Mini Vapor tip - Prop Repair and Balance

Mini Vapor tip - Prop Repair and Balance

Postby SamIam » Sun Mar 31, 2013 6:01 pm

If there's a weak point in the Mini Vapor design, it has to be the prop. It's a delicate piece of plastic, particularly at the hub. It's prone to breakage and unbalanced vibration.

Here's what happens.

Screwing the prop on may cause the hub to split as the shaft cuts its way in (I suggest warming up a new prop before putting it on so it's more flexible). Or maybe there's been a ground strike or two. Either way the hub cracks. Initially the crack is small. You might not even see it without a good magnifying lens. But now that stress on one side of the hub has let loose, the 'good' side pulls both blades closer to that side. Again, it's barely visible without squinting real hard, but now the blade is split AND unbalanced. There's the slightest hint of 'banana' where there should be 'straight as an arrow'.

This is when you start to notice the power isn't as good as it used to be (vibration sucks energy). The split continues to widen. Now the prop can't even grip the shaft anymore. I thought my motor was dying, but it turned out the motor was humming along fine, it just wasn't turning the prop with it. This is when you've finally realized your prop is shot and it's off to the LHS for a new one. The prop may even have broken in half at this point.

But the fix is easy, even if the prop has broken clear in half.

Broken Prop! Oh Nos!
prop_fix_1.jpg (47.99 KiB) Viewed 6826 times

First glue up the prop with super glue. Whether the prop has broken clear through, or merely split at the hub, glue it up straight as you can. Now use thread to bind the hub together. Bind it tight on both the top and bottom of the prop, and criss cross between the blades. Use a little super glue to get the thread stuck down to start it, then soak the thread in super glue once you're done. Don't go crazy with the thread. It only takes a few wraps to keep things held together. The hub won't be going anywhere now and the thread will keep it from splitting in the future.

And fixed!
prop_fix_2.jpg (34.03 KiB) Viewed 6826 times

Balancing a tiny prop like this might be a problem (friction), but here's a trick.

Screw the prop back on the prop shaft (it's going to be a little tighter now that the hub is properly strengthened). Now unscrew the prop shaft rear thrust block (it's a reverse thread so you have to turn it backwards to loosen it). Unscrew the rear block 'just enough' for the shaft gear to clear the motor pinion (see the picture). Now the prop is free wheeling. You might be able to do some 'rough' balancing like this, but the shaft friction is still going to be too much to get an accurate balance, so here's what we do. Plug in a battery and throttle up the motor at the slowest possible speed, keeping the Mini pointed down slightly so that gravity keeps the prop from sliding back and re-engaging the motor. The vibration from the running motor will loosen up the prop shaft just enough that the prop will always turn heavy side down. Now it's easy to see where the balance is at.

In mount balancing. Make sure the motor pinion doesn't engage the main gear.
prop_fix_3.jpg (35.72 KiB) Viewed 6826 times

If the prop is properly balanced it will come to rest in any orientation (basically stay where you turn it). But chances are good, the prop is going to come to rest in a horizontal position. Even if you turn it by hand it will return to the same horizontal position. This means the blades are still 'banana' bent and not straight across from tip to tip. They're drooping like a moustache and that makes them want to rest in a horizontal position. Pinch the blade firmly at the hub with one hand and gently force one of the blades 'away from the droop' with the other hand. Try to grab and force the blade as close to the hub as you can so you don't bend out the rest of the blade pitch. Except for the hub, these blades are remarkably resistant to 'permanent' bending. Go slow, before you know it, the banana will be gone and the blade balance perfectly.

You may find that after you straighten out the 'bend', that one blade is still heavier than the other. You could correct this too (trim the heavy blade or add weight to the light blade), but worst of the balance came from the bend in the prop and I just let the rest go.

Finally tighten up the prop shaft rear thrust block and enjoy your new smoothly flying Mini. You can check your prop balance at any time by loosening the rear thrust block and getting the motor running again.
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Joined: Mon Feb 13, 2012 5:46 pm

Re: Mini Vapor tip - Prop Repair and Balance

Postby filbert » Sun Apr 14, 2013 8:18 pm

excellent tip for us night vapor fliers, thanks for posting.
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Joined: Fri Nov 09, 2012 11:07 pm

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