FlooredCOBRA

I talked to a god friend of mine today about engines unloading in air. He has been in the hobby for years and he was the one that got me into flying 13 years ago.

I was telling him about how my GRRT will not spin the rpms and not unload in the air. One thing that came up was my fuel type. I been using 15% OMEGA and from what i hear is the wrong kind. So I purchased some 5% to help this out some maybe. Also I was telling him about how I can not get my engine to unload in air the way it should be. Something that came up was that I may be running to lean on the ground and the engine is already maxed out. Therefor when in air it can not rev anymore from there so the power gain is to nothing. I thought they way I had it set up I did have it a bit rich due to I could pinch my line and it rev up a bit more. We was thinking I don't have it rich enough on ground so it never has a chance to unload properly. So far I am totally confused about this engine. I can tune any other engine and tear it apart and back together, but this one has been a huge pain.

My question is this, lets say I have it rich enough so it does unload in the air. Will I be getting more RPM's this way than the way I have been running it? Meaning from what I think it may have been maxed out on the ground already. Now if it unloads in the air will the RPM's be greater than the other way I been running it. I always been afraid to have it to rich and not have enough power for it to get going with the 7.2 x 8.6 prop I been using.

At this moment I am lost about this thing. One I cant seem to get it to spin at 20,000 as other have been. Next it does not unload correctly. Hopeful my 5% fuel will help me out in this case. Another thing is my pipe is fixed so it makes it hard to tune it as I would a macs with header.

PAINLESS

My Feedback: (19)

I'd be curious as to why 5% fuel would let your engine rev more than 15%, did your friend give any reasoning for this?

There is no way in he!! you can peak an engine on the ground and expect it to reach it's potential in the air. Every other form of carb adds fuel as the throttle is advanced to full open.

The only way to tell when its right is by measuring the planes speed, either by radar or timimg it some how.

FlooredCOBRA

thats not exactly what I was saying.

My engine has the same tone on ground as it does in air. it never seems to rev any higher where ever it is at. Yes radar gun would be an answer but I don't have one. But on any other engine it is very obvious when the engine unloads in air. As for this one it does not.

PAINLESS

My Feedback: (19)

Try this:

When the plane is in the air, make a pass down the runway listen while slowly advancing the throttle. If the there is no increase in rpm (or sound) from 3/4 throttle to full throttle the engine is set too lean.

One of the problems is people equate more sound with more speed. This is not always true.

FlooredCOBRA

One of the problems is people equate more sound with more speed. This is not always true.

I have to agree with that.

I will try out your suggestion about making the passes. But for sure I know my engine is supposed to be turning more RPM's than it is. Folks on here with same engine are turning in the 20,000's on ground and here all I can get is 16,900 on ground. Something is not right.

PAINLESS

My Feedback: (19)

Those RPM's don't sound right for your combo, if your running a tuned pipe, my guess is its too short, and not coming on the pipe. You might also check your head clearance. These are some generalizations based on my experience; a stock engine with a regular muffler and 15% fuel, set the head clearance at .010 to .014. As RPM increases to 18 to 20 k, increase head clearance to .017 to .020 to prevent detonation.

You can tell if your engine is detonating using a tach. You will get very eratic readings, jumps of a few hundred rpm will show up, you will also see pitting on the piston top or inside of the combustion chamber, (bottom of the head).

Hope this helps.

jlong34016

My Feedback: (1)

My nelson is doing the same thing. Tech support told me to cut the prop until it turns the desired rpm on the ground. I bet the guys that have quoted their rpms on here have shaved the prop and didnt mention it. I went from 21700 to 23000 by cutting 1/16 off each tip. Dont forget to balance.Have not flown with modified prop yet.

PAINLESS

My Feedback: (19)

"Tech support told me to cut the prop until it turns the desired rpm on the ground"

What is the desired RPM on the ground? How is this number determinined?

2 stroke engines produce their max HP and torque at a certain RPM. Not to say that this can't be increased, by a tuned pipe or more nitro. I'm not sure if you should prop an engine for max torque or HP, my gut feeling is HP. Once this is done, going for another 1000 rpm wouldn't gain much in the air. I think this is why your not seeing increases in the air, your engines are maxed out as it is.

