Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

cant get RPM readings

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Old 10-18-2002, 10:00 PM
  #1  
FlooredCOBRA
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Default cant get RPM readings

well today during lunch hour I decided to pick me up a Tac. I purchased the Hanger 9 Tac/ Volt meter one. This is not really the one I wanted but all they had there. Anyways I just tried to get a reading on it and it did not seem to work to well. I am running the GRRT and using a 7.2 x 8.6 prop on my Diamond Dust. My question is when you go to get a reading does there have to be daylight behind the prop? The way I was trying to get a reading was by having the plane on the stairs with prop over hanging. My concern was that it was a bit shady in that area. I was wondering it daylight needs to be in line with the tac and not the stairs. I was running it this way due to no landing gear.

Anyhow I was only able to get 10,000 rpm to 11,500 at just under half throttle. I could get a reading at idle to under half throttle and that was it. After that the readings showed it was turning 300 to 1300 rpm. It fluctuated back in forth. Any suggestions?
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Old 10-19-2002, 01:32 AM
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Default cant get RPM readings

Or any ideas how to get full throttle readings? What am I doing wrong?
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Old 10-19-2002, 01:49 AM
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Default Tach...

If you are in the shade..just point a bright flashlight at the tach...it is DC sunlight...

And I'd say that I think you are having a bit of trouble with the GRRT..Andy Steer and Carl E. both run these with great success, never heard anything bad..search for theses guys.
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Old 10-19-2002, 02:47 AM
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Default cant get RPM readings

okay I will try that with the Tac.

As for my engine its running really good since I put the new prop on it. I'll just hve to see how they perform in air. Tommorrow I will know how it works.
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Old 10-19-2002, 01:39 PM
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Jeff Leavitt
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Default Tach's

Hi,

Just for grins you might want to check your batteries in your tach to be sure that they are good. Low batteries can give you some wierd reactions. The flashlight in back of the prop is an excellent suggestion. I have a TNC tach and I have to use the light sometimes if the light isn't right. I'm using a kind of transparent composite prop on a 1/2a and it's sometimes hard for the tach to "see". Rgds, Jeff....
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Old 10-19-2002, 03:59 PM
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Default Tach readings

Floored Cobra,

I had the same problem of sometimes not getting a reliable reading with my "Intellitach."

It took me a while but I finally figured that for the best tach performance you need light reflecting off the side you are reading, not shining through it. For example, if you are taking a reading from behind the prop, you want the sun to be behind you.
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Old 10-19-2002, 08:52 PM
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Default cant get RPM readings

okay I see what you mean. I just got back from flying and we did tried out the Tac again. Another guy had the same Tac as I did and ours read about the same thing. I turned the plane so the sun was facing prop and tried again that was the key. Thanks guys, its been a pain but got it now

I ended up getting 16,980 with a 7.2 x 8.6 on my GRRT 40. I was not really impressed with these numbers. I thought it would swing that smaller prop a bit faster than that. I am not really sure what else to try to get it to run faster. I may try to shorten my pressure line to my pipe alot more. The wayI have it running is a bit long as it wraps around the pipe. Maybe with a shorter line I can get more pressure with it.

I kept asking the guys at the club if it looked fast. They all said it was screaming and was hauling. I don't know but it just does not seem to impressive to me. Maybe when flying the plane and watching it from ground is 2 totally different things. Guess watching it the whole time you cant get an idea how fast it is as a spectator would. All I know is a while back I seen DD run and it WAS hauling. It looked like a VCR on fast forward. Thats what I am trying to get it to fly more like.
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Old 10-19-2002, 10:05 PM
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Default RPM

Floored,

Those don't sound like terribly good numbers for a racing engine. As a reference, with the Jett FIRE 50, which is just a hot sport engine, I could turn a 9X10 at 16.5 and am currently turning a 7.5 (cut down 8.75NN) X 8 at 20.7.

