Guys,

In two weeks I've lost two Dyanamax fans running on an OS .91. I looked at the crankshaft and it seems straight to me. What could be causing it?

The first time, the shroud and all stators broke. This time, only the rotor blades are gone. I cranked up the engine and set the mixture. I did not feel any strange vibration. I lined up with the runway, accelerated to "Vr" and all sorts of crap starting coming off the exhauast (impeller blades). I was able to do a soft landing on a grass field, and the airframe didn't suffer any damage.

I'm starting to think FOD. But I don't know, the runway looked pretty clean to me.

Any ideas?

did you use the same parts for the second fan or is this a new fan?

In the 15 years that I flew DFs, the only Dynamax rotor that I saw come apart was one that had the starter probe stuck through it after the OS.91 started.

I can't imagine why you have had 2 come apart. Did the blades break or did they just come loose from the hub? I have heard of some blades coming loose from the hub if the hub bolts were not tightened properly. The bolts need to be assembled with locktite, also.

I never heard of FOD until the turbines came on the scene, but I guess it's possible to have FOD with a DF.

Hi Gary,

The hub was tightened alright, it still is. Only the blades came off, the hub is intact. The engine does not make any noticeable vibration or noise while its running.

Ram-bro, it was a totally different fan. I will try using a Ramtec I have sitting on my bench.

Is this the same engine on both fans?


In 12 years of flying fans I have never seen one come apart. Other than a Byro fan that came unbolted from the mounts.

Ronnie, yes it's the same engine. Like I said it was runnig solid until take off roll. I doubt it was an engine problem. Weird isn't it?

Sorry to hear this man.

Is it possible that the first fan it shredded bent the crankshaft? I am pretty sure we have a run out gauge. If you bring it in we could check it.

Seems odd this happening. The Dynamax unit is a solid unit. I am wondering if the main aluminum mount might be cracked or something. There could very well be an issue with the motor, if it isn't in the mount. Do you have a different motor you could try?

For the blades to let go, they would have to hit something, and I sincerely doubt it is FOD. It takes a large hard object to do the damage you are stating here, liek putting a starter wand through at full throttle. I would guess the fan is hitting the shroud or, the mount is loose causing the fan to come back or twist and hit the stators. That would be where I would first start looking.

Hi Les,

I will bring it tomorrow. Like I said the crankshaft loosk straight, but only to my eye. Also I sassume that if the crankshaft was bent the Fan sould have been destroyed right away.

John R, Yes I have another OS engine (New) on a Ramtec. I will try the Ramtec on my F-15. Im positive it will fit OK.

David

David,

The only fan I ever had shed blades was a early Ramtec, and there was an obvious flaw (air bubble visible at the break point) in the molding at the base of one of the blades.
In my experience JMP's quality control was always second to none.
Is it possible that the 3 small screws that hold the rotor hub together were not tightened properly or vibrated loose?
I was always told to use a liberal dose of loctite on those screws to keep them from vibrating loose, and even with loctite I did find one that had come loose on occassion.

Kevin,

To be honest I've never messed with the rotor screws, not even to check weather they are tight or not. These fans have been running for years Maybe that's my missing link. Better start checking for stuff like this.

David

I used blue loctite on every single screw in the fan and the engine, except for the rotor nut and header bolts which got red loctite, and the engine mount bolts got nothing since they thread directly into the plastic centerbody.

Take a look and let us know what you find when you check those screws.

PMFJI but are you using Tom's pittot tube? The one that mounts into the fan. If you are, could you check and make sure it that at full power it will not rotate and bind with the blade.

Yes I am, but the pitot tube is way behind the rotor.

Have never seen one of Tom's JMP Dynamax ever come apart. This unit is tested far above anything your OS will ever acheeve.
I agree with the others that something is array. I think you should send the whole unit into Jet Model Products for inspection and set-up. Then also have someone check the engine runout .

Is this a unit that has the blades repitched? The heat they use to repitch will make the carbon fibers delaminate from the matrix and cause failure.

I still would send it in for a going over .

Marty O

Some of the very early Dynamax fan units had problems with the rotors coming apart with the OS91 (the early Dynamax fan was designed around the OS 65/77 VRDF). JMP modified/improved the rotor design to make it more robust and never looked back.
When you buy "very old" used (or NIB) Dynamax fans you have to be wary of this issue. I speak from experience and had to send mine to to JMP to get upgraded. It is possible that the fan unit in question might have been one of the older fan units that had not been upgraded.

Peter
Fort Worth, TX

The only problem I ever heard Tom talk about with the early Dynamax fans, was that some purchasers were assembling the rotors incorrectly and some rotors came apart. Tom started assembling all rotors himself and the problem went away. I have owned some very early Dynamax fan units that were built before the OS.91 came out and have never had any problem with the rotors, even after upgrading to the OS.91.

Gary,

I seem to recall that the early dynamx rotors that where coming apart utilized an aluminum rotor retaining plate and JMP subsequently upgraded it to steel. It has been several years since they retrofitted the parts on my fan unit but they definitely had to change out parts to get my unit in compliance with the current configuration for OS91 use.

Regards,

Peter

Ha,ha,ha the *******. Broken and Fragmented - all over the place- front bearing. Must have caused the fan to vibrate to destruction. Crankshaft is still straight and rotor hub is in place.

Also, the hardwood Fan mounting clocks were loose, must have been from the vibration. The inlet is cracked all around the fan section, but I fixed and reinforced last night.

Ramtec fits perfectly, but I had to increase the height of the fan mounting blocks because the soft mount sits higher than the Dynamax hard mounting plates. Looking good for this weekend.


David