A hot plug will advance the timming, and it will help to avoid what is happening, I believe.
However, that is like a bandage, since the real problem seems to be insufficient load for the engine and excessive revs.

Note that OS recommends 13x8 and 13x9 props for this engine.
The attached graphic shows that the same engine works properly within the range of 13x6 to 15x8 props.

Please come back to us with the results of the bigger prop.

I have seen this chart... I didn't think by "underpropping" something like this would happen... I just wanted more revs... will get back after the weekend only... no time in week to fly...

??

this has nothing to do with the idea that the piston is travelling faster than the speed of sound.

What you are refering to is the pressure wave and the reason is due to attenuation of the pressure wave in water because of the energy released and the mass (density) of the medium through which it is travellinginvolved.
the sound wave is less attenuated since it requires less energy to drive it through air.

This is now off topic. BTW try not to type in caps,its like shouting. If you're going to shout, then at least spell correctly

More revs not always make the airframe move faster.
The limit to higher speeds is the drag force, which grows exponentially with the speed.
The only thing that can increase speed is an opposite force: thrust force.
(The most common thing that can stop high speeds in a dime is called flutter, watch for it!!).[sm=75_75.gif]

When your model flies level at max speed, the prop-engine is developing the max forward force that it can.
When you dive from that condition, the component of the weight force adds to the thrust force, increasing the forward speed of the model for your low pass.
At that point, the AOA of the blades of the prop decreases, they lift less (less thrust), and tend to move faster driven by the engine (rpm's increase much).
Then, the air pump that the crankcase of the engine is, sucks faster, the carburetor throat becomes a bottle neck (sorry, no more air can go thru), vacuum sucks more fuel from the spray bar, which makes the mix too rich for timely combustion (rich mix burns slower and cooler).

The result is what is called "four-cycling", the engine actually is firing every other revolution.
On one stroke, the combustion is interrupted by too rich a mixture.
That rich mixture gets ejected on the following downstroke, after which a fresh charge enters the combustion chamber that the plug may be able to fire, if it is not too "oily" and cool.

If you have available power, like the case of your fast Revolver, achieving the highest thust force is the way to go for max speed.
Unfortunately, the relation between rpm's and thrust force is not lineal, as any engine performance chart shows.

The right combination of engine, prop diameter and pitch will provide the max thrust force, which is seldom found at hyper-rpm's.
It has been said that the max thrust is found at a range of rpm's at which any engine can develop max torque.
That range of rpm's is always below the max rpm that the manufacturer recommends as "safe" for the engine.
In practical terms: select the max prop diameter that your airframe (ground clearance) can take, and increase prop pitch until the engine is loaded to rev's in the range of max torque.
From that point, follow a fine tune, playing with diameter and pitch in small increments, until finding the sweet spot that pleases you.

Just experiment, this staff is fun!

I hear what you say, I started off with the 14x8... was too much load for the engine... I live at sea level... then I went to 12x8 which it certainly doesn't like because of little drag I would say being the airframe so light in comparison with the engine power... 13x8 is my next choice for the weekend...

That just doesn't sound right to me...

Max thrust is obtained at maximum engine power not torque
"Ground clearence" has nothing to do with the prop selection for maximum speed. [X(]
You'll need the smallest diameter that will still pull your airframe so that you can have a high pitch...

I think Switch and Mr. Cox hit it , tuned pipes require running on the rich side as more fuel is required when the pipe "kicks" in.

My 91 fx liked a 13-6 APC prop for speed and I would run it at 12,500. I tried up to a 14-8 on it but wasen't what I was looking for. I also got rid of the OS #8 plug and put a Fox standard long glow plug in mine and made it run better.

Later on my 91 fx also had about the same problem you'res is having. I found out the bearings were going bad in mine so I had them replaced and it ran like a new engine again.

Hope the bearings are ok in yours.

good luck

o well... I would have to check the bearings then, thanks for info... I have hit 128 with the engine before by using a 13x8... wanted faster, thought the 14x8 would do it... but the engine doesn't like the coastal air it seems... so tried the 12x8... think its too little by what people have described... so I guess the 13x8 it is... so 128 is almost max it will reach... don't have cowling on yet, so it could help when I install it finally... but will need to check those bearings for sure...

Instead of investing more money in a rossi muffler, I just took the baffle out of my stock 91fx muffler and it increased my speed quite a bit and was easy to tune. I was half afraid most of the time to open up my 91fx running the 13-6 prop thinking it would tear my wings off! I wouldn't even consider running a 12-8 on the 91fx unless you want to put a lot of unnecessary wear and tear on your engine at full bore.

Maybe you should consider putting a rossi engine on your plane if you like the high rpms.

Hope you get your 91 fx running right.

One thing about bearings, a guy tried to tell me in RCU, if the bearings are bad in an engine you can here them rattle. That's not always the case. Mine weren't rattling, they were just brown and a tad rusty from not using after run oil; well, that's what they told me anyhow after I sent my 91fx in for new bearings. If you want my opinion, I think its that rossi muffler your using. I've never been wrong in my life, but in this case, I could be as far as the rossi muffler goes!

big dan

I had to think about that one and push the numbers, but yeah, mach 1.2 or more is not unusual.... these things are moving much faster than I ever thought!

input noted... I will try the stock muffler again with the 13x8... its leaking at moment... so I don't use it... the 128 was with BCM pitts muffler btw... made enough RPM's... only recently have I tried the rossi... the stock one I must fix the leak and then also try it again... I only had 2 runs with the 12x8 and rossi and the engine is still really new... so I would be disappointed if the bearings are already failing...