To get your airplane in the 180+ range based on very rough math I believe you need to be turning around 25K in the air with an 8 pitch prop, a little less in your case. To do that, based on an airborne unload of 3-4 thousand you need to be turning 20K plus on the ground.

If all goes according to plan I plan to get a radar reading on my Dust Monday and will let you know how those numbers work out in real life.

I don't know anything about the MVVS but I would think a little screamer like that should be turning well over 20K with that prop without even breathing hard. Figure a 3-4 thousand unload in the air and you should be in the ball park.
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Old 10-20-2002, 12:58 AM
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Default cant get RPM readings

Thats exactly what I was thinking. It is just not turning as it should. The specs for it says 2.7 hp. and 26,000 RPM. That is the whole reason I decided to get it in first place. Seemed good numbers for the money. It just does not seem like its doing what it should. Then again maybe this is all it will do, but I kind of doubt that. Should be a bit better than that I would think.

A while back I had my OS 46 FX with Mac's pipe on it and a 9x7 on my Scat Cat and ran it it like that. Now that engine loved it, you could really hear it open up in the air and especially in a dive. No as my GRRT it has the same tone in air no matter if in a dive or going straight. I have no idea whats is going on with it. But I still think it is not getting on the pipe as it should be. As for my OS you could hear the transition. I'm am totally lost here with his thing, kind of wish I would have got the Jett or and OS DF engine.
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Old 10-20-2002, 01:04 AM
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Default cant get RPM readings

I am now thinking I need to go back to the obvious. Maybe I need to check for fuel line leaks or or check my flow on my pipe on pressure fitting. Or any kind of simple causes at that rate.

I was even thinking of putting the stock WOT venturi back on it and trying that out. I may be sacrificing with the use of a carb maybe, not sure. I know the size of the carb and the stock venturi is a bit different in bore diameters. I will try it out for heck of it and see how it goes.
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Old 10-20-2002, 11:45 AM
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Default nelson

My nelson qm40 does the same thing. Turns 7.4x8 @21300 with no unloading in the air at all. They told me to cut the prop down so i get 23000 on the ground. Havent done this yet but sounds like you are still overproped. Takeoffs are a little shakey with diameters this small and a 4 1/2 pound plane.
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Old 10-20-2002, 10:30 PM
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Default cant get RPM readings

What size is your carb bore? It should be around 9mm. That's the Q500 carb, which is the one I have and can peak 23~23.5k on the ground with using a 7x9 prop. The venturi bore is 11mm, which will give you a little more power, but think about it: that's like driving a ferarri with the speedo buried all the time. Not exactly a smart idea.

If your carb is only 7mm or so, call MVVS and ask for a modified Q500 carb for your GRRT. It may take a couple weeks to get it, but it's WELL worth it! Also make sure of the obvious--is the carb opening all the way? It can be really hard to tell with the rear-induction.

Carl

PS- What fuel are you using? The GRRT is an FAI pylon engine, designed for ZERO nitro. I run 5% but with a shim or 2 to keep it happy. Make sure you are shimming it enough for your nitro content!
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Old 10-21-2002, 12:04 AM
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Default cant get RPM readings

I have the Q500 carb with it. Well at least thats what I told them to send me. How can I tell if it is 9mm? Would a micrometer work? not sure how else i would know because there are now markings on it to tell the difference.

Well as for the fuel I am runing I am using 15% OMEGA, the pink stuff.

An as far as the carb actually openning WOT i am pretty sure it is but it is a bit hard to see being that the fuse is right in the way.

