CustomPC

Please post your 75AX stats here.

I'd really like to see an apples to apples comparision between the 75-76 motors (OS, Tower etc) using the same size props. I think OS have done the AX a dis-service by recemmending such large props in the manual (13x8-10 and 14x6-8).

Try to keep on the subject of tach figures and not drift into debates such as "the Tower motor is cheaper" or "why buy the 75AX when the .91FX is lighter and more powerful".

asmund

I really hope you will get some good replys on this one as I am awaiting a 75 to arrive in the mail for myself (I`m not telling you which brand) But it will be interesting to test some props and compare performance. I guess I too will shoot for a 13-6

Iflyglow

My Feedback: (79)

Asmund,
I think I remember you mentioning a TH .75. I think you will find that they are awesome, and you can buy two of them for the price of the OS, and you do not have to worry about the silver glitter (ABL SLEEVE) on your leading edge.

CustomPC

I've just returned from the field where i ran mine on the test stand today.

I was intending to test a whole bunch of props but i had problems with the fuel tank on my stand causing bubbles in the fuel lines which gave me some tuning issues. It was very difficult to get the right tune as the bubbles were causing it to run leaner than you'd expect for any given needle setting. This is purely a problem with my test stand and in no way reflective of the motor's quaity.

I did manage to get some indicative numbers with a Bolly 13.5x6 prop.
With the standard muffler i got 11,000rpm and with the Turbo-Jett muffler i got 11,500rpms with the same prop.

The Turbo-Jett muffler gives it a real nice boost and very crisp throttle response.

[email protected]

I have a .75 AX on a Cap 580 and I also agree the manual calls for too large a prop range. The engine makes peak power at 14000 to 16000 rpm. I turn a 13x4 APC wide blade at 12600 rpm using 15 % nitro and 18% synthetic. I tried 14x6, 13x8 and 13x6 pattern props, but did not get good rpm. I can't see running this engine less than 11-12000 rpm. I have not found a two stroke yet that will turn their recommended props at max rpm.

DarZeelon

CustomPC,


500 RPM is a nice boost to you???

The .90 size tuned muffler from Nelson (UltraThrust UT4) will work from RPM as low as ~10,500.

The .90 size JettStream and probably also the Turbo-Jett require a higher base RPM, for the engine to get 'on the pipe'.


For a .91 size engine the prop size range for the Jett is 12x6-13x6, suggesting it will get 'boosted' only if able to spin beyond 12K, with the normal muffler...
...And you are talking about a smaller .75 engine with a bigger, 13.5x6 prop...

A 12.5x6 Bolly Clubman will no doubt show a much higher RPM gain than 500...

bbagle1

I know Jett makes a muffler for the 120ax which is specifically designed to give you a boost at lower rpm's. I am not sure if it will work or if they have one for this engine though.

CustomPC

I've been told by bob27s (who works for Jett) that the Turbo-jett muffler is more for torque than all-out rpms.

He told me that i should be able to run a larger prop. At the moment i have an APC 14x6 fitted and it's all ready to go for the maiden flight of my Super Skybolt this weekend.

I got 10,400rpm with that 14x6 prop using a dodgey tach in fading light. I don't know how accurate that result is. I've now bought myself a Fromeco TNC tach as an early xmas present and will try to test a few different props during the weekend.

bbagle1

Just a side note here. I get 10,500 with an apc 14x6 on my saito 82. I would think this engine would pull harder than a similar sized 4 stroke.

CustomPC

Just an update now that i have a decent trustworthy tach.

I measured the APC 14x6 and an even 10,000rpm today using the Turbo-Jett muffler with 15% nitro, 15% Klotz and 3% castor. The plug was an Enya No4. Temp was in the low 20's Celcius and i'm at sea level.

DarZeelon

CustomPC,


Your engine is obviously not 'on the pipe'... As written in post #5...

It is in even worse circumstances than it was with the 13.5x6 Bolly...


If you want real power from this essentially tuned silencer, ignore OS' prop recommendations and let this engine unwind!

A 12x6 and a 12x7 (or a Bolly 12.5x6) sound good to me.

CustomPC

Hi Dar,

You're probably right, but i'm happy with how the motor is running.

The 14x6 at 10k was more than enough for the Super Skybolt it was hauling around the sky. I spent most of the time around half throttle except for the uplines and it had plenty of vertical. I left at the end of the day considering a finer pitched prop.

