Marc

I'm a relative newbie, but it seems to me that measuring rpm, amps and prop speed of a static model might all be a deception. My experience with high performance nitro planes suggests that when a plane takes to the air everything changes. As the prop begins to screw through the air, the motor begins to unload. With a decreased load on the motor, rpms increase, amps drawn decrease, and actual airspeed might well exceed the prop speed of the handheld model. Controlline speed planes are a rather extreme example, but they can barely stay in the air until the 3rd or 4th lap. When the prop catches, the engine leans out, and the plane leaps to 200 mph.

The hypothetical is a sport plane with minimal frontal area and a wing thickness of 10-15% of its chord. If proped for maximum static thrust, what percentage change should be expected in terms of rpm, amps, and speed? Does anyone have any empirical evidence related to speed since rpm and amps are difficult to measure in the air.

CHELLIE

Hi Marc your right, there are a lot of different things going on, Electric is a lot different than Nitro, thats for sure with electric if you get 90% of the power from your motor, your doing good, then drag like you know plays a big part, I have flown nitro control line for many years, and nitro rc, I really like e Power, but it took me a long time to learn about it, you have amps, mah, esc, bec, C ratings, watts, Etc, it drove me crazy at first but i asked a lot of questions, and learned a lot, Now I am Comfortable with e power, and Have a FunJet doing 103Mph on E Power, and hopeing to go 125 to 150Mph with a Stryker with 100Amps and over 1 kw, I have been involved with e Power since Oct of 2006, and i am Still learning everyone here on wattflyers, is very helpful, and willing to Share their Knowlage, If I can be of any help, PM me, I may or may not know the answer, but I will know of someone that Does Take care and have fun, Chellie

BradT

Marc, short answer is YES, the prop unloads in flight, just as you experienced with nitro engines. The exact rate is highly variable, of course, depending on prop, and airframe, but I've found 10% to be a good starting point. There are data loggers available to measure in-flight power, but I have no experience with them.

The static numbers are only useful as a rough comparison between systems, and inflight testing is the only way to tune for optimum performance, just as with any power setup, model or full scale, air, ground, or water. You can often get a more precise reply to your specific needs, if you post your exact setup, and what you're trying to achieve with it. Chances are, one of us will have tried something very similar, and have some real world data for you.

Hope that helps,
Brad

eflight-ray

As already put, knowing rpm, amps and prop size is very useful as just data.

The variations of electric motors just do not compare to I.C's. Most 40's will tend to run a very similar prop size, (obviously there are exceptions). But what could be called a 40 size electric motor, could run quite a range of props because it can be run on different voltages, (perhaps a bit like standard and high nitro fuel, only an even bigger range). Then of course gearboxes can be added to increase the range even further.

So it's always handy to know what combination of prop and voltage, (number or type of cells), gives what rpm and current draw.

I wouldn't wont to be without my wattmeter and tacho now.

Lieutenant Loughead

You were absolutely correct, right up until your final statement.

With electric motors, you're going to get more thrust in a static environment -- so, when the motor "unloads" you actually get LESS thrust. (So I suppose an electric controlline plane should have an easier time getting started than an IC controlline plane.)

The critical link you left out of your argument is DRAG. When you're measuring prop pitch speed on the ground, there is no drag involved with the power system. When you start flying, the power system starts to experience drag on the airframe. As the airplane goes faster, the drag increases exponentially -- so (in my experience with electric power systems), the actual airspeed is usually about 15% to 20% slower than the measured prop pitch speed.

If you really want to get into the nitty-gritty, I can e-mail you a couple of electric power system spreadsheets I've been keeping for the last several years, for your review. The main spreadsheet currently has about 200 entries...

Alternatively, you can simply buy an Eagle Tree Data Logger, with the GPS option, and actually measure your top speed, and compare it against your calculated prop pitch speed.