Please help with my first scratchbuilt 3D foamie, it flies like a dog - RC Groups
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Oct 30, 2011, 12:31 PM
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Please help with my first scratchbuilt 3D foamie, it flies like a dog

Hello everybody, I'm pretty new here, I just started flying in July.

An older gentleman from church helped me cut out my first 3D foamie from 1.3# 9mm EPP. The airplane is his own design, and looks like a Yak. The wingspan is 40". The weight with the current motor and a 2200mah pack is 680gm (1.5lb).

He used a hacker motor on his depron version, it was the A30-28S 1140kV and 3S, but I wanted to save some change so I purchased a few Turnigy Aerodrive SK3 motors with plans to use the others on more airplanes.

The motor I currently have mounted on my EPP plane is this:

1500 kV, 370W 8T
31A max

Wouldn't this give me 370w/ 1.5 lb = 246 w/lb? Shouldn't that me more than enough for a 3D plane? Why does it fly like a dog and it seems like I don't have enough power?

I took it out to the field and one of the other members helped me load test it and we had to downsize the prop to an APC 7x5 to stay under 30A. I've got a HK Tower Pro 30A ESC on the airplane. The problem is that the airplane flies like a dog. It has hardly any vertical thrust and it is very difficult to fly.

I'm having trouble finding the pages with guidance on how much power I need per lb for a 3D plane. I know I should be swinging a bigger prop and lower kV. But if two motors have the same watt output, I don't understand what the difference really is to have:

bigger prop & lower kv
smaller prop and higher kv

I also have this motor:

Would it do any good to switch to this motor? it seems like if these two motors both have 370w and 375w, then shouldn't they have the same performance?

Do I need to purchase a more powerful motor like one of these?
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Oct 30, 2011, 12:58 PM
Registered User
My initial reaction was stopped short by the 1.5 pounds!
What? have you assembled?
IMO 1/2 that weight is on the edge of the typ weight range.
Not surprising that it performs Poorly.
Rather than buy a whole bunch of 'new' parts trying to make a clearly too heavy model work 'better'.
Perhaps you might spend some time reading the threads in this forum ( Scratchbuilt Foamies :-) on Models that are of similar intent to understand about their components, constructions and weights.
G 'luck
Oct 30, 2011, 01:32 PM
Registered User
flydiver's Avatar
You don't have a dog, you got a pig. It should literally be half that weight.
Smaller motor, smaller lipos. You NEED to get the weight down so it floats.
You can try smaller 2S and a larger prop (you REALLY ought to have a wattmeter for this kind of work.) This is just putting the pig on a diet though.

You want just enough power, and no more, to fully power a lighter plane. Grossly over powering it means excess weight.

Lg. props moving slowly have torque (grunt power) which is needed for this application.
Small props moving fast can have 'horsepower', but not much torque. They are more suited to fast moving planes; sport type, not 3D.
A motor suitable for a jet (small prop moving fast) can have exactly the same amp/watt specs as a slow moving 3D motor. You have to know how to use the tool.

Totally rethink your build strategy. If you over built that model you may need to start over.

Do some reading here:
and here:
Oct 30, 2011, 01:48 PM
jackerbes's Avatar
"..1500 kV, 370W 8T
31A max

Wouldn't this give me 370w/ 1.5 lb = 246 w/lb? Shouldn't that me more than enough for a 3D plane? Why does it fly like a dog and it seems like I don't have enough power?..."

Your plane sounds like it is a little on the heavy side, the prop might not be optimum for the motor, and your expectations for the motor might be a little high.

Hobby King is notorious for overstating the capabilities of their merchandise. And like nearly all sellers, they present specs as sales data in a manner that lets potential buyers take the specs out of context and get unreasonably high expectations. If they got 370 Watts out of that motor it was only a momentary peak reading. It is an 80 gram motor, it is capable of about 3 Watts per gram or 240 Watts.

If you want to get the kind of power you're talking about out of a SK3 motor you should consider the 3530 or 3542 sizes

The Hacker motor your friend used costs about three times as much, weighs about 71 grams, and is rated for a peak Watts of 260. That is a little over 3 Watts per gram and is a credible number. The Hacker is an much better quality and honestly rated motor.

Down rate HK motors and ESCs to 75% of what they say they are capable of. If you want a 375 Watt motor buy one they advertise as being capable of 33% more than that and you can probably get the 375 Watts when it is down rated to 75%. If you want a 30A ESC buy a 40A or higher rated ESC.

Oct 30, 2011, 03:51 PM
Registered User
Dr Kiwi's Avatar
I have just test run a 76g Turnigy SK3 3530-1340 (~1380Kv) and with a 9x5 GWS HD it got me 1350g/47.5oz @ 54.3mph from 28.00A/308W. It was getting a bit warm after a brief WOT burst.

That's close enough to 2:1 thrust to weight for a 1.5lb plane... but if the plane weighed only 0.75lb... then it would really perform!