I do have a bunch of props to try. I'm thinking an APC 14x5 Narrow prop might be a good match. The narrow profile will allow motor to rev out and the 5" pitch will be better suited for the speed envelope of the biplane. If i get home in daylight tomorrow i might quickly test it with that prop.

DarZeelon

CustomPC,


I wrote what I did, since I am inclined to think most people would like to get their money's worth...

You paid a pretty large sum for that TurboJett, so I was just telling you what you should do get the best performance from your engine with it...


But hey, it's your money, your win, or your loss...


Cheers.

bob27s

My Feedback: (19)

The turbo-jett works over a far broader rpm range than the jett-stream mufflers.

I think he has the 15cc size turbo-jett. Anything in that 10-13K rpm band is pretty much where that size runs best. The OS75 engine seems to like that range too. The 10,000 on the ground is probably getting up closer to 10.5K or so in the air once the prop unloads a little bit.

DarZeelon

Bob,


As someone who also sells this item, I look at the Jett web site's muffler page...

The .91 size TurboJett 'boosts' the OS.91FX by 800 RPM on the 14x6 prop and by 700 RPM on the 13x6 prop.
The 15x8 and 12x6 sizes, at both extremes of the scale, got only 500 RPM more with the TurboJett than with the standard OS muffler.

I believe CustomPC, with the prop he's using on his engine is at one extreme, not getting the higher, middle of the range boost.
I am just trying to steer him in the right direction.

CustomPC

I pulled the plug out last night and found that it had a coat of oily carbon.

I sure that it's still on the rich side. I haven't adjusted the mixture needle from the rich factory setting yet so there is some room to lean it out a little. The motor runs and idles 100% reliable out of the box so i was reluctant to fiddle with it until i'd maidened the plane.

It could also be that the motor is mounted on it's side and or the fact that it spent most of it's flights at half throttle. I did tend to gurgle a bit if i cruised around below half for too long. More throttle cleared it out nicely.

Now that i have the maiden of the Skybolt out of the way i plan to test a number of props next outing. My previous test sessions have all been jynxed with this motor with test stand problems, tach problems and forgetting to take my prop bucket to the field.

Bob, if you want some pictures of how well the Turbo-Jett fits the skybolt, take a look at post #385 here: http://www.rcuniverse.com/forum/m_42...6/key_/tm.htm#

bob27s

My Feedback: (19)

Aircraft came out nice Im pleased to hear it flys as good as it looks.

CustomPC

Hey DarZeelon,

You may be right about the Jett Muffler not stepping onto the pipe with the 14x6.

I had the motor off the plane the other day and had a few minutes to kill so i put it on the test stand and tried 3 different mufflers.

APC 14x6 with stock muffler = 10,000rpm, needle at 1 1/4 turns when 300rpm rich side of peak.
APC 14x6 with Turbo Jett muffler = 10,000rpm, needle at 2 turns out when 300rpm rich side of peak.
APC 14x6 with Bisson .61FX Pitts muffler = 10,200rpm, needle at 3 turns out when 300rpm rich side of peak.

I was surprised at these figures. Firstly that the Turbo Jett showed no improvement over the stock muffler with the 14x6 and secondly that the Bisson that is designed for a smaller .61 beat them both. The standard muffler is the quietest of the three. It has a nice and subdued exhaust note. The Bisson is by far the loudest but the sound is quite pleasing to a rev-head although the dB level is uncomfortable on the ears. The Turbo Jett falls somewhere in between.

I also tried 4 other props on the plane with only the Turbo Jett.

APC 13x8 10,900rpm
APC 14x5N 12,000
Zinger 14x5 10,500
Bolly 13.5x8 9,900rpm

Of all the props, i still prefer the way the plane flies with the 14x6 so the Turbo Jett is not providing me with any benefit over the stock muffler.

jettlag

Here are the exact data from our tests.

OS .91 prop rpm muffler
14 x 6 10700 stock
13 x 6 11600 stock
12 x 6 13100 stock
14 x 6 11500 turbo Jett
13 x 6 12300 TJ
12 x 6 13600 TJ
15 x 8 9600 TJ
15 x 8 9100 Stock

OS 61 12 x 6 12200 Stock
13 x 6 10200 Stock
12 x 6 12800 TJ
13 x 6 10900 TJ

MDS 68 13 x6 10700 Stock
13 x 6 11200 TJ

Magnum 75 13 x 6 10900 Stock
14 x 6 9900 Stock
13 x 6 11300 TJ
14 x 6 10400 TJ

We also tested the Bisson and found it to be as good or maybe 100 better than the stock mufflers.