Motor stand offs or boxes?

[Matt Carlton]

Hi chaps.

Looking at converting IC models to electric, often it seems that an offset/stand off is needed to get the prop driver in the right place.

Is there a 'best way' to do this?

I'm assuming it depends on the size of motor, but I'm contemplating building a hollow ply 'box' which can be bolted to the firewall using the existing blind nuts for the IC mount. Made long enough and robust enough, I imagine that should work? Maybe 1/8" ply for smaller motors up to 300W or so? 1/4" for more than that. I guess they don't need to be as hefty as they would for IC? Less vibration and all that.

I'd rather not start ripping out firewalls and rebuilding if I can avoid it!

Edited By Matt Carlton on 07/01/2021 21:05:24

[Trevor]

1/8 ply will be fine. You might want to use triangular balsa strip inside the joints to increase the gluing area.

[Martin Harris - Moderator]

This was the solution chosen by Extreme Flight for their electric conversion option for several of their models. Don't forget to build in any desired side and/or downthrust while you're making the box.

Edited By Martin Harris - Moderator on 07/01/2021 21:51:53

[Matt Carlton]

Excellent, thanks chaps.

I am on the verge of going all electric. Didn't think I would, but the lack of noise, mess and faffing about is very appealing for the smaller models I prefer.

[Capt Kremen]

Whilst alloy stand-offs are nice and invariably strong for any size motor, a custom made ply box can be tailored to the exact size and distance for the prop driver/spinner backplate.

[Stuphedd]

I used to use "boxes" but I have found that threaded rod , from B&Q 4,5,6 mm allows you to play with the down and side thrust settings and get them to your preference ,

cheers

[leccyflyer]

Like Stuphhed for a time I also used to use boxes but now greatly prefer the ease and adjustability of threaded rod with aluminium tube stand offs and radial motor mounts. These replicate the excellent Ernie Moorcraft squirrel cage mounts that I used to use years ago.

[Geoff S]

Funnily enough I've just done that to my Li'l Cub build. The AXI motor I picked up from a sale at the club is rather longer than the recommended one and the Peter's dsign calls for a 4Max motor stand off. I opted to make a box.

MIne has a 6mm 'firewall' so that 4mm T nuts fit easily (3mm is a bit shallow) but the 3 sides are 3mm ply and the 4th side will have 1.5mm balsa once it's all wired up (it can be easily broken away if I need to do any maintenance). I was concerend that simply glueing the box to the 3mm firewall of the already built fuselage might not be secure, so I added 4 x 6mm dowels.

I íntend to put a battery isolating/arming plug to the side of the box because the battery is fitted from the front right in arc of the prop. I don't normally bother but in this case it seems a sensible precaution.

I've used long 4mm bolts and locknuts in the past but to do that you need to be able to fit T nuts in the firewall and, as I said above, 3mm is too shallow. Plus I needed a means of mounting the dummy engine.

Geoff

[MAD Dave]

Has anyone tried 3D printed stand-offs and/or boxes? Obviously structural considerations to be thought through but exact size - no problem!

[iflylilplanes]

I've had a thin plastic mount, (looks 3D printed), the motor got just hot enough, the motor mounting screws pulled through the plastic. I was odd, the plastic where the screws went through the mount was 3 times as thick as the rest of the mount. I think maybe the wrong filament was used for the print, needed a higher temp plastic.

Dave

[SIMON CRAGG]

I have now converted virtually all my fleet of 18 models.

From 1/4 scale to 3D models.

I have used stand offs from Spalding Fasteners every time.

Next day delivery.

[Chris Bates]

Ply box much more rigid than all thread rod for longer stand off.

[Stuphedd]

A box would have been a bit difficult with this one !

[Geoff S]

Posted by SIMON CRAGG on 08/01/2021 08:25:36:

I have now converted virtually all my fleet of 18 models.

From 1/4 scale to 3D models.

I have used stand offs from Spalding Fasteners every time.

Next day delivery.

Those spacers from Spalding Fasteners look very good with a wide range of lengths. Not seen them before and I may well use them in the future. Thanks for pointing them out.

