Jun 30, 2001, 09:02 PM
Registered User
e-sailpilot86's Avatar

Cessna 172 Electric

I have a 36" wingspan Cessna 172 that I converted to speed 400 power. It weighs about a pound, and I can see that it will take off, but my 20-amp ESC is giving me problems. Whenever I throttle to full power, it dies on me, and I'm asuming it's because of an auto-cutoff feature that will prevent the motor from being over-worked. I am using a master airscrew 7x4 prop in direct-drive, and I have tried this with a 6x4 Zinger. Same thing happens, the motor cuts off. When I run the motor with "no-load", or no propeller, it doesn't cut off. If I use a smaller propeller, I'm concerned about not getting enough thrust. Am I actually using too "big" of a prop on the motor? The ESC and motor are from FMA direct. Another thing is that it takes half as much distance on the throttle stick to reach full power. Is this normal for electrics? By the way, I'm using an 800mah 5-cell AA rechargeable with Sanyo cells.

[This message has been edited by e-sailpilot86 (edited 07-01-2001).]
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Jun 30, 2001, 09:15 PM
Tiger Moth World Tour
Clydewinder's Avatar
I'm pretty sure that what's happening is the AA cells you have are quickly dropping below BEC cutout ( probably around 5v ) under load. I think you may want to use a beefier cell like the KR600AE in a 7 or 8 cell pack with either a smaller prop or a gearbox on the prop that you have. It seems like most AA and AAA cells can't handle much of a current draw and the voltage drops very quickly.
Jun 30, 2001, 09:19 PM
Registered User
I'll take a shot at an answer (then the experts can chime in . You may have three problems consiring against you.

First, rather than using 5 cells, you should probably be using 7 or 8. Five cells under high amp load will drop the voltage below the ESC BEC cutoff limit (4-5V) very quickly. Note that while the speed 400 motors are nominally rated at 6V or 7.2V, we still use 7-8 (9 or 10 geared 7.2) at 1.2V (unloaded) and live with the reduced motor life for the improved power.

Second, the AA cells you have may be fine for low current draw (2-4 amps), but may not be up to the 10+ amps you need to pull. A lot depends on the specfic cell, but "typically" long thin cylinders are good for high capacity at low current and short fat cells are better for high current.

Finally, your prop selection is very important for getting the motor to run with reasonable efficiency at high power. You might want to switch to a "zagi" prop (5.?x4.?) or possibly a 6x3 cam prop.

If you have a local hobby store with an electric plane section, it might be worth a visit.

Jun 30, 2001, 09:22 PM
Official Boat Bum
Eddie P's Avatar
Wow, a C-172 of that size at 16 oz. That's great.

I wouldn't think your prop is the problem (though the 7x4 is too much for the typical speed 400 at 7 or 8 cells - mabey ok at 5 cells....) I'd go with a 6x4 to start with anyhow.

Buuuut, i'm wondering about your 5 cell setup. You may be activating the BEC cutoff under load with only 5 cells. Especially if you are running old cells or have a bad cell in there. Let's hear what other more "learned" guys have to say here - you will get a better response but this is my first thought.

Have you tried running with smaller cells like 600AE's? You could be lighter with 7 cells and have plenty of voltage to go around. It would be worth just a test to see if it will work that way. And a 600AE would do well in this setup with the higher cell count.

As far as the FMA controller goes, I'm not familiar with that brand enough to say why it is giving you an apparent full throttle response at half stick. Have you gone over your radio setup to be sure you don't have any old program peramiters in there?

Wadda you guys think?

Jun 30, 2001, 09:24 PM
Official Boat Bum
Eddie P's Avatar
Funny, when I went to post this there weren't any other replys! Now I've been redundant!

You can't say Ezoners don't try to help!

Jun 30, 2001, 09:37 PM
Registered User
Make sure your speed control is designed to operate on 5 cells. 6+ is common.

A 7x4 on Speed 400 6v will draw around 16 amps static, a 7.2 motor 10 amps, according to MotoCalc. 10 amps is good for a speed 400. One of those nice speed400 gearboxes for around $20 will get more thrust and efficiency.

The BEC speed controller doesn;t protect the motor. It will gladly feed a speed 400 20amps until it smokes if thats what it's propped for. Instead, it shuts down the motor when it detects the battery is almost dead, to save power for your receiver and servos, so you have control to land.