Now as for the shims what size are the shims? and how would I go about getting a few of them? I had no idea that i needed shims with this engine. I figured that it would work as it is. But maybe that is my problem why I am not getting over 16,000 rpm
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Old 10-21-2002, 05:42 PM
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Default cant get RPM readings

maybe of interest for you:
http://www.rcuniverse.com/showthread...81&forumid=162

mecc
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Old 10-26-2002, 01:25 AM
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Default cant get RPM readings

I think I found my problem why I am not getting the RPM's I am supposed to. I did measure the diameter of my carb and I do have the q500 carb set up. It measured out to be 9mm. But here is the thing that I think may be causing my problems, I think. My engine is rear intake and rear exhaust. Well my intake or carb is pretty much butted up to my fuse and I have the thickness of my ply engine mount covering up directly across my carb. My ply engine mount was also built up and is a bit thicker than the stock size. I am thinking that this is causing obstructions on my carb and not allowing proper airflow to the carb. So I am going to cut out some more of my engine mount and free up some of the area there. Because the way it is now its butted right up to it. I did not realize this until I got to looking at it.

Has anyone else had the same problem on their DD? I mean did the ply engine mount cause and performance difference? Meaning that it was directly in way of airflow.

Hopefully this will correct my problem.
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Old 10-26-2002, 12:19 PM
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Default engine stand

Put the engine on a stand an do it the right way. Dubb Jett says the test stand is the only way to work out engine problems like your having. This makes sure everything is optimal for evaluations.
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Old 10-26-2002, 03:47 PM
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Default cant get RPM readings

I prefer doing it on the plane it self. What may work on a stand may not always work the same on the plane. Meaning the fuel tank location may be at a different height causing different settings. That tuning it on the plane will ensure it will run the same on the plane. Now if we was talking about cars engines it may be different.


Now this gives me an idea. Has anyone ever tried out a car engine say like a 15 or 21 size? they have a lot of power and tons rpm's, say in the 36,000 to 44,000+
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Old 10-26-2002, 05:24 PM
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Default cant get RPM readings

Yes, that would definitely cause some performance loss. On my Dust, I cut away the middle 1/3 of the firewall to allow the carb breathing room. I then built a little balsa air duct to help the air flow get down into the mouth of the carb better. The GRRT carb is really squished in there. Look around this site; there are a few good pics of D-Dusts with rear-induction engines and shows their installations.

Carl
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Old 10-26-2002, 06:20 PM
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Default cant get RPM readings

I actually thought about making a duct also to help it breath. Do you have any pics on how you did yours? Curious to see. I was thinking to use like lexan or use fiberglass wheel pants and use the end of it cut in half. Not sure guess it is going to take some imagination
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Old 10-27-2002, 05:33 AM
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Default cant get RPM readings

It wasn't fully ducted...I guess a duct is the wrong word. It's more of a ramp; like 1/2 a duct. I just made it out of balsa wood sheeting. I'll see if I could get pic of it tomorrow for you...

Carl
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Old 10-27-2002, 05:43 AM
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Default cant get RPM readings

I am still not getting the RPM's i thought I would. Still in the 16,000 range. I am using 15% omega fuel as of now. I know alot of people was saying to use 0% with the GRRT to get it to run right. That may be my problem right there. Just not sure if using 0% will jump my RPM's that much. At this moment I am totally lost on what to do with it now to get it to turn faster.

For some reason I think I posted something like this already, just forgot where.

Anyhow if anyone else has any suggestions let me know. Because at this rate I think my OS 46 ran faster than this thing. And it should be the other way around. Maybe shims? I did see someone mention that before also.
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Old 10-29-2002, 12:14 AM
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Default fuel % that important?

Well I went out and bought me some 5% nitro finally. The guys at the hobby shop was saying that some engine hate nitro. Hopefully this may help out my problem.
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Old 10-29-2002, 12:28 AM
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Default What does...

What does your pipe look like?? Sounds like to me you aren't getting on the pipe. Can you post a pic, or give a link to a page with a similar set up??
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Old 10-29-2002, 12:30 AM
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Default cant get RPM readings

sure let me take a close pic real quik like
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Old 10-29-2002, 12:35 AM
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Default cant get RPM readings

here is one
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