Hi all,
I presume that this has been covered before somewhere, but haven't been able to find anything.
I'm just wondering if anyone knows how much extra power is drawn by a model plane when full throttle is used on the ground for throttle testing compared to when the plane is free to fly?

Presumably there is more resistance on the prop when the air is not pushed into it as there is when the plane is moving.

I know that wind conditions and other variables could affect the answer here but is it possible to give a figure on average to this question? (Someone who has a onboard amp meter could probably work this out I'm thinking)

Cheers!

VERY approximately, the amount a plane unloads in the air is the prop pitch divided by the diameter.

Sorry, what are the units of measurement? Degrees for pitch since it is an angle right? Then does the answer become the factor by which the prop is unloaded?

Herer there are some measurements made both, in land and airborne, you can make your own conclutions:

http://www.rcgroups.com/forums/showthread.php?t=1101699

Like diameter, pitch is measured in inches/mm... thus a 5.1x4.9 Zagi Carbon prop is 5.1" diameter and 4.9" pitch... as an enlightened Kiwi, you'd read it as 130x125 (mm).

What Vintage is saying is that a 10x10 prop (ratio 1:1) may not unload in the air, but a 10x5 should do so (I am a bit skeptical of his full 50%, but 20-30% might be reasonable).

Originally Posted by Johnnz

I'm just wondering if anyone knows how much extra power is drawn by a model plane when full throttle is used on the ground for throttle testing compared to when the plane is free to fly?

The difference is only a few percent and if you depend on that slim margin to keep your plane in the air you're asking for trouble. Better to have at least a small safety margin and static testing provides just that.

For a given prop, loading is really a function of blade angle of attack. Thus how much a plane unloads depends heavily on drag (both parasitic and L/D). Here's a page that describes what I'm talking about:
http://home.scarlet.be/comicstrip/En...Efficiency.htm

...and the key image from that page, which IMO embodies a lot of ideas an advancing hobbyists should know (TAS~aircraft velocity vector):

When calculating such things, we usually speak of pitch in terms of degrees. Using trig you can easily convert pitch inches to pitch degrees (hint: slope of the helix). Given that hobbyists aren't equipped to accurately measure/estimate drag, trying to pin down an unloading factor is a long trip through muddy water. It's suffice to say: put setup X and prop Y on a bulky/draggy trainer, and it may not unload appreciably. Put setup X and prop T on a slippery pylon racing airframe, and you might see quite a bit of unloading.

Cheers,
Kev

Originally Posted by rich smith

The difference is only a few percent and if you depend on that slim margin to keep your plane in the air you're asking for trouble. Better to have at least a small safety margin and static testing provides just that.

Its estimated between nothing at all anmd +- 50% actually. A 10x20 prop was estimated to nearly double its draw at or near pitch speed.

A 10x5 will drop to nearly half.

But those with loggers on their planes show that the angle the plane is at makes a lot of difference. A full bore WOT climb is nearly full draw but flat out horizontal is a lot less unless the model is very steep on pitch.

I don't know about all this.

According to NACA the power absorbed by a 10x6 prop at Pitch Speed drops to about (0.3/0.375 = 75%) of static, while a 10x11 prop drops to just a little less (0.55/0.8 = 68% ).

The power absorbed at Pitch Speed is even less when expressed as a percentage of max power absorbed (which occurs at approximately 40% of Pitch Speed). Maximum power absorbed compared to static increases with P/D ratio and is about (0.0975/0.08 = 1.2) or 20% more than static for a prop with a P/D ratio of 1.1.

The X-axis of the Advance Curves represent airspeed.

Pitch Speed (RPM X Pitch) occurs at an Advance Ratio equal to the P/D ratio.

Cp is the Power Coefficient which would be proportional to current for a constant RPM.

In real life the RPM and Pitch Speed would increase because of unloading resulting from the lower current drain.

Originally Posted by vintage1

But those with loggers on their planes show that the angle the plane is at makes a lot of difference.

I did measure this with a logger onboard and it did not make a lot of difference. Never close to 50% or even 75%. Square or over-square, vertical or horizontal. Apparently YM does V.

This theory, of unloading based on props is strange. So if we static test a setup and are getting 95% effieciancy and then fly it we could get it to go better than 100%..

I dont know, I think this thread is more relevant and was more to do with how effiecent a motor is designed versus prop diameters. The less eff, the higher it could unload. the more eff, less unloading.

all the experts weigh in here very interesting.........

http://www.rcgroups.com/forums/showthread.php?t=662235

Originally Posted by c/f

So if we static test a setup and are getting 95% effieciancy and then fly it we could get it to go better than 100%..

Huh???

Uh oh... Martyn's confused. Taste of his own medicine.

Read the link it ends up boiling down to an electric motors design and how effiecient it consumes energy as a constant resistance. Unlike like a fire breathing slimmer........

If a super effieceint motor/prop combo where to unload at significant values based on a prop/air/angles/attitudes etc it would turn into a generator to produce power , that aint happen as of yet........

Speaking from personal experience...
It all depends on the setup. It's hard to make a mathematical model. I have seen one setup unload more than 100% in the air. It was a 16x10 prop on an axi outrunner which drew close to 60 amps on the ground and less than 30 in the air on a slick pattern ship with a ground speed of about 60mph. That's an extreme case.. But most ofthe time I've sen about 20% decrease.