how much does a prop "unload" - RC Groups
Sep 07, 2001, 10:02 AM
Registered User

# how much does a prop "unload"

My electrostreak is drawing 30-35 on the ground, my speedcontrol is a 30 amp unit. How much of an amp drop can I anticipate as the plane "unloads" in the air???
 Sep 07, 2001, 10:19 AM Registered User Depends on the prop and how it's set up - high pitch on a draggy model will never fully unload, but may be stalled to begin with, giving you a lower current draw than when it's working in flight.. In general, however, 10% or so.. ..a
Sep 07, 2001, 05:00 PM
Boffin
Quote:
 Originally posted by bpu6999: My electrostreak is drawing 30-35 on the ground, my speedcontrol is a 30 amp unit. How much of an amp drop can I anticipate as the plane "unloads" in the air???
It is the prop that would unload, and you didn't mention which it is. The short answer is not much (less than 20%). A square prop (i.e. 7X7) will be stalled at static and will initially increase the load in the air.

To save your control, you'll have to use full throttle for seconds only.

 Sep 08, 2001, 04:20 AM Registered User @rpage53: Problem is at partial throttle the ESC gets even warmer than at full throttle. @bpu6999: Use a different prop which draws less amps. Jürgen
 Sep 08, 2001, 04:43 AM Registered User astro bob say's go down an inch in lenght for static testing, this be close to amp draw with the bigger prop in flight.
 Sep 08, 2001, 07:45 PM Rocket Scientist Sure it might be ok but just barely if at all and a speed control with a higher rating should have a lower resistance and thus better performance. I would save the 30 amp control for another plane and get a 50 amp control for the E-Streak. A fried speed control can cost you a plane, either because of the unexpected loss of power or if the thing really fries the heat may cause the voltage regulator in the BEC to cut out as well causing loss of control. I expect that because you are asking the question that you do not use a fuse between the motor and the battery. There was one time when if I had used a fuse it would have saved me a speed control. Does the speed control have overcurrent protection. Tim Knowles Tim Knowles
 Sep 09, 2001, 01:53 AM Boffin >Problem is at partial throttle the ESC gets even warmer than at full throttle Not if full throttle exceeds the ESC's specs. Most ESC's can handle "momentary" currents well above their rated value (i.e. 30A continuous, 40A momentary). So a 35A static draw for a 30A ESC is likely OK if: 1) the prop isn't stalled at static 2) the voltage is within the specs 3) you limit the time at full throttle 4) the ESC has adequate cooling. Keep in mind, you still might burn out your ESC. If you don't mind that risk, go ahead. [This message has been edited by rpage53 (edited 09-08-2001).]
Aug 07, 2006, 04:00 PM
Dave North
There's also a problem of the actual flight usage; what kind of flying you do. And what you're measuring on the ground. Here's an inflight plot of a motor/battery/prop (8x6 APC SF) that measured 12.7amps 11v on the ground.

The first curve is "static test in the plane." Full throttle just before hand launch. Note that it pretty much says the same as the ground measurements (I ignore sharp spikes).

The second blip is takoff into an initial climb straight up, followed by coasting back down, then full throttle into the wind, coast around the corner, full throttle with the wind.

Note the upper registers of power use are in all cases quite brief, and most of the time this 12.7-amp system is well under 11 amps, and rarely over. I wouldn't be surprised if a good 10-amp ESC could handle this load.

Dave

### Images

Aug 07, 2006, 04:02 PM
Registered User
Quote:
 Originally Posted by Andy W Depends on the prop and how it's set up - high pitch on a draggy model will never fully unload, but may be stalled to begin with, giving you a lower current draw than when it's working in flight.. In general, however, 10% or so.. ..a
I did Motocalc simulations and got between -10% and +70% depending on the power, the model, and the prop.

Impossible to say without more detail.

Motocalc will give you a general feel if you plug the 'streak in with the right prop etc.
 Aug 07, 2006, 06:18 PM Registered User Dave, why did you revive a 5 year old thread? Good Luck!
 Aug 07, 2006, 07:25 PM Dave North Beav, I hate starting new threads to address old ideas. Doing this gives it a bit more perspective, don't you think? These problems and questions have lingered for half of forever, and now we're starting to see some data flow in. I'm thinking of starting a new thread on the subject, though. Data only. Dave
 Aug 07, 2006, 10:14 PM Registered User As you say, Dave - the question of unloading recurs again and again. In-flight telemetry is the way to find the answer! More power to you!
 Aug 08, 2006, 12:11 AM phillyphly A very recent thread from people using the Eagletree system logger reported around 10% unloading in real-life flying. Also, quite a few spikes above static amp draw during aerobatics and sailplane climbouts etc.
Aug 08, 2006, 09:57 AM
Registered User
This has been posted before, but it shows power drawn as a function of flight speed for props with Pitch/Diameter ratios between 0.6 to 1.1.

The X-Axis of these Advance Ratio plots may be considered airspeed.

Pitch Speed occurs at an Advance Ratio equal to the Pitch/Diameter ratio.

The Coefficient of Power Cp demonstrates the power absorbed by the prop at different flight speeds.

Of note and mentioned previously is the fact that the power absorbed can actually be greater than static at some airspeeds.

Also of interest is that the power is not zero at zero thrust because of the profile drag of the prop.