Geoff

[Nigel R]

Used both.

Standoffs probably quicker. ESC can be tucked inside the standoff area.

Ply box allows the lipo to slide forward; might be useful for CG location.

Converting an existing model, standoffs are probably easier.

[Matt Carlton]

Those standoffs look pretty good Geoff. I assume I can put them straight into the IC mount blind nuts and fashion a mounting plate for the motor to fix to. Easier than drilling new holes in the firewall.

The box might be a better option on some though as the motor is a good deal lighter than an IC engine so as Nigel says, CG location might benefit. I'll have a think before I start pulling things apart.

[Gary Manuel]

I'd be a bit concerned about using the spacers on anything but the smallest of electric models. Unlike a properly designed standoff, there is no widening of the diameter towards the ends of each piece. This means that they will be more prone to "biting" into the firewall and eventually working themselves loose.

Remember that standoffs rely on having zero free play for them to be effective. Even the slightest bit of free play will result in movement, which will result in wear, which will result in more movement etc.

[Geoff S]

Gary, you can minimise any biting into the firewall simply by using so-called penny washers (Modelfixings stock them). I do that when I use long bolts and nuts (usally 4mm) and I haven't had a problem.

Geoff

[kc]

I notice that the pic of Geoff's model does not have the traditional triangular reinforcement to front bulkhead. Reinforcement of that joint is essential in my opinion and i recall some designer mentioning that it had inadvertantly been omitted in a plan but should be used.

A year or so ago i had a 500 watt electric model throw a prop blade after the prop just touched the grass on takeoff and this vibrated the engine bulkhead off completely within a few seconds. Bulkhead came off with balsa triangle half on bulkhead half on model. Model was landed OK without motor which greatly changed CG! Replaced with birch ply block instead of balsa triangle. Beech 3/4 inch square might be better for my future models and could be trimmed to rough triangle if space is tight. A couple of small screws into ply sides might help too if the motor box is birch ply.

[Gary Manuel]

True Geoff, but I still think that the spacers should be used with care. Threaded rod with nuts and washers may be a better solution.

[Mike T]

Horses for courses, innit? Box for longer installations, standoffs for the shorter, or a combination of the two for adjustability.

I'd save hard ply for the 'face' of the box. I usually use 6mm liteply for this kind of thing. Light, stiff, easy to cut (with a razor or shark saw) and easy to square up the joints with a bench hook and a permagrit.

With standoffs, (ref. Gary's post above) you need a suitable load spreader where they meet the timber airframe - an alloy plate or "repair washers" from the DIY

Edited By Mike T on 08/01/2021 12:21:39

[Gary Manuel]

Is there enough thread at the end of the spacers to allow a washer / plate of sufficient thickness to be added?

Just a thought.

[Mike T]

Looks sufficient if T-nuts are used. No problem if you use studding, which I would probably sleeve with alloy tube. (Studding on it's own looks like a bit of a lash-up).

[leccyflyer]

Posted by Gary Manuel on 08/01/2021 12:03:43:

I'd be a bit concerned about using the spacers on anything but the smallest of electric models. Unlike a properly designed standoff, there is no widening of the diameter towards the ends of each piece. This means that they will be more prone to "biting" into the firewall and eventually working themselves loose.

Remember that standoffs rely on having zero free play for them to be effective. Even the slightest bit of free play will result in movement, which will result in wear, which will result in more movement etc.

I use two radial mounts. One on the firewall and one on the motor, with threaded rods through both, into captive nuts set into the firewall. That replicates the squirrel cage mounts that i have used previously, with no chance of biting into the firewall.

[Geoff S]

Posted by kc on 08/01/2021 12:16:49:

I notice that the pic of Geoff's model does not have the traditional triangular reinforcement to front bulkhead. Reinforcement of that joint is essential in my opinion and i recall some designer mentioning that it had inadvertantly been omitted in a plan but should be used.

It doesn't but it does have 6 x 2mm dowels (3 each side) and it's glued with 60 minute epoxy. I think it should be secure.

Geoff