Then I guess I'd rule out the bearings if your engine is still pretty new. I think using your 13-8 or even a 13-6 as I did and your stock muffler without the baffle in it might do the trick.

Also try a new glow plug. You can always stick the original one back in it. I've had OS #8's go bad after a couple of flights, mainly from running too many rpm's from the start. But, you say yours wasen't too lean and the engine wasen't too hot so I don't know what to tell you from here on in.

Ask some of the experts in here, they might know.

I'm just one of many "RCU dummys" in here trying to help you out.

thanks, I have loads of options and opinions to follow... sure something has to work and get it running like it use to...

12,900 rpm on the ground.
Asumming 10% increase in the air = 14,200 rpm = 236 rev/s

128 mph = 188 ft/s

13x8 prop can advance a max of 236 rev/s X 8 inch = 1,893 inch/s = 157 ft/s

128 mph is achieved helped by gravity, and the model is moving faster than the propeller pitch would do at blades' AOA = 0 (zero thrust).

Unless rpm's reach 188 ft/s X 12 / 8 = 282 rev/s = 16,920 rpm!!!

Don't you mean leaner?? That is the reason you tune to peak then back off to the rich side for flying, the engine leans as it unloads in the air.

Don

Do you have muffler pressure on the tank? This really helps with fuel draw. Also, you need a tach to set the engine right. I set mine to top rpm and then back off 300 to 400 rpm. Doing it by ear may be alright if you have been doing this for a few years, but it really takes a good ear and Inever did get mine tuned right that way. It's much better using the tach. If you were using a 14X8 and proped it down, you may be over reving the engine. From a 14X8 you should go 12X10 or 12X11 to keep close to the same load. For each inch you change down you add one inch of pitch. Sounds like you are over reving the enging in the air once you get it in that dive. How steep a dive are you doing anyway. You just said dive, not how steep.

Ed

where do you get all these theoretical calculations

in theory yes it should get those rpm... don't think it does... I measured with GPS in plane... was about a 30 degree declined dive downwind... this was still when I had the BCM with 13x8 prop on and no issues... this engine was running in another airo... took it out and needed to fit to revolver... BCM didn't fit unless I cut out the whole bottom end of cowling...

Sounds right AND this engine has a history of being lean in the mid range. I believe the heli version has a mid range needle.
They need to be set a bit rich with a standard muffler so set it as rich as it will run reliably for a pipe. Also some of the older 91 FXs had the needle behind the piston causing heat build up in the needle
assembly itself. OS fixed this with a bracket that moved the needle assembly out to the side and into the air stream. Mine likes 10% dosen't seem to like 15%. I have Macs muffler on it.

Model max mph = (engine max rpm X propeller pitch) / 1056

Note that this is max reachable speed, for an ideal condition in which the propeller does not slip respect to the airstream.
Real or aerodynamic pitch depends on the AOA of the prop blades, being zero for static tests, and max for diving flight.

Hence,

Model max mph @ 8 pitch = (14,200 X 8) / 1056 = 107 mph

Model max mph @ 8 pitch = (16,920 X 8) / 1056 = 128 mph

All these theoretical calculations indicate that for speeds higher than 128 mph, pitch 10 is the key for your Revolver.
Pitch 8 and a 30 degree dive cannot move your model faster than the speed you have measured.
I believe that having so much available power, and willing to push the speed envelope seeing no engine puffing, 13 x 10 is your best prop, as confirmed by this thread:

http://www.rcuniverse.com/forum/m_94...tm.htm#9426330

That 13 X 10 would load your engine in static test just as the 14 X 8 did (8500 rpm).
Did you measure in-flight max speed for the 14 X 8 that you statically tested?
As you know, the 13 X 10 will give you more top speed than the 14 X 8, but it would be less efficient for take-off’s.

More theory, if you don’t mind:

• Check attached nomogram to select pitch based on speed.

Full article here:

http://www.rcmplans.com/issues/reque...011990-1-1.pdf

• Clarence Lee’s review of your engine:

http://www.rcmplans.com/issues/1999/...-081999-1.html

The static rpm results per propeller were:

13 x 6 - 12,500
13 x 8 - 10,400
13 x 10 - 9,500
14 x 6 - 10,900
14 x 8 - 9,500
14 x 10 - 8,400
15 x 6 - 10,200
15 x 8 - 8,100
16 x 6 - 8,300

• A good calculator for static thrust:

http://adamone.rchomepage.com/calc_thrust.htm

• For a 30 degree dive, a force equal to half the total weight of the model adds to the engine thrust.

Additional thrust force = sin 30 X weight = 0.5 weight
Necessary wing lift = cosine 30 X weight = 0.86 weight (then wing AOA and drag get reduced)

If you really want speed, then I'd just sell that reactor and get a jet.

I seen one advertised in RCU a while back for only $34,000.00

What a joke!

big dan

its not that I want speed as such... at moment I just want the engine running happy again like before... it will go fast enough with the 13x8, must just get a pipe tuned in correctly it seems... or else like someone stated before it could be a bearing issue...

well today I got round to trying out the engine again... no success... I changed to 13x8 APC prop... the engine starts but seems to starve itself from fuel somehow... now I've checked the tank pressure by taking it all out and closing all pipes blowing into tank, tank was fine no leaks... so then after a while I got it running but once it goes beyond half throttle it dies... glowplug is good, fuel flow is good but its not getting sucked into carb... could this be something to do with the bearings also? crankcase pressure being lost? I want to get diagnoses correct here... don't want to strip the whole engine down and in the end it was something smaller and easier...

I have had a small piece of matter in the spray bar area and that will restrict flow substantially. pull the needle and run a very light wire thru the opening. Sometimes the object is hard to see but will cause some of the problems you discribe.