DA 100 throwing props/spinners

[Gary Powers]

I have a brand spanking new DA 100 on an Aeroworks X260. Each of the first two flights ended quickly with a thrown prop and spinner. When I checked the needle settings Monday, I found the low end at least a full turn richer than factory recommendations.

As a starting point, can I assume that I have found the problem with the engine, as in "back firing" and shearing the prop bolts (forgot to mention that)?

Gary

[Jenko]

I have an idea, ask the people who know best, Desert Aircraft.

[3dubya]

Get howie to help out jenko..

[GremlinX]

How tight are you or not tightening the bolts? The only time I have seen them shear is when they losen, or are not tight enough... carbon prop? Wood prop?

[Freddy Warbird]

I wouldn't think a backfire would cause prop bolts to shear off. IMO, Prop bolts don't know whether the engine is rotating one way, or the other.

I would look into how your fastening the prop to the engine. Not every propeller has the same hub thickness, some are thicker, and some are thiner. The fasteners may not be fully seating against the plate. They could be bottoming out -ever so slightly- against the rear engine hub giving a false indication of torque. Likewise, over tightening bolts with a wooden prop could crush the hub ruining the prop. Just my two bits!

[JAS]

Start with factory settings. Since you found the low end out, I'm assuming you have checked and reset both? Be careful not to over-tighten the needles as this will ruin the seats in the carb.
Make sure that your prop bolts are not bottoming out (not sure if the hub is drilled/threaded all the way through). Screw a prop bolt all the way in and measure the length that remains out. Then measure your set-up (prop/spinner back plate/washer) and make sure it isn't to thin. If bolt isn't actually tightening against the prop it's tightening against just the hub.
Call DA in the morning, I'm sure they have a more detailed list of things to check.

[GremlinX]

As for the backfire... I think it would not losen them. As for a backfire, that can shear off flywheel bolts on a car, but it's generally when there is a severe change in rpms... ie nitrous backfire. I think you would know it if it really did backfire. I think it is in the bolts/prop/hub. I do not think it is internal... give DA a call like they have mentioned.

[dmcquinn]

Almost certainly your prop bolts were too loose. Needle valve settings will not cause this. I use a torque wrench calibrated in inch-pounds (not foot-pounds). I use 37 inch-pounds for wood props. Re-tighten them after fllights 1, 4, 8, then every so often.

[Kevin Y]

	Quote: Originally Posted by JAS
	Make sure that your prop bolts are not bottoming out (not sure if the hub is drilled/threaded all the way through). Screw a prop bolt all the way in and measure the length that remains out. Then measure your set-up (prop/spinner back plate/washer) and make sure it isn't to thin. If bolt isn't actually tightening against the prop it's tightening against just the hub. Call DA in the morning, I'm sure they have a more detailed list of things to check.

You beat me to it Jason. Thats what I would look at!

[Gary Powers]

	Quote: Originally Posted by GremlinX
	How tight are you or not tightening the bolts? The only time I have seen them shear is when they losen, or are not tight enough... carbon prop? Wood prop?

Appreciate the responses. Wood prop, very tight! I should have mentioned that in addition to the two lost in the air, the first time I cranked it on the ground, it also spit the prop/spinner off and sheared all six bolts. I am assuming a backfire, since I really have never heard a bang or such. It happens as I throttle up.

Gary

[Gary Powers]

	Quote: Originally Posted by Jenko
	I have an idea, ask the people who know best, Desert Aircraft.

Well, I intended to put the question in the Desert Aircraft Support forum. In fact, I did. Hmmmmm.

[Vic3D]

	Quote: Originally Posted by Gary Powers
	Appreciate the responses. Wood prop, very tight! I should have mentioned that in addition to the two lost in the air, the first time I cranked it on the ground, it also spit the prop/spinner off and sheared all six bolts. I am assuming a backfire, since I really have never heard a bang or such. It happens as I throttle up. Gary

Hey Gary!

I'm starting to think that your ignition sensor might be off, which makes your timing bad. I would get that checked out with DA, also.

Vic

[Gary Powers]

	Quote: Originally Posted by Freddy Warbird
	I wouldn't think a backfire would cause prop bolts to shear off. IMO, Prop bolts don't know whether the engine is rotating one way, or the other. I would look into how your fastening the prop to the engine. Not every propeller has the same hub thickness, some are thicker, and some are thiner. The fasteners may not be fully seating against the plate. They could be bottoming out -ever so slightly- against the rear engine hub giving a false indication of torque. Likewise, over tightening bolts with a wooden prop could crush the hub ruining the prop. Just my two bits!

Good thoughts Freddy and thanks. If anything I am over tightening. This started with some thoughts posted by KIWI regarding the wisdom of using anti-seize or lithium grease on the bolts to prevent them galling. When it threw the prop on the engines initial start, I got to thinking maybe that was a bad idea, so I cleaned the stuff out of each hole, but it has happened twice since. Rich as the engine was, the power was very impressive. Can't wait to get a full flight in. :-)

Gary

[Gary Powers]

	Quote: Originally Posted by dmcquinn
	Almost certainly your prop bolts were too loose. Needle valve settings will not cause this. I use a torque wrench calibrated in inch-pounds (not foot-pounds). I use 37 inch-pounds for wood props. Re-tighten them after fllights 1, 4, 8, then every so often.

I do have an inch-pounds torque wrench and will check it out tomorrow. And I will give DA a call in the morning; I thought they might be monitoring this forum.

And thanks to everyone who responded! I've gotten just enough time on this airplane to know that I want a lot more and I only have the coming weekend to get this solved before its off to Albuquerque for their IMAC contest.

Gary

[Gary Powers]

	Quote: Originally Posted by Vic3D
	Hey Gary! I'm starting to think that your ignition sensor might be off, which makes your timing bad. I would get that checked out with DA, also. Vic

Ah, Vic, something I had not considered at all. OK, a call to DA first thing in the morning. And again to all, thanks.

Gary