I am currently setting up a Phoenix Topstar 60. This is similar to UCD 60 or Flip3D. The manual recommends 2 Deg downthrust and 2 Deg right thrust. Its an ageing aiplane using a old resin impregnated mount. Two questions here

1) As you can see, the mount deforms when the mounting screws are tightened. I am using 4mm machine screws with spring and flat washers both sides and locknuts. Is this OK? The mount goes back into shape when unloaded.

2) As you can see, the gap between the spinner backplate at the top of the cowl is more than that at the bottom. Is that ok?

If you hold the engine and press it down hard, it actually causes the mount to bend, firewall remains ok. I have already flown the airplane like this six months ago without any problems.


Ameyam

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Mount may well be absorbing some vibration.

Tom

The thrust values sound like they would be ok.. I am not sure Iwould trust that mount if it were me. How much pressure is it taking? a light press or something more significant? It wouldnt be any trouble for you to get a new one would it?

Managed it now

Downthrust too much?

Nope

When a motor mount isn't flat against a firewall, however, you should shim or trim so it is flat. The motor should settle down onto the mount flat without having to tighten the motor hold-down bolts. Other than that, your 6 months worth of "testing" should have shown you any problems that might be present.

Da Rock,

the motor sits flat, thats not the problem. You see, the FS91SII is inverted, the screws that are seen in the above pics have the nuts on the top side of the mount. Once the finger tightening is done, I use a spanner to tighten further. Normally you would tighten till it wont turn any further But, even with a little tightening, the top surface of the mount begins to sink under the pressure of the washer and the sides begin to bulge out. I have highlighted this with the yellow circle in the attached image. All four nuts have this problem and I have tightened as far as they go (or as far as I dare to). The problem is that this resin-FRP mount is very soft which is good, as it absorbs some vibration. It sits flat agains the firewall as well

By the way, I havent flown the airplane for six months but rather I flew it only one day six months ago before the monsoon. Then too, the mount had bent as seen here. Now, I am setting it up again to fly it next week. Should have flown it today but for unrelated problems on the field (horse race)

Ameyam

The bolts that you are using look rather large, so you may be squeezing the plastic more than what you could with smaller bolts. I normally would use a 6-32 size. You could make a couple of metal plates to spread the compressive load over a larger area on the bottom (top?) of the mount's beams.

I am using the same bolts that are recommended by the manual.

Spread the compressive force...hmm... isint that what the washers are supposed to do? In any case, could the mounting I have really cause any problems?

The airplane has been through numerous heavy landings when the 75AX wasnt running properly on it. I have another airplane of the same sitting in a box and another that has caught my eye in the LHS at a bargain (box damaged). I want to fly this one till it files no more, I have put in a lot of hours setting it up right (after it was test flown before which it was extensively strengthened with fibreglass cloth). It looks quite horrible by today's monokote standard but flies like nothing else- it needs no forward speed to fly. I have had the engine quit quite low in the hover several times when with the 75AX and all it does is plops the nose down from vertical and is immediately flying again. A similar situation crashed my Reactor 46. The Topstar is half the price of the Reactor and much more fun to fly

Ameyam

Ameyam,

Such soft and flexible engine mount, any thrust angle that you trim will change in flight under the forces of the propeller.
So don't worry too much about the correct angles.

You don't need so many nuts there.

In order to avoid the compression of the mount and eventual failure of one bolt due to fatigue, you will need to make two thick pieces of aluminum, just like the base of the crankcase.
Similar oversized washers (aluminum or steel) should be added under the bolts that are holding the mount to the firewall, or similar fatigue failure may happen.

Bolts holding vibrating loads MUST BE tight; otherwise they will break off.

Best advice: install a harder engine mount.
If vibration absorbtion is desired, use one designed for that.

Ok I started with a 75AX on this airplane. When it wouldnt run properly, I tried horizontal mounting, so made new hole for passing the pushrod. Then I gave up, lowered the tank and installed the FS91 for which I had to drill new holes for the tank as well as the pushrod (the pushrod hole for a 4c is different from that of a 2C) . Now there are so many holes in the firewall, I dont want to drill new ones for a mount. I purchased the Dubro flexible mount but held off using it for the same reason. I will check if I can get metal flats made as shown by you.

There arent too many bolts there, just the nut and the lock nut. The black nut on one of the screws is to hold the terminal of a remote igniter in place si I can use a small hotshot igniter

Ameyam

That's just one of the old nylon mounts and they are soft. The plates LN is showing will do the trick. Fire wall holes are easy to fix, just drill them out to the size of a dowle and epoxy in the dowels in all the holes and you have a new fire wall after you epoxy over them to fuel proof. Thrust angle is something I work out during trim flights and I make up shims to go behind the mount like Rock mentioned. What degree the thrust comes out to I haven't a clue. At the field I use washers to find the thrust angle the plane needs then when I get home I measure and make up the shim.

These work the best for every engine 90 and bigger.

http://www.horizonhobby.com/Products...odID=HAN242024

I did make a reduction in the down thrust by inserting a washer between the engine and the mount beam as I mentioned above. But I think that those may have sunk back into mount during tightening. Will fly it next week and see

Ameyam

Just epoxy some dowel rod in the holes you,re not using and cut them off flush with the fire wall problem solved.

That looks like a good bird. Where can I get one?

PA BEN, its discontinued[&o]. But you can get the Phoenix Funstar. Its the same airplane in a 40 size and its really great fun. Its also fragile but doesnt have the breakable wing tips like this one

Ameyam