iflyg450

I am running a plane called a scat cat with a TT pro .46. I do not want the pain of tunning a pipe. In the past I have used MACS one piece tuned muffler. Now I want to try something different. I was looking at mufflers from nelson and jett. Which one is better and what prop to use?

bob27s

My Feedback: (19)

Good quesions.....

First, the MACS one piece is not a true tuned muffler. It is light weight, low-loss muffler and a good product... but it is not tuned and does not actually 'boost' much if any.

The jett-stream muffler .... and also the nelson ultrathrust muffler (same technology) are true tuned exhaust systems.

The TT46pro and jett-stream muffler is an excellent combination. It is my favorite of the econo-speed setups.
Just bolt it on and go.
On the scat cat..... just run a standard 9x7 APC prop to start with.
Find peak rpm, back off rich on the needle about 500-800 rpm (dont need a tach, just listen so its a touch rich)
Then make sure someone takes your picture after you land that first time in the air ...... its hard to beat that big smile [&:]

Performance with the ultrathrust muffler is pretty much the same.

I hope this is helpful
Bob

iflyg450

Thanks for the help I just placed an order. Any other tips to pass on? You said a 9x7 on the tt .46 what kind of rpm will it be turnning also how long will it last?

bob27s

My Feedback: (19)

expect somewhere around 16,500 rpm (maybe a little more) with the 9x7

Engine will last quite a long time.... as like any other engine, it all comes down to how you take care of it.
Use good fuel, always error on the rich side for the mixture.... make sure the fuel system is good.... avoid lean runs.
Good treatment with after-run oil will give the bearings a fighting chance against corrision.

My one TT46 has been run hard.... nearly 4 years old now. Couldnt say for sure how many runs.
Probably better now than when new

Barticusmax

Greetings all, First, Bob27s, your sharing of expertise, with very limited bias, is GREATLY appreciated!!

Second, I am reading all the tech data I can about Jett engines, I have my first one, and am just awaiting the carb (It was a qm? so just an open venturi, no throttle). Having never had this class of engine before (34 years of flying now, and I just got my first Jett?) I am taking all the advice I can get! This engine is going on a Scat Cat as I had a new one still in the box, and it's my 4th or 5th one.

I am also getting a bladder tank so as to reduce any risk to the motor.

So, How do Most folks get a speed indication? I would like to use a recording GPS, but I don't want to add any weight at all. I have seen "Speed Traps" at jet rallys and the like, but I neither have one, nor know anyone who does. Are there standard techniques out there?

Again, any and all advice I can get to go fast and survive.......
Bart

bob27s

My Feedback: (19)

A carb on a QM engine ..... well.... ya will have some fun at least!
Biggest thing with that engine will be prop .... even with the carb, use one of the QM40 props from APC (carbon) 7.6x7.8 is a good start. And very important -- use good ear protection ... both you and the individual launching the aircraft.

Speed..... sometimes hard to really measure.
First, the air data and GPS systems are pretty useful. Thereare a couple on the market.
Actually using both GPS and air data give you correlation between the two, and a better reference on actual speed.

Id not worry about weight all that much. It does not really affect speed on a Q500 airframe as much as it does turning radius and acceleration. A Q500 for racing weights 3.5 or 3.75 lbs..... but they fly just fine at 4 - 4.25 lbs too. Finding room inside the Q-500 airframe would be the real trick

Probably the most useful tool we have found of late, and othere on here have found, has been the sound-dopler software. All you need is a good stable recording platform, and some good video and/or audio recordings. The sound doppler software can calculate the speed and usually the engine rpm from the recorded sound. This is the technique used to measure speed at the big Euro speed events, but they do it live, with good audio equipment.

A good radar gun works well.... but they cost a bit, and you usually have to get pretty close to the line-of-flight, or have a remotely mounted radar gun out on the field.

Another common and very useful indication of speed I've also found...
.... is how many club members head to their cars after your first high speed pass down the back edge of the runway

Btw..
Thanks for the postitive comments. This for me is a sport and a hobby.... fortunately there are many good thigns available in the market place, many of which are now even less expensive than when I started flying in 1976 ! So with that, there is something good to suit everyone, and every budget. Thing I try to do is make sure the information I share is as accurate as it can be.... too many folks like to speculate and guess ... and that is just not right.

