14x7 and the 2820-950 with a 13x6.5.
bryansifsof44 is offline Find More Posts by bryansifsof44
RCG Plus Member
Latest blog entry: Weights for planes
Reply With Quote
Old Apr 28, 2012, 06:44 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Aberdeen
Joined Mar 2006
11,402 Posts
Quote:
Originally Posted by stgdz View Post
you island people can enjoy this but I am off to bed, after pulling the the 32gage servo wires out of both my edge and mxs, and then swapping the esc's and bec's for both (the edge had a 50 and the mxs had a 60) I pulled the following on the sunnysky 800-2820.

13x6.5 xoar
8200rpm
523watts
36amps
Those numbers are way low for a 800kv motor. Your battery voltage is a bit low (14.5V) but even considering that I'd expect more RPM and more amps.

For comparison my 750kv Motrolfly spins the very same prop at 9570RPM and pulls 845W and 55A (peak values). Your kv is higher so on the face of it you should be turning the prop faster and pulling more power than my motor. Because mechanical power is proportional to the cube of RPM, the Motolfly is in fact putting 60% more power to the prop (and consuming 60% more electrical power).. these are huge differences.

Seems to my non expert thinking that there are a few things that might explain this:
  • The motor is much lower kv than advertised (not necessarily a bad thing for our applications)
  • The windings have small cross section area of copper, so resistance is very high.
  • Magnets and/or stator iron core are not very efficient.
If you could check RPM with no prop then that should give some confirmation of kv and rule that one in or out.

Steve
JetPlaneFlyer is online now Find More Posts by JetPlaneFlyer
Last edited by JetPlaneFlyer; Apr 28, 2012 at 07:46 AM.
Reply With Quote
Old Apr 28, 2012, 07:20 AM
Not as Good as The Kid
Aeroplayin's Avatar
South Pasadena, FL
Joined Sep 2009
6,391 Posts
Quote:
Originally Posted by stgdz View Post
I pulled the following on the sunnysky 800-2820.

13x6.5 xoar
8200rpm
523watts
36amps
If it really were an 800Kv motor, and if you were getting anything close to the efficiency rating we use for the Torque, Motrolfly, and Hacker, you should see about 10,100 RPMs with the 13x6.5 prop. That's about 780 Watts-out, which will require about 50 Amps from 4S (figure 15.6v).

8200 RPMs means a 65.7% Kv efficiency, which is pretty much useless because a 13x6.5 prop rotating 8200 RPMs is 50 Watts-out less power than my Omega 103g motor spinning the APC 13x6.5e prop on 3S.

You're generating about 420 Watts-out, which really only needs 27 Amps at 15.6v. That's 120 Watts-out per pound.
Aeroplayin is online now Find More Posts by Aeroplayin
RCG Plus Member
Reply With Quote
Old Apr 28, 2012, 11:59 AM
VOLTS > AMPS
stgdz's Avatar
United States, MN, Buffalo
Joined Jul 2011
3,865 Posts
Sunnysky 2820
no prop info
12000rpm
1.5amps
25watts

put a apce 14x7 on it for 4s
7800rpm
38amps
570watts


DM28020 600kv 5s
13x7 Xoar
40amps
720watts
8800rpm

13x8 won't fit as the hub is to thick, ken said don't go above 13x6.5 and it looks like it is a good idea. I dropped about 700rpm maintained the same watts, but jumped by 5 amps. I would hazard a guess that 50-70watts is being converted into heat.
stgdz is offline Find More Posts by stgdz
Last edited by stgdz; Apr 28, 2012 at 12:33 PM.
Reply With Quote
Old Apr 28, 2012, 12:43 PM
Team Hillbilly (UK)
Space Bat's Avatar
United Kingdom, England, Cambs
Joined Oct 2011
1,396 Posts
A closer equation to obtain kV

Can be seen as rpm / (volts - (current x motor resistance)

for example if the sunnysky had a resistance of 50mOhms...

We could say from stgdz results

12000 / (16.6 - (1.5x0.05)) = 726kV

Do you know the Rm for this motor as can't find it...
Space Bat is offline Find More Posts by Space Bat
Last edited by Space Bat; Apr 28, 2012 at 03:30 PM.
Reply With Quote
Old Apr 28, 2012, 02:48 PM
bryansifsof44's Avatar
United States, AK, Anchorage
Joined Oct 2011
4,554 Posts
Quote:
Originally Posted by stgdz View Post
Sunnysky 2820
no prop info
12000rpm
1.5amps
25watts

put a apce 14x7 on it for 4s
7800rpm
38amps
570watts


DM28020 600kv 5s
13x7 Xoar
40amps
720watts
8800rpm

13x8 won't fit as the hub is to thick, ken said don't go above 13x6.5 and it looks like it is a good idea. I dropped about 700rpm maintained the same watts, but jumped by 5 amps. I would hazard a guess that 50-70watts is being converted into heat.
Did you happen to get voltage without prop. If you had simular voltage sag as me of 16.7volts. 12000rpm / 16.7volts = 719kv
But those numbers are low with the 14x7 prop... Battery good?
bryansifsof44 is offline Find More Posts by bryansifsof44
RCG Plus Member
Latest blog entry: Weights for planes
Reply With Quote
Old Apr 28, 2012, 07:41 PM
VOLTS > AMPS
stgdz's Avatar
United States, MN, Buffalo
Joined Jul 2011
3,865 Posts
.15 volt drop with no battery.

Battery is fine as far as I can tell. Ir is still low
stgdz is offline Find More Posts by stgdz
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Question RC kits for experimenting with flight control systems SubZer0 DIY Electronics 11 Oct 12, 2011 08:58 AM
Wanted left wing for extreme flight yellow and black 74 yak hondaboy92hatch Aircraft - Fuel - Airplanes (FS/W) 0 Sep 29, 2011 10:18 AM
Discussion Extreme power systems & Royal Evo 12 rbehrends Radios 5 Oct 11, 2007 05:55 PM
Discussion Battery for Logo 10 - Thunder Power "eXtreme" 5S 4600 or Flight Power 5S 3700? WeatherB Electric Heli Talk 6 Jan 25, 2007 10:29 PM