There are many factors to consider here. At some point increasing RPM makes the prop less efficient, the trailing blade travels through the proceeding blades vortex, causing cavitation. A singleblade prop is more efficient than a 2 blade prop because of this.

jlong34016

My Feedback: (1)

Tech support at nelson i guess has data on the engine. They told me to get 23,000 on the ground. Its range is posted at 23,000-27,000 rpm. This is the QM40 model. For other engines you need to experiment or find someone else who has done it. For flooredcobra he is trying to duplicate numbers posted here. I may end up getting more on the ground than this but only one way to find out if its better or not. Try it and fly it.

PAINLESS

My Feedback: (19)

It would be interesting to see a dyno sheet on some of these engines mentioned here with some different props.

I don't think simply increasing RPM is the answer, how high do you go? Till you break a rod or piston, there is a limit.

FlooredCOBRA

I will try the shims out and see how that works. It seems I am the only one here with the GRRT that spins at slower rpms on ground. I wish I could adjust my pipe length but being that it is fixed I can not do this. The shims may help me out to get my engine to spin more or come up on the pipe. It just does not seem to be doing this the way it is. It just revs up to a mono tone and thats is it. On the other hand I could tell a huge difference when my OS 46 FX w/macs pipe came up on pipe.

This thing has been a real pain. Especially knowing it has way more potential in it.

FlooredCOBRA

Another thing is I am a bit paranoid about cutting my prop. I would be afraid that it would fray or come apart on me. One reason I am just using what props I have and trying to get my numbers from there.

If nothing else it will make a nice paper weight.

PAINLESS

My Feedback: (19)

Its possible you just got a bad one. One in which all the tolerances were stacked in the wrong direction or all the tool bits were dull from the end of a production run.

Is there a warantee on it?

FlooredCOBRA

you know that could be a possibility. I would for that to be my problem though. It took me forever to get this engine. Maybe now they are not having as much trouble keeping them in stock as before.

I will try out the shims idea and if that does not work heck with it I'll just have to send it back for another one. I didn't think about that being an option. Good idea though. I'll give them a call once again and see what they say.

PAINLESS

My Feedback: (19)

I'b be curious to know what the head clearance actually is.

I've had 2 engines over the last 10 years that were absolute junk, and will not buy those two brands again. You get what you pay for definitly applies to engines.

jlong34016

My Feedback: (1)

Removing shims is alot more risky than trimming a prop. You can ruin an engine increasing compression and getting predetonation. Sanding a prop is the safest and easiest thing in the world to do. You should be sanding new props anyway. If its wood use thin CA when your done. Always balance like you do new props. Do you balance your new props? How do you balance them if you dont sand or cut them? Once again i reccommend you put that engine on a stand and trim the prop to get it running correctly. Once it is running on the stand correctly then put it in the plane and fine tune it. It will turn less on the plane. This will let you know if its the carb being blocked ect.

PAINLESS

My Feedback: (19)

On a high performance engine like a Nelson or Jett, I wouldn't expect to have to adjust head clearance. I'm sure its optimized for some RPM and nitro percentage, probably 15%, since thats what pylon racers have to use.

Most all cheap sport engines have too much head clearance, at least the ones I've bought lately. I think this makes them more user friendly, reliable and long lasting, but is not optimum for best performance.

Your right, detonation will kill an engine pretty quick. I wouldn't start removing head shims without knowing the clearance first. I'm not comfortable cutting the tips off a molded composite prop, such as APC.

FlooredCOBRA

I sand mine and balance them with my balancer. I am not sure what brand balancer it is. I got it a long time ago. It is the one that has the four big metal disk wheels and a shaft that lays on top of them. Only thing is I have not cut my props down or sanded them to a shorter length. I am bit paranoid about them flying apart when running.

FlooredCOBRA

About dang time! I finally got my engine to get on the pipe. I ended up calling Morris Hobbies and talked to one of the guys there. He had the exact set up as I do but he was getting the rpm's. He is using 15% fuel along with a 8 x 5 prop. It seems that the FAI GRRT is a bit different than the others. Due to the fact that it does not have a header on it so the power band is in a different area because of its small pipe length.

