Electric Stunt > Gettin all AMP'ed up!

Selecting Motor Size

<< < (2/2)

Ron King:
Dick,

I'm not sure if that's necessary on a Cardinal, but I did take care to give the bellhousing plenty of clearance to rotate and as I also mentioned, I'm now running Tru-Turn 6mm prop nuts on my profiles.

I went back and reviewed some notes and remember Dean Papas saying, "You cannot run your motor too cool." or words to that effect. Dean flies electric RC Pattern ships and they fashion some serious air deflectors and create air shrouds that closely match the surface of the motor.

BTW - the Cardinal is an excellent choice to learn about electric power. It's an excellent profile model to begin with and has plenty of wing area to carry the extra weight. Converting it to electric is easy. I have two of them and the magic numbers are:

AXI 2826/10, Castle Phoenix 45 amp ESC, Thunder Power 14.8 volt, 4200mah batteries, and a Gator 11.5 x 4.7 inch three blade carbon prop. My controller design flies this package on 61 foot lines at 5.0 - 5.05 lap times. It is a marvelous machine.  <=

I've attached a picture to show you the front end. All you really have to do is open up the engine mount area all the way back to the wing LE. This becomes both the motor mount area and the battery pit.

Hope this helps,

Ron

Dick Fowler:
Thanks Ron... a picture is indeed worth ten thousand words.

How about another question? I found the weight of the AXI motor is 6.3 oz. which is a bit lighter than comparable glow engines but the battery pack you are using is about 13.3 oz!!

Does this much weight placed forward  in the airframe (just  about where the fuel tank would be) create a nose heavy airplane? I noticed that Bob Hunt's plane has the battery compartment under the wing just about on the CG.

Ron King:
Dick,

Good question - and you may be surprised at the answer.

There are two things to consider: total weight and moment (arm). The total weight went up (my total package - batteries, motor, ESC, controller, and connectors weighs 19.75 ounces), but the moment moved BACK toward the CG.

So what happens?

1. Yes, we have a slightly nose heavy aircraft.

2. We have reduced the nose moment (or barbell effect).

3. The airplane turns much better, even though it's nose heavy.  #^ 

Sounds strange at first, but makes sense if you think about it. With the glow set up, the motor is the heavier piece and keeps the moment out near the front of the model. With the electric, the batteries are tucked in front of the leading edge. So even though the model will balance out nose heavy, the heavy piece is close enough to the CG that it doesn't hinder the turn - it actually helps the turn.

There are some other things to consider as well: One is CG shift. On a glow fuel model, the CG will move aft as fuel is burned off. Thus, your turn and your trim will change as you go through the pattern. Some people wonder why their ships get jumpy and feel lighter in the overhead eights. This is one of the reasons. The planes are lighter weight and the CG has moved aft.

With an electric, the "fuel" is EMF (voltage) and the weight doesn't change. Your CG and trim will remain the same throughout the flight.

The other thing to consider is vertical CG, although it doesn't come into play as much with profiles. A side mounted engine vs. an electric dosen't give much contrast, but there is a great deal of difference between an electric setup and an inverted piped stunt engine.

An inverted engine with a pipe lowers the vertical CG enough that some guys put their leadouts on the bottom of the wingtip. With my electric stunter, I place the batteries to balance the motor in line with the wing and have no tilt or hinging tendencies. Bob told me that changing to an electric motor in his Genesis cured the tilting problem and he was able to remove the flap tweak he used to try and compensate for the problem.

Hope this helps,

Ron

Dean Pappas:
Hello,
I would use a figure of 125W/lb as a minimum for Stunt, and shoot for 150 W/lb.
That's Watts at the battery.
Hope this helps,
Dean Pappas

frank carlisle:
Hi Guys,

I've been watching this thread and it's very interesting.
I haven't flown my electric Smoothie for a month or so now because I've pretty much given up on it. As far as I can tell I've gone as far with it as I can. Mainly because I just am not an electric genious. I'll never come up with an inovation in the electric department.
BUT...................I have been thinking about the plane. So far we've all been converting I.C. models to electric and to me it seems we should rethink the plane.
I'm thinking that since we don't have to tame or hold the engine back we should build a model with less drag. It doesn't have to be vibration proofed as much and the structure doesn't need to be so beefy.
Getting an electric motor to behave like an I.C. engine in an airframe that's had 50 plus years of refinement to perform with an engine rather than a motor is in my opinion what the drawback is to electric at this time.
Well that's what I've been thinking anyway.

Navigation

[0] Message Index

[*] Previous page