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-   -   why are the motors SO far forward? (http://www.rcuniverse.com/forum/aerodynamics-76/11231434-why-motors-so-far-forward.html)

guver 09-17-2012 01:02 PM

why are the motors SO far forward?
It has always intrigued me why motors/props were so far forward on many of my planes. Engines for that matter too. I realise that we need to get a good CG balance , but many of mine seem to really be hanging further forward than they need to. I've not had much experience moving them , maybe only moving props a few mm forward or backward. Although one mount I installed on a nexstar mini-ep wound up being way further forward than stock. It crashed.

What started this was my e-stik may need a new motor and I like using lightest stuff available. Well it is one that actually hangs the motor out front with nothing else there. Given that I want to keep the same CG and motor weight I could conceivably mount the motor BEHIND the mount (instead of ahead of it) thus moving the prop back a few inches. I could then move the battery forward to keep same CG. I guess being a car/truck nut convinces me that all weight should be concentrated in the lowest center of vehicle for a good polar moment of inertia and handling.

Is it not the same with planes? (other than ability to be in many various orientations) Is there a significance to having the prop far away from LE of wing? I do have a couple of props behind the wing planes.

I notice that many of my planes have the battery exactly on the CG , yet some of them are very much forward of that point. Let's say I needed to install a heavier motor instead of a lighter one. Would I then be ok to move it far back? AND the prop?

Hope you kinda get what I'm asking and why. It doesn;t have to be very scientific.

da Rock 09-17-2012 01:36 PM

RE: why are the motors SO far forward?
Why? Because they have to balance the weight on the other side of the CG, and the CG gets it's location based on aero considerations. The area of the wing, it's chord, how far back the horizontal tail is, it's area and chord are those aero considerations. Once they're built, they and the weight of the plane lock in where the motor goes and how much it has to weigh. That's it.


If you're replacing a motor with another, the usual choice is a replacement that weighs the same as the old one. If not, then you work out where the replacement should go based on how much it weighs compared to the old one. Pretty simple to do actually.

As for placing things like they are in cars, that idea doesn't fly. Almost nothing is comparable.

So what does the old motor weigh, and what weight is the new one? Yes, shifting a battery can make up for a change in motor weight.

guver 09-17-2012 01:40 PM

RE: why are the motors SO far forward?
It's that simple then? For my particular electro-stik I am getting a slightly lighter motor and will be forced to mount it the same far forward position because I will be able to move the battery a bit forward. Thanks for your response. It is simpler than I expected.

Leadershobby has WAY too many motors to choose form. :D

BMatthews 09-18-2012 10:40 AM

RE: why are the motors SO far forward?
Yep, it's that simple.

The last thing you want to do is have to add weight. And if the nose becomes TOO long then it does have some downsides as well. As a general rule of thumb the prop is best off when it's about one wing chord in front of the leading edge or less. But this is not chiseled in stone by any means. Folding props on electric models will often foul the wing when folding unless the motor and prop are a bit more forward. And if you look at Open Class "Mulvihill" rubber models you'll see noses that are almost silly long. Yet they fly just fine.

Also there is simply nothing at all sacred about having the battery pack located right at the CG. And in fact it is one of the primary things to move as required to get your CG into the right spot. ESPECIALLY if you fit a lighter than stock motor and need to shift the CG.

guver 09-18-2012 01:18 PM

RE: why are the motors SO far forward?
Thanks guys. I'll not worry any about it. I'll just continue to put my CG where I want it (never adding wight)

jester_s1 09-18-2012 06:36 PM

RE: why are the motors SO far forward?
FWIW, moving the motor behind the mount would cause cooling issues even if you compensated for the CG without adding weight.

David Bathe 09-30-2012 10:03 AM

RE: why are the motors SO far forward?
There Isn't a Stick I've built that hasn't had the firewall chopped and repositioned further back. At least 1/2 of the front between the F1 and F2 removed. Looks better, balances better.<div>A great many of todays sports models still follow classic RC model designs. Back in those days, engines where smaller, simpler, lighter.</div><div>Today we want bigger, heavier more powerful engines,  2 stroke and 4 strokes. The older designs need modification to enable this.</div><div>Very simple to establish on a new model before assembly... strap on all the tail feathers, fuselage mounted gear, throw in some servos and pick up the body (no need to attach the wing unless it's significantly swept) and lay the engine, complete with prop.spinner and mount along the front of the fuselage untill you feel it balance. Chop and build a new firewall at the point.</div>

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