AR cells and others can handle 10+ amps, but "normal" AA rechargeables are happier at half that, and will drop voltage severely under load. The speed control intereprets low voltage as discharged state and shuts down. If you have a way to measure amp draw or pack voltage you can find out just what is happening. 10 amps max and 1v/cell you should be fine. If your batteries can;t handle it, try some 600ae or the 1600NiMh from radio shack. If you can charge NiMh that is.
Hope this isn't too basic, just all for clarity.
Jun 30, 2001, 09:39 PM
Registered User
e-sailpilot86's Avatar
I can see how the AA's are dropping below the BEC range now. Although, because of my battery setup, If I can keep the plane up at low current draw, I'd guess I can get some pretty long flights! I first tested the motor to break it in on a test stand, and it ran for more than a half hour! I think I'll go with more "fat" cell's. It was bugging me when it would just quit.
Jun 30, 2001, 09:45 PM
Registered User

This reminds me of the first question on "who wants to be a millionaire". Who can buzz in with the correct answer the fastest. I lost this one cause i was looking up specs on the LM2940CT-5 drop out voltage regulator for BEC. I think it needs 6.25 v min to work. Maybe the new switching ones can work on fewer cells... like the slowflyer controllers do.

At any rate, seems like the consensus is inadequate batteries. What kind of model is it? a 36" cessna seems like a nice project. any pics?
Jun 30, 2001, 09:47 PM
Registered User
I have a 36" foamie Cessna 210 that I fly with a 6v sp400 on 7 600AE's and a Gunther "toy" prop. Flys great, ROG, loops, touch & go. 6 & 7 inch props are too much load for a 6v 400 and you need a 4 or 4.5" pitch.

The batts I use are from FMA.

The FMA ESCs only use about half the throttle stick travel to reach WOT.

What kit do you have?


Jun 30, 2001, 09:48 PM
Registered User
e-sailpilot86's Avatar
the ESC says it's built to handle 5-10 cells, now I don't believe that. it's more like 6 or 7-10 cells. I chose the 5-cell pack because an electric similar to this called the "Aeroneca Sedan" used only 5-cells. It sounded right, but they may have been using a 10-amp, not a 20. But boy, that electric motor sure puts out a lot of thrust, more than I originally thought. I typically fly thermal/slope planes. Everyone seems to complain about electrics at my club, I think that should change soon. Thanks!
Jun 30, 2001, 09:58 PM
Registered User
e-sailpilot86's Avatar
The plane is a Guilliows Cessna 172 skyhawk, 36" span, could easily use a geared speed 280. Has a wing area of about 150-200 sq in. It's a pretty plane, but takes a while to build. You could give it ailerons, they are pre-cut for the plane. The kit is about 20+ years old, and is a conversion. It is originally Control Line, or a free-flight. It just needs a little thought to make R/C. Lousy wood though, if you build one, reinforce the center wing panel, where it mounts. I don't know how to post a picture, sorry, explain to me and I will asap.

I made up a quick webpage with photos for those of you who'd like to see the plane. Here's the link:

[This message has been edited by e-sailpilot86 (edited 07-01-2001).]
Jun 30, 2001, 11:05 PM
ὅπερ ἔδει δεῖξαι
Gerald's Avatar
Hi e-sail. I also did a conversion of this plane last year. It flew really well 'til I decided to increase the elevator throws for better aerobatics. That's when I discovered the meaning of "high speed snap-stall-spin'. It bit the pavment from about 50' straight in. I'll get it back in the air one of these days.

Anyway, mine flew fine at 16-18 ounces and it too had a direct drive 7.2v speed 400. I used 7 cells of 600AE and a graupner 6.5x4 JU52 prop. I added the ailerons too. You can see details of it at this web page: Cessna Skyhawk 172
Jul 03, 2001, 03:06 AM
Registered User
That really is a nice looking plane. So... have you flown yet with the new batteries? how 'bout an update?
Jul 11, 2001, 10:05 PM
Registered User
e-sailpilot86's Avatar
I've just come back on vacation, and I will put in the new cell's and a prop combination soon. I have a 7-cell 500 mah pack now, and an APC prop. Should fly well.
Jul 16, 2001, 01:41 AM
Registered User
e-sailpilot86's Avatar
I flew it a second time after having crashed it, and history repeated itself. I don't recommend making this plane only rudder/elevator, use either tailerons or the standard configuration.

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