Electric motor mounting for side-thrust

[Spikey]

Which do you guys do? Fit the bulkhead into the fuselage at the correct side-thrust angle either with the motor central on the bulkhead or off-centre to bring the propshaft back onto the central axis? Or do you fit the bulkhead square to the fuselage with the motor central and then pack one side out as required?

I try to get both side- and down-thrust angles right when I fit the bulkhead, then mount the motor slightly offset to one side so as to bring the centre of the prop back onto the longitudinal axis.

The big question is, though - am I right to be doing that?

[Bob Cotsford]

as far as I'm concerned that's the right way to do it. Even if you fit the bulhead square you still need to off set the motor to allow for the sidethrust, why not make life simple and set side and downthrust into the bulkhead in the first place?

[avtur]

What a brilliant question!

My, slightly, engineering background would suggest that adjusting the motor position to get the prop position bang on the centre line would be the right thing to do. However that is opinion and not based on any practical experience.

Given that predicting sidethrust / downthrust isn't a exact science then you would have to be prepared to 'adjust' the motor mount a time or two in order to get the prop in the 'exact' right position.

I'm looking forward to the replies from those who at better informed !

[Alan Gorham_]

How do you know the sidethrust/downthrust angles you build in right for your model and power combo?

I'd be tempted to fly the model prior to final completion e.g. fitting cowlings etc and seeing what thrustline changes are necessary in an ideal world.

The reality is that it's not that simple and thrust adjustments are a compromise at best so both of your methods are sound, it's just a case of horse for courses...

[Tony Bennett]

i set my firewalls dead square and adjust after flying a few times.

quite often i find that the triming needed on the controls to compensate is minor enough to not worry about adjusting the side/ downthrust.

[Spikey]

Posted by Alan Gorham_ on 05/07/2014 18:27:34:

How do you know the sidethrust/downthrust angles you build in right for your model and power combo?

How indeed! On my previous Bees I've used what the plans call for i.e. 6° down and 2° right, simply because I know no better way.

Let's say that this one flies for the first time well enough, considering that I built it, and needs very little trim to fly straight and level hands off at half throttle. Given that the Lazy Bee is essentially a horizontal barn door with a fuselage hung under it and is pretty forgiving generally, how on earth might I establish whether or not the thrust angles are in the right parish? Might it not be the case that by doing that initial trim, I've actually compensated for a wonky thrust angle rather than, say, a slight wing or control surface issue?

[Alan Gorham_]

Well I see what you are saying but the term "trim" is quite a loose term and mainly comes from free-flight where you had to trim the model to glide safely and then trim it to fly under power safely.

If you intend to fly the Lazy Bee around for most of it's flight at half throttle because this gives you the type of flight you desire and you find it flies straight and level, hands off the sticks at this power setting then great, why bother to do anything else?

Be prepared for the nose to rise under more power though...

And to pick up on your wing or control surface issue then surely you can detect this by climbing to height under power and then stpoping the motor and checking how the Bee glides, so ruling out any power trim issues.

My main point is that ANY thrust angles given on a plan are what the designer used on their prototype to suit their power system, their airframe with possible warps or bends and maybe different incidences to those you have ended up with and also their style of flying. I.E. they are a compromise.

What you are discovering now is that YOU may have your own ideas and preferences and they may be different.

[Pete B]

If the design calls for down/sidethrust, I'll build that into the bulkhead as the motor mount, of whatever type, will then need less offset in the event that I need to add a little more - not unusual but must be the way I fly....

Rather than build a ply motor box, I usually use the threaded studding system, as here:

This mounting method makes fine adjustment of down/sidethrust much easier to achieve. You can also see the pencil marks which indicate the offset from the fuselage centre line on which the mount is located. This ensures that the prop driver emerges roughly where it should, in the middle of the hole at the front (can't think of more technical term!). It may look a little flimsy here but once it is all nutted up, it's surprisingly rigid.

BTW, in case anyone thinks that that bulkhead looks a tad thin to support the motor, it is doubled to 1/4" behind the bulkhead

Pete

[Spikey]

Thanks again gents. I think I'm finally getting used to the idea that at least with the sort of models I like, few things are as critical as I imagined them to be ...

[Andy.I]

This might be an academic point, but I think there's some relevance...

it dawned on me recently while pondering over side thrust and motor offset that they are both actually the same thing. For example, a motor offset to the left is the equivalent of right thrust and a motor angled to the right is the equivalent of being offset to the left.

From this I conclude that offsetting a motor to allow for its thrust angle is a cosmetic exercise so the centralised prop and spinner just look right.

[Spikey]

Andy, I wouldn't know about motor offset per se. All I've been concerned about is that if the motor's mounted central in a bulkhead which is sloshed relative to the fuselage, the centreline of the propshaft is both offset from and at an angle to the longitudinal axis of the fuselage.

What I do is slosh the bulkhead then offset the motor on it so that the angle is maintained but the centre of the prop is put back on the longitudinal axis of the fuselage. And I've only been doing that because it seemed logical to me - not because of any theory behind it. That's why I was asking if that's right!

[Stevo]

Pete B's got it right on the money. I've done this before...

Perhaps you cant see it in the picture but I've shortened the appropriate arms for side/down thrust...

[Gary Murphy 1]

Being a new builder and new to the hobby this post was just right.I will be needing to suit out the motor position for thrust to and I have the choice of both ways,mount or fire wall.For me the mount method will be easier BUT it is hard for me to work out the system of moving the motor in 2 plains and stll get the shaft centra.,so I go to all that trouble and then might need to adjust and the shaft is now off center.Would a couple of degrees be noticeable OR look dodgy?

[Bob Cotsford]

It's very easy to get tunnel vision when we're building a model. Generally speaking, if the motor ends up so that the prop isn't quite on centre the only one who will notice is the builder. I've certainly built more than one model where I didn't allow enough offset or had to add extra sidethrust and whether the cowl is offset to the rest of the fuselage or the prop is offset to the cowl as a result, nobody notices!

As for working in 2 planes - simple. Don't. Treat sidethrust and downthrust separately. Measure from the back of the prop driver to the motor mount in inches and call it L, then offset=no of degrees*1/64*L" or degrees*0.016*L".

So if your motor is 3" long and you want 3 degrees sidethrust, offset = 3*3*1/64 = 9/64" or a smidgeon over an eighth of an inch. If you want 2 degrees downthrust, offset= 2*3*1/64 = 6/64 = 3/32"

You motor mount needs to be 1/8" to the left of and 3/32" above the centrelines.

Courtesy of Bodgit & Co engineering dept.

Edited By Bob Cotsford on 10/07/2014 12:01:29

[Gary Murphy 1]

Bob,thanks for the great tip.Most helpful.

[Pete B]

As a co-proprietor of Bodgit and Co, I'd go along with Bob.....

If you look at the front end of the plan in the background of my pic above, you'll see a pencil line drawn at a 2deg angle to the centreline. Follow that pencil line back to the bulkhead and that distance from the plan centreline ( surprisingly about 1/8"!) has been transferred to the bulkhead itself - you can see the offset from the centreline.

Easier to see than explain...

Pete