Bob

Xardas26

I've been running a TT Pro 40 with a Nelson Ultrathrust muffler on my Viper 500 for half a year now, and never had any problems until recently. As it turned out, the retainer of the rear bearing broke. I bought some better bearings from rc-bearings.com, and everything is okay now.
If you're running your TT at rpms higher than 16000, (and the JETT and Nelson mufflers will get you there), I highly suggest to update the bearings as well.

Barticusmax

Hmm, gonna hafta take a nerve pill..LOL The more I read about these engines, the more I pucker! A 7 inch prop on a .40? ok, RPM is the name of the game it seems.
Is it really unusual to add a carb? Call me chicken, but throttle seems important to me. I had a throttle linkage failure on a Skyward .46 with a tuned pipe, and the carb spring pushed the throttle open until I ran out of fuel. That was on a 'Snapdragon", old plans from an old magazine, but basically a 25 size version of the Scat Cat, and a lot faster than I usually fly, but I am working my way up...

What RPM am I likely to see with the 7.x prop? Or, is it hard to estimate because of the addition of the carb?
Thanks again,
Bart

bob27s

My Feedback: (19)

Hi Bart....

I dont want to get too far off topic on this thread..... but just to provide some info and clarification.....

The quarter midget .40 engine is a very, very unique engine. It is designed for QM40 racing. It was not designed to have a throttle, we fly QM with venturi only, and wide open using a fuel-pinch shutoff. It is in few ways similar to a sport OS46, skyward 46 or any other sport engine you have used before. (in contrast, the sport-jett engines are very user friendly... thus sport engines. The QJ-40 racing engine is also easier to operate and set up).

A few folks do fly it with the throttle carb for sport speed and practice just to have some in-flight speed control. Works great for that, and it is a lot of fun once you understand the engine. Dub sells that setup over seas quite often to guys who want to practice with FAI aircraft, but want a reliable but fast engine. Do not in any way expect it to "idle" - but you will have some effective throttle from about 35% power to full power. You will still want to start it and set the needle at full throttle.

The props used are very specific, and matched to the engine design. For QM racing, the greatly reduced prop-disk area trades nicely for extra speed, and the planes take full advantage of the prop designs and rpm involved. For a Q-500 application, it is not optimal, but it is similar.

Props you can use on this engine available from APC
7.4x7.5C
7.4x7.6C
7.4x7.7C
7.4x7.81C

These are the only suitable commercially available props for the QM40 engine.
Do not under any circumstances use the gray APC props, or any other molded prop.
If you have local help who can supply you with or teach you to make decent wood props in the 7.25x7.8 QM design ..... those are good too.

Trust me though, even using the 7.4 diameter prop you will in no way be underpowered on a scat cat

The port timing is VERY high, trading most torque for rpm. The crankshaft timing is equally as agressive. The engine and muffler (it is a combination set) are tuned for approximately 27,000 rpm in flight. With the 7.6x7.8 APC carbon prop, you want to hit a ground peak of around 23,500 rpm, and a launch rpm of 22,500 rpm. When running the engine on the ground, you will only see that peak rpm very briefly, and then you have to rapidly open the needle valve. If the engine gets hot or even the slight bit lean during ground run, you will never see that peak rpm again.

When I race these engines, I do not even approach peak rpm.... I just warm the engine up so it stages on the muffler (start 4 turns out, you will hear the rpm jumps... around 18,000, 20,000 and then up over 22,000). Once it hits the last stage, I know (from bench time and experience) exactly where my needle should be, find 22,600 with my tach, and I can adjust very slightly depending on the conditions..... and sometimes how stupid I am that day if I choose to try and lean it out just a tad more.

Key to note here.... the RPM is considered a fixed number. You adjust the engine by adjusting the prop, or changing the prop to meet weather/air/location conditions. You still want to set the mixture to hit that 22,500 number at least to start with.... maybe go just a tad leaner with experience.

If Dub provided the carb specifically for the QM40 engine, you will be getting the "big" carb, so the rpm loss will be minimal at best.

And just in general, if the throttle linkage on a skyward .46 scared you.... think twice about flying that QM40 engine until you spend a few tanks running it on the test stand --- so you fully understand what you are dealing with.

I hope this is helpful
Bob