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Oct 24, 2020, 05:18 AM
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from gas to electric Pietenpol air camper

I am finishing a build on a Pietenpol air camper. The model came designed to have a 40 gas engine. I want to install a G46 electric motor and related equipment. The G 46 is about 1" shorter than the electric motor and so the firewall placement is to far to the rear for the propeller to clear the nose of the plane. My thoughts were to move the firewall forward for clearance or to use standoffs as described by my hobby shop. Which is the best solution? Thanks for any help. I am new to this and only built models that hang from the ceiling.
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Oct 24, 2020, 05:40 AM
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scirocco's Avatar
Moving the firewall would probably be the hardest option and quite unnecessary when there are several ways to get the motor forward where you need it.

Stand-offs are usually pretty easy, but if it's only about 1", even a relatively crude solution like putting a block of wood the right thickness on the front of the firewall might work for you. Or if you need a bit of nose weight, stacked steel nuts could substitute for aluminium or nylon standoffs.

Google image searches on RC motor standoffs and rc motor mounting will give you some ideas.
Oct 24, 2020, 06:00 AM
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perttime's Avatar
Many like to make a box to extend the firewall forward.

BMatthews recently posted a rough illustration in a balsa builders thread:
Oct 24, 2020, 11:40 AM
B for Bruce
BMatthews's Avatar
Looking at some test specs for the motor at Hobbyking I see that even on a 4S setup the motor can run at 740 watts pretty easily . Depending on the size and somewhat anticipated weight for the model the G46 might easily end up being way more motor than you need for scale like flying. .

What's the size of your Piet' and do you have any idea at this point of what it'll come out at for weight?
Oct 24, 2020, 04:38 PM
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Thread OP
Its a 50" wingspan and too far out to tell about the weight yet. I may try to weigh all the individual parts and get a tally.
Oct 24, 2020, 04:41 PM
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scirocco's Avatar
I look at motors a bit differently.

The maximum power the motor might run is only relevant if it's not enough for the desired performance of the model. Otherwise, because the actual full throttle power depends entirely on prop and battery choice, the motor is only too much if it is physically too big to fit or too heavy to balance the model without excessive tail ballast. The point is that the "46" number is only a label and a max power rating is only a potential and neither should be a criterion to exclude a motor, especially one you have, without assessing it more closely.

Motors larger than the bare minimum size required to handle the required power can make very good sense for gentle scale flying as the typically lower Kv opens up a wide range of larger more efficient props while keeping full throttle power appropriate for the application, with potential benefits in both motor and propeller efficiency.

Probably less than required for a model designed for .40 glow but as examples, 10*7 on 4S makes the G46 only a 300W setup. 13x8 on 3S is also a 300W setup but develops more thrust for that same 300W input.
Want a 500W full throttle setup - 4S 12x8 or if the gear is short, 5S 10*5.

The flexibility of electric motors is a key advantage and imo its worth looking beyond the label to exploit.
Oct 24, 2020, 07:23 PM
B for Bruce
BMatthews's Avatar
Originally Posted by oldtruck54
Its a 50" wingspan and too far out to tell about the weight yet. I may try to weigh all the individual parts and get a tally.
I'm a bit confused now. I've never seen a 50" span Pietenpol that needed a .40. That would make it more like a pylon racer than a lazy floater.

Looking at Outerzone I see a few... This one is 65" span and recommends the older loop scavenged OS 30 or .35. And this one has an 84" span and uses an OS60 four stroke. So roughly the power of a two stroke .40. And roughly a match for your G46 motor. And in fact it would be a nice but still overly strong motor for a 8 lb 84" span model. But workable for sure.

When I've seen 50" span Piet's they usually had a .15 or .20 in them. Like the next link for a 56" span and intended for the old loop style .19 to .25

Who's kit is it and can you take a picture of the plans? And if you have not ordered the G46 yet I'd hold off until you clear this up. It'll be a totally unrealistic motor for a 50" span Piet.

The only answer I can see is that the .40 that they suggested is a .40 four stroke. Even then that's a lot of engine for a 50" Piet. It would certainly be the maximum size. I'll bet that the plans call for more like a .20 or .25 two stroke with the .30 to .40 four stroke as an option.
Oct 24, 2020, 08:53 PM
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scirocco's Avatar
An electric motor on its own cannot be overly strong.

It might be too big, too light, too heavy, the wrong colour or have a misleading label but it can only "overly strong" in the context of powering a model when the complete system including battery voltage and prop are considered. Then and only then can its overall suitability be assessed.

A Saito 30 4 stroke weighs 9.28 Oz or 262g and claims 0.5 hp and turn a 10*6 at 9800 rpm. An OS 25 weighs the same.

A G46 weighs 303g, so it's unlikely to be too heavy in a short nosed model. We already know it fits because the OP started out asking how to move it forward off the firewall.

Use 4S and a 10x8 prop and it will only use 300W, or just enough for a 4lb model using 70W / lb, and quite a bit less power than the Saito 40 with a 10x6.

But if prop clearance allows and the model is heavier, 11x8 makes it a 400W system, 12x8 makes it 500W. At 500W input, it's about 0.5hp at the prop and on the cubic inches displacement x 2000 rule of thumb, about equivalent to a .25 2 stroke.

So about the same weight as small glow alternatives and less or the same or more power depending on the prop and battery - hardly too much motor.

Edit: the above is not to suggest that the G46 is necessarily the ideal or optimum motor, but that if the model balances well with a motor that weight and kv, then it can be set up to do a very good job, from barely waft around the circuit, to lots of reserve power, whether the original design was for a little 20 glow or indeed a 40.
Last edited by scirocco; Oct 24, 2020 at 10:47 PM.
Oct 25, 2020, 06:52 PM
Balsa Flies Better!
I agree with Bruce- something seems off here. I think a 50" Pete should probably be less than 2 lbs ready to rock- these airplanes have a lot of wing.

Scirocco- no doubt that a 13 x 8 on 300 watts should work well in a Pete- but it'd probably have closer to a 72" span...

Oct 25, 2020, 07:37 PM
B for Bruce
BMatthews's Avatar
To be fair a motor that will spin a big powerful prop will also sip at the pack and turn a small prop just as well*. And as mentioned in a few other threads if the plane needs a bunch of nose weight anyway and if the motor is already in the box above the bench then there's a good reason for using it rather than buy a motor. But in this case where I gather the OP is looking to buy a motor I'd say it would be better to buy one that is closer in size, weight and power that will be more in tune with the Piet style of flying.

*- I'm "guilty" of this right now. I've got a small 5 oz model with a motor that'll handle a whopping 55 watts of power. But due to the 2 amp limit of the onboard ESC I prop'ed it down to a measely 15 watts with a small prop. (a touch under 2 A on a 2S pack).
Oct 25, 2020, 09:31 PM
Balsa Flies Better!
I also don't disagree with Scirocco about buying a somewhat larger motor than "needed". Larger motors operated not at their limits are generally more efficient than a smaller motor at the upper end of it's operating range. The only advantages of a smaller motor are size, weight and conceivably cost.

But my biggest concern here is that something seems off in the specs of the airplane...


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