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Old Jun 14, 2014, 01:42 PM
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United States, WA, Monroe
Joined Dec 2013
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Question
Engine mounting measurements

I'm at the point in the PT-60 kit where I'm getting the engine mount in place. Reading ahead, it calls for positioning the engine so that the backplate of a spinner will be 4-7/8" in front of the firewall.

I haven't picked up a spinner yet (somehow managed to purchase parts and tools I won't need till much later in the build, but neglected to buy an item I would need sooner). I'll order one (or more?) today, but I don't want to get hung up on this step waiting for it. My question is if I can get the same measurement by using the front of the drive plate/washer as a stand-in for the back of a spinner as it seems the two would be in contact anyway? If it makes any difference the engine is a TT Pro 61 and I'm using the GP mount that came with the kit.
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Old Jun 14, 2014, 02:55 PM
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United States, WA, Port Angeles
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it really depends on the spinner you plan on buying!

I have made that mistake, most aluminum sinners are flat on the back of the spinner!
and some plastic spinners have a recessed back plate!

since it is a PT not a scale project, I would give it an extra 1/8th inch!
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Old Jun 14, 2014, 04:29 PM
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Letchworth, Great Britain (UK)
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If you get a backplate spinner, like this one http://www3.towerhobbies.com/cgi-bin...?&I=LXE117&P=7 and most of the others at Tower Hobbies, where the backplate is mounted first, then the prop goes on, then the spinner fastens over the top, then the back of the spinner is the same as the front of the motor's drive plate.

If you have the kind of spinner where the prop goes on first, then a sort of spinner skirt, like this one http://www.jperkinsdistribution.co.u...tname=Spinners on top of it, the distance can vary according to the depth of the skirt and the thickness of the prop.

So I would work off the front of the engine drive plate, and make sure the spinner you get is the backplate variety
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Old Jun 16, 2014, 12:08 AM
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Arvada, Colorado
Joined May 2004
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Hello again, Sparky!

I just picked up the 2-1/2 size spinner made by Du-Bro. I talked over the options with my local hobby shop owner (who fortunately is an avid flyer and builder with nearly 45 years of experience in RC). I went with the 2-1/2 inch rather than the 2-3/4 inch recommended spinner shown on the plans; primarily to give myself that extra 1/4 inch of play. This type follows the recommendation abenn mentioned in post #3 above.

You can see in one of the attached photos below, that I had some variance between the side panels of the fuselage and the nose blocks. I ended up with a mismatch on the top edges of the front of the fuse and will end up scabbing some small pieces of scrap beginning at the fire wall and tapering out to where I can sand the mismatched edges to the same size. I don't think anyone is going to notice the variance from the plans, nor do I think it will affect the flight characteristics of the model. We'll soon see.

Regards, Chris
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Old Jun 16, 2014, 02:36 AM
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Quote:
Originally Posted by Jugjock View Post
... I don't think anyone is going to notice the variance from the plans, nor do I think it will affect the flight characteristics of the model. We'll soon see. ...
Just sand out the mismatch, and all will be fine. What is more important is to make sure you mount the engine with the correct right and down thrust, as shown on the plan.

And while you're at that stage, don't forget to fuel-proof all the wood in the engine bay.
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Old Jun 16, 2014, 12:40 PM
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Hello, abenn!

Thank you for your immediate reply and sound advice.

Not having built a nitro powered model previously, I've been researching other modeler's fuel-proofing techniques. From what I've found, most simply use an application of epoxy glue; brushed on, heated with a heat gun, and excess removed with alcohol soaked paper towels. Paint can then be applied over the cured epoxy.

A few have used epoxy paint, sprayed or brushed.

Spraying epoxy paint requires some significant care, preparation, and safety equipment. I like the idea of matching the engine compartment, and other interior areas subject to fuel exposure with color-coordinated epoxy paint.

Nevertheless, since this is a first-time trainer, and will likely be subjected to some rough use, I keep telling myself that "quicker and cheaper" are really the more practical solutions. So, I'll probably go with the simple epoxy glue application.

Of course, other opinions, comments, and lessons-learned from experience are most welcomed.

Regards, Chris
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Old Jun 16, 2014, 01:33 PM
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United States, WA, Monroe
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Hi Chris!

Thanks for taking the time to look everything over and offer the advice you have. I'll be interested in watching your kit come together as well.

I ended up ordering some Great Planes the day after posting the question. I'm confused though about the purpose of having extra play by dropping the size 1/4". I must be missing something...

The noseblock pic you posted looks exactly like how mine ended up, I just somehow managed to make it occur on the opposite side instead. I like the scabbing idea, but I'll certainly give it another look today and if the profile won't look too odd, I will just sand it.

Thanks!
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Old Jun 16, 2014, 01:48 PM
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Quote:
Originally Posted by whiskykid View Post
it really depends on the spinner you plan on buying!
I ended up ordering several GP 2-3/4" spinners. I couldn't determine what the backplate is made of (some descriptions say it has an aluminum backplate, others say nothing about it, and others simply say "nylon spinner"). I was hoping to not get hung-up waiting on that, but it ended up being a busy weekend so I wouldn't have had the time anyway and now the order is only a few more days out.

I like the idea of moving the engine forward another 1/8" as it sounds like I would be able to use a broader variety of spinners in a pinch.

Thanks!
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Old Jun 16, 2014, 01:53 PM
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Thanks abenn!

I think I ordered a backplate type, but now I'm not so certain, they'll be here in a few days along with a pile of other items I hadn't thought to take care of before.
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Old Jun 16, 2014, 02:16 PM
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Some backplate spinners have aluminium back plate and plastic spinner cone, some have plastic and plastic, and others are all aluminium. I think GP do all three varieties.
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Old Jun 17, 2014, 01:31 AM
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Arvada, Colorado
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Hello, Sparky!

Regarding my comment about having some extra play: I was thinking (perhaps erroneously) that having the diameter of the back plate slightly smaller than the interior sides of the nose blocks might eliminate any dimensional errors in my build. I ran into some problems with the fuse sides, fuse doublers, and fuse formers that required some removal and re-installation. The smaller spinner will probably increase drag a little, but I thinking the effect won't be noticeable. If later, I want to increase the size, it shouldn't be a problem.

I'm tempted to start my own build thread for the PT-60, but I've started several threads in the Scratch Built electric plane forum and never finished them, so I'm reluctant to start posting before I'm sure I'll complete the project.

You've posted some really nice work on your PT-60. Again, I really appreciate your taking the time to do that.

Regards, Chris
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Old Jun 20, 2014, 07:35 PM
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For fuel proofing I sometimes use epoxy thinned with a couple of drops (it doesn't take much) of alcohol-the 90%version-. Works great, and drys quick .
As far as where to mount your engine you might want to wait till you can check the balance in case you need to move it out a little instead of adding lead weight.
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