I put on a 8.5 x 5 and that was the ticket. This whole time I been flying my DD and it never came close to running on the pipe. At first that prop size seemed very odd with very little pitch. But with my type of engine set up it definitely kicks in. I did not think It would have much thrust with it being so little pitch. But it has plenty and will for sure be a lot easier to launch now.

I put the tac on it and got from 17,600 to a sudden jump to 22,950. Now I can tell it is up on the pipe. Finally after all the problems I been having it is finally set up. One thing I noticed is that there is about a 2 second delay before the it comes up on the pipe.

Now All I need a decent weekend so I can get the thing in the air.

jlong34016

My Feedback: (1)

Cobra. You seem a little confused. Thrust is from diameter which you have alot of, speed is from pitch which you have little of. You'll be lucky to get 100 mph.
What you need to do is decrease diameter and sacrifice thrust and increase pitch so you are not getting passed by trainers. If you dont want to cut props youll need to buy them of the right size.
7.5x8 is a place to start. It will still come on pipe and give you potental for 150+ mph and 5 pounds of thrust for takeoff.

FlooredCOBRA

I am just going by what the guy told me to use. He has been using this prop set up for a while and it is working the best.

Also I have tried a prop close to the one you mentioned 7.5x8. But the one I used was a 7.2 x 8.0 and It did not come up the pipe as i mentioned before. That is when I had a constant 16,000 or so rpm's. But now that I noticed that the pipe kicks in at about 17,000. I can mess around with the 7.2 x 8 maybe shave some off and try to get some more out of it. As odd as the 8.5 x 5 sounds this is supposed to be a good point to start at. Like the guy said he has been running this prop for years on his DD and likes the setup. (speed wise also) So for now I am going to take his advice being that he has the exact plane,engine and pipe combo I do.

PAINLESS

My Feedback: (19)

FC,
You prabalby realize this now but your pipe was too short to begin with. Going with a smaller prop let the engine rev up more inspite of the pipe length, which got the rpm up to where the pipe was effective.


"FAI" really means no nitro. Nitro is hard to get in Europe and even then very expensive. I think all FAI catagories of competition require no nitro fuel. Running more than 5% nitro in an "FAI" motor usually decreases performance, the first MVVS engines in this country where like this. They may have changed them since then.

FlooredCOBRA

Actually I did try out a lower nitro - 5%. I did this today when I was using tac to see what the difference was. The 5% did spin a bit slower than the 15%. It was a bit hard to tell due to it was dark by the time I got home from work. I had to use a shop light on my prop to get tac to work. I will try this again in the day light and see figures I get then.

ChuckAuger

My Feedback: (12)

If you look at the advise that painless and jlong gave you, you will see a common thread. Even though the prop you were running was just a 7.2 X 8.6 (I think I'm right ), the load on the engine was greater than the load of the 8.5X5. So it got on the pipe.

And resonated like they do when all the bits come together!! So that is GOOD!

But I have to say that painless and jlong are right..you just need to get a smaller diameter/higher pitched prop to get on the pipe.

And I think now that I have heard where your pipe comes on, you ought to put the 7.2X8.6 back on there and do a pinch test.. I have to think you should have come on the pipe but were just too rich. If you are getting on the pipe at all with some 8.5" dia prop, you should be able to get on in a dive with a 7.2X8.6 IMHO. The GRRT will pull a 7.2X8.6, just has to.

Happy trails!

FlooredCOBRA

Actually I did put the 7.2 x 8.6 back on and tried again. Now I know I don't have a defect in my engine so I started to fiddle with the prop some. I sanded off just the very tips of the prop, balanced it out and tried it again. It seems all the other times I was very close to getting on the pipe but never did. With the tips sanded off some It did get on the pipe this time. All it took was a very small amount off the tips and it was working. I had no clue that a very small amount would make that much of a difference like it did. I mean it was very little that I sanded off too.

At the end the 7.2 x 8.6 spun to 23,900 to 24,100. I did do the pinch test and I was still on the rich side so I plan to keep it at this setting. It did take a second before it did come up on the pipe guess since it is a bit rich. But when it was on it stayed there and did not come off until I throttled back.

So I have a question now. When you guys launch do you have it on pipe or right before it hits? Either on or off? Anyways Saturday cant seem to come any faster.