wyldrydes

My Feedback: (4)

Guys,
Intend to put electric power on a 6 lb Pitts which should do IMAC (non 3D) mostly as a parkflyer.

Question- Is there a conversion chart out here for nitro engines to electric motors? AC profiles and parsitic drag are understood and can be taken into account when the approximate motor is known.

Also, if the model says it uses a 12x6 prop with nitro (13K RPM) what would be the formula to get that performance from electric?

Thanks,
Jerry

Pete Bergstrom

Jerry,

There is not a simple conversion chart to do what you ask but I'll try to provide some guidance.

Aerobatic power requirements. For IMAC style flying with your airplane you will need 150-175 watts per pound of power coming from your electric power system. Your stated weight is 6lbs, but that is generally empty without fuel. So if you have a 16 oz fuel tank the fuel will weigh about 14.5 oz so let's call the all up weight 7 lbs for this discussion. You need a power system that will deliver between 1050 and 1225 watts to get the performance from the airplane you are asking.

The Eflite motors have a numbering system designed to provide a baseline for the conversion you are asking about. They suggest their Power 46 for a 46-50 size glow airplane, Power 60 for a 60 size, etc.

As for the propellers, you will learn a whole new set of rules regarding propeller selection. In a glow or gas engine, as the airplane moves forward and the propeller unloads the rpms increase. In an EP powered system the rpms are a reflection of the kV (rpms per volt) and as the propeller unloads the rpm does not increase, but the amp (current) draw does, effectively reducing the horsepower being delivered. The EP motors don't turn near the rpm that your glow motors will either. As a result, you will most likely end up with a 14 or 15" prop with an 8-10" pitch to get similar (actually better) performance from your airplane.

One last thing ... a 6-7 pound model is hardly a park flyer, not matter how quiet it is. Parks generally have uncontrolled pedestrian traffic that would be in danger with your model flying there. I suggest you reach out and join one of your local clubs for flying this airplane.

Pete

wyldrydes

My Feedback: (4)

Pete,
Thanks.
Thats just the data I need. Nice work.
Re Parkflyer- I agree. 6-7 lbs dramatically exceeds the LETHAL THRESHOLD limits critical for a safe Parkflyer program. Will club fly the 7 lb Pitts.

When calculating LETHAL THRESHOLD (for AMA insurance purposes) I'm using AC speed squared x weight (lbs) divided by 32 x 1.07.

That would mean a Parkflyer operating within the LETHAL THRESHOLD should be 24 oz AC flying normally at 35 mph.
Close?
I left RC 20 yrs ago and now find electric flight with 2.4 radios make it great (quiet&safe) sport today. Appreciate the back and forth.

Jerry

Pete Bergstrom

Jerry,,
Common sense should always rule the day. Even a 24 oz model at 35 mph can do some hurt if you are unlucky enough to get hit but the prop hub hub.

I just re-read my post and I made one error. "In an EP powered system the rpms are a reflection of the kV (rpms per volt) and as the propeller unloads the rpm does not increase, but the amp (current) draw does, effectively reducing the horsepower being delivered." This read "In an EP powered system the rpms are a reflection of the kV (rpms per volt) and as the propeller unloads the rpm does not increase, but the amp (current) draw also decreases, effectively reducing the horsepower being delivered."

Have fun!

Pete

dealwithit

My Feedback: (55)

the rpm of the motor will go up as speed increases, especially if its a direct drive motor, up until it hits a point where the kv vs the air drag equals out, similar to a gas engine wouldn't it? if the throttle is full bore, from start to full speed the current draw will be highest from dead still to lowest at full kv top rpm. if this is totally wrong please post up, i haven't been running electrics for that long, but i did go to school to be an electrician and that just seems abit more along the right lines?

tIANci

Jerry ... you have 2 options for a 6 lbs plane for IMAC. You can go 5S or 6S. For a 6 lbs plane 6S will usually give you 3D capabilities. So you need to decide 5 or 6 and also think medium term. I go with 5 and not 6 beacuse I also fly 10S. Anyways, in trms of lethality, a small light place into a kid's face esp the eye is 'fatal' enough. He may not die but he will lose and eye. So its about ensuring we fly at a place where there is as little possiblity of crashing on someone. That is the best formula.

Mustang Fever

My Feedback: (2)

This section on Great Planes seems to have good info: http://www.electrifly.com/motors/gpmg4505.html

The rimfire 80 seems to meet your requirements.

I'm going to build a Wing Mfg P-39 soon, and I'm going to use their ".32" size. Seems to meet all the requirements and besides I have a nice 4250 3S pack that should work great with it.

Androgeny

Selecting the correct prop in electric requires that you have an amp meter. First, you can get a good estimate of the correct prop for your motor by using one of the many prop calcluator programs on many websites, or Motocalc. Next, buy that prop, and maybe one above and one below. Then, insert your ampmeter and test the current draw on a bench. At wide open throttle, you should not exceed the motor or ESC recommended amperage. Thus, if the motor should not exceed 45 amps at WOT, then this should be the case on the test bench. If your draw is too low, then move to the next higher prop. If too high, then move down a prop. If you increase prop diameter, you get better torque, if you increase pitch you get better speed, so you can increase or decrease either diameter or pitch to get the current draw right depending if you prefer torque or speed. Then, what you get in the air may differ because the prop unloads in the air. Thus, you may want to get a logging amp meter that you can leave in the plane when you fly it and then read the results when on the ground. I use the logger from http://www.eagletreesystems.com/ to fine tune the correct prop. With nitro, you do not have to do all of this. With electric it is essential. If you draw too much current, you will smoke your motor and your ESC quickly. Also, pay attention to the "C" rating of your lipo. That sets the limit on what current the lipo can dispense. If you have a 3000 mah lipo with a 30C continuous rating, you multiply 3 amps times 30 to get the maximum of 90 amps continuous current draw. So the entire chain from lipo, ESC and then motor amp rating must not be exceeded by the prop selected. At the end, current times voltage equals watts at WOT. You want between 75 to 175 watts per pound depending if you want docile or aggressive aerobatics. I keep a box of various props handy to prop up or down when needed. If you do not have enough power you can also increase voltage by going to a 4S, 5S or 6S or so lipo system, keeping the current constant, provided your ESC and motor can deal with the higher voltage. Remember that above a 12S you are above 50 volts. At 50 volts you exceed skin resistance and can get a nasty shock from your power, so it is unwise to play with more than a 12S system to get power up. However, now there are guys who have in excess of 7000 watt power systems in RC planes using 12S lipos and 200 amp ESC/Motor combinations. That could fly a 70 pound plane! Many times if you visit a forum for someone who already has your model, and your power system, the prop they use is a good starting point, assuming they put an amp meter on it to get it dialed in right.

jack sanzoni

I FLYING A SPOT ON 50 WITH A SATIO 62 ENGINE I WANT TO GO TO ELECT. ANYONE DONE THIS WITH GOOD RESULTS JACK

Dr Kiwi

Also, if the model says it uses a 12x6 prop with nitro (13K RPM) what would be the formula to get that performance from electric?

Using the Badcock calculator you can see that to get a 12x6 APC E to 13000rpm from a reasonably efficient motor (80%)... you need about 1800W...

6s (21V) and 85A might do it.