A lower Kv version of this same motor with something like a 13x4 might be the ticket.
Oct 30, 2011, 04:36 PM
Registered User
meshyx's Avatar
Originally Posted by jetada
bigger prop & lower kv
smaller prop and higher kv
You've gotten good help here, but I'd like to answer this in a different way. Think of the KV of the motor as the "electrical gearing", like on a bicycle. Forget about pedaling fast or slow, or changing gears, just think of the effort needed and the speeds you get in different gears.

For the same amount of watts drawn:
A low KV motor with the appropriate large prop is like first gear, you get good pulling power and acceleration, but low top speed. This is ideal for 3D and hovering.
A high KV motor with a small prop is like top gear, you get higher top speed, but worse acceleration. This is good for prop-driven 'jets' and strykers and the like.

What is low and high KV depends on the weight of the motor. For a 50 gram motor, 800kv is really low and 3000kv is really high. For a 250 gram motor it might be 200kv and 1000kv.
Oct 30, 2011, 07:34 PM
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faststick's Avatar
prop's on backwards.

Oct 30, 2011, 09:33 PM
Registered User
Thanks for the replies everybody. I was thinking it was a little heavy, I think he goes a little overboard on the hot glue. Also, I should have weighed it before painting it; I'm not sure how much weight I put on it with paint either.

meshyx, thanks for explaining the acceleration. I did notice that it was difficult to pull out of slower speeds.

I think I will rebuild the motor mount; it had a nasty crash on the maiden voyage that I had the instructer fly and it has quite a bit of hot glue.

I may just try to shave some foam off the fuselage and use a 1550mah pack to see how it flies. It'd be a shame to start over.
Oct 30, 2011, 10:04 PM
Registered User
At the risk of being Pedantic :-) Go to the Scratchbuilt Foamies Forum.. Find a decent 3D design (there are LOTS of good ones).. build one.. and ....Fit... the suggested proven power equipment.
There's No amount of lipstick you can stick onto your existing model that will stop it from being a Pig (to paraphrase:-)
Start OVER.. Why make life difficult?
Oct 30, 2011, 10:06 PM
Registered User
Could somebody please link me to a decent reasonably cheap wattmeter?
Oct 30, 2011, 10:22 PM
Registered User
I've read many pages in the leadfeather Yak55 thread, but I just didn't pay attention to the weight.

The guy whose design this is also used depron and I'm sure his plane is lighter. He's been flying for over 20 years and has about a dozen gassers with 110" to 130" wingspans. His 3d foam plane design works great; I've seen him fly his. I think it's just too much hot glue and perhaps a little heavy on the paint. I don't see any harm in spending a little time reworking the nose & motor mount if I can lose 5 oz and shrink the battery to lose another 3 oz.

You've got to cut a little slack about "reading the scratchbuilt forum". I've read hundreds of pages in there but there's a lot to learn when you're new.
Last edited by jetada; Oct 30, 2011 at 11:18 PM.
Oct 31, 2011, 12:06 AM
Registered User
I've seen quite a few posts where the 40" 3D planes are about 16-20oz.
Are these planes grossly overweight too? it looks normal for a 40" wingspan.

15.7oz without battery
480 with 1300 mah 2S battery
20oz with battery
15.5oz without battery

I just took off 2 oz by changing the motor mount and shaving of some glue & foam and taking off one of the wing spars (it still has a flat on the leading edge and one spar extending the within 1/2" of each tip).

My plane now weighs 16 oz without battery. With a 1550mah battery it's 20 oz. I think I'll try the other motor with 1340 kV.
Oct 31, 2011, 05:51 AM
Risto Hölttä - F3A/P Team FIN
Finnspeed's Avatar
Yes, try the lower kv one with 9 x 5 GWS HD as suggested by Dr Kiwi, it will be better for 3D. Even the original motor might perform reasonably well with 8 x 4 prop and the weight reductions.
Oct 31, 2011, 03:03 PM
Registered User
flydiver's Avatar
Original motor is 1500kv.
Using 2S on it will essentially be like making it a 1000kv motor so you can use larger props, but that means possibly buying new lipos.
That's one of the things the 'mid-KV' motors are great for. Pick a KV~1500-1600, use a 2S, and prop it like a 1000KV motor on 3S.

About as cheap as you can go for a wattmeter:
TURNIGY watt meter

FWIW, rattle can paint on a plane that size that is (very) weight sensitive can easily add an unacceptable amount of weight. This is why you see them very lightly airbrushed or done with permanent marking pens.
I had a buddy rattle can paint a small plane and made it completely unflyable. No way to 'fix' it after that either.
Last edited by flydiver; Oct 31, 2011 at 03:47 PM.
Nov 01, 2011, 01:43 AM
Registered User
Dennis Everett's Avatar
my 40 in foamy 3d plane is a bit over 9 oz rtf , using a razor motor , gws gearbox , 12x6 prop , 300 mah 3s lipo 3 servos, castle phnx 10 speed control , xps receiver ...

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