I changed my setup from a 15X8 APC prop to a 15X6 APC and shortened the header accordingly. I had to lean about ½ a turn. ( I am using a perry pump) and it peaked at 10800 RPM with a sharp change in exhaust note when it goes past 10600 RPM. The problem is that if I tune it on the rich side it battles to “get onto the pipe” and takes about 30 seconds after take off before it peaks with the throttle open. If I lean it more it peaks sooner but then seems to fade later in the flight. The question is should I lengthen or shorten the header to overcome this problem?

Piet,


Just four words: Tuned system too short.

You shortened the header too much!
It is not an engine problem, but a user problem...

The cost of an extra 200 RPM, could be the untimely death of your engine...from you setting the mixture too lean for flight, under these conditions.


In any non-race tuned-pipe setup; the engine must get 'on the pipe' immediately, as the throttle is advanced.
It is even better for the engine for get into the pipe's effective RPM range, shortly before full-throttle is reached.

The way some carburettor work, the engine may become a bit rich, around ¾ throttle and become right at full, causing a momentary 'hiccup'... But this does not seem to be the situation you are seeing.

Thanks for the advice Dar, luckily the header is as far in as possible ( no gap between pipe and header) I will start with a 1cm gap and see how it goes.
Cheers
Piet

....you're prop is way too big. It will never "come on the pipe".

FBD.

Quote:

ORIGINAL: Flyboy Dave

....Your prop is way too big. It will never "come on the pipe".

Dave,


That is true for sure, if a standard .91 pipe is installed on this engine, like some "pre-tuned" pipe sets...

But... if a real 'loooong' pipe is installed, as Piet hints was previously installed, with an 15x8 APC prop size (first post), it did get 'on the pipe' and there is no reason it would not do it on this 'smaller' prop size.

Although [link=http://www.supertigre.com/engines/supg0235-man.html]this MAN review by Mike Billinton[/link] discusses another engine, this OS is also a bored and stroked .60, which should respond similarly, to using a similar tuned exhaust arrangement.

With a long enough pipe, there is every reason to expect the OS.91FX to get on-board as well, even with huge props...

....you will need to get the RPM over 13,000 to have any chance, assuming
the prop, pipe, and fuel are correct....just that simple. [sm=75_75.gif]

“You will need to get the RPM over 13,000 to have any chance, assuming
the prop, pipe, and fuel are correct....just that simple.”

What I need is an engine that would power my 3.8 Kg pattern plane at not more than 100Kmh and have enough torque for a steady vertical climb.

The 15X8 prop had some good qualities: very low noise, power at 9200 (the same as a 125 Saito) and a very smooth power curve. But it was too fast and did not have the vertical climb that I need.

The 15x6 prop has phenomenal vertical power when it revs past 10600 (about 2 Hp) I hope the judges will accept the noise level and that I can smooth out the bugs. I will test it again on Wednesday and see.

Piet, you're on the right track. Dar's advice is good. Work out the rpm band you want to work in and tune the pipe dimensions to suit.

The OS FX .91 should easily get 2 - 2.5 hp (using the Pe Reivers spreadsheet) at rpm levels in the 9500 - 11,000 rpm zone with the right pipe and header dimensions.

That's the neat thing about sport two-strokes - they have a very wide band of rpm where good power can be coaxed from them with a tuned pipe if you're prepared to try different length pipes. I've seen an OS .50 SX put out nearly 1.5hp at a very low (for a .50 SX) 11,000 rpm on an APC 12 x 8 with a long pipe. With a shorter pipe, the same engine on the same day put out 1.66 hp at 14,300 (APC 11 x 6). Sure, this engine was happier doing 14K than 11K but if you need to worry about noise and reduce rpm without compromising power too much (as you do in pattern), this pipe tuning flexibility can be very handy.

I've done quite a bit of taching on several FX .91s and measured their power output. In stock trim, the OS .91 FX is a relatively low revving engine. It's happiest with its stock muffler running anywhere from 9500 to about 11,000 rpm. With a tuned pipe at the right length for its application, this can be extended from as low as 9K to as high as 13K or more.

Piet....if you want to try and coax low speed power from your engine
you will have to use a long headpipe. Lenthening the headpipe is for
low prm use.....longer is for low speeds.....set-up a "torque pipe".

You said the engines battles to "get onto the pipe". A pipe set for low
RPM will not give the type of RPM boost that is associated with an
engine "coming on the pipe"....at which time the engine will jump into
a higher RPM range, and much more power will be produced. This jump
can be 2,000 RPM, or as much as 4,000 RPM.

I Have an OS .61 engine set for low RPM with a long head pipe, the blue
plane....at 11,400 RPM. I have another engine, an OS .90 that will be set
for higher RPM....at around 14,500 to 15,000 and greater speed than stock.
I will probably have to cut off the headpipe to get the system where I want it,
but the engine on the white plane will "come on the pipe" when I tune it to do
so. When it "comes on the pipe", it will jump from a normal RPM of 12,500
to 14,500 or so.

I will be using probably a 12-7 prop on the .90 engine. The .61 engine uses a
bigger prop....a 12-8.

FBD.

Hi Fellows,

What do you think, should I try to tune a Macs pipe (# 1190, TowHob LXZ563) for relatively higher speed
flight with OS.91FX on a propeller APC 12.5x12.5 ? Do you have some suggestions how long the header is
needed to be, or I better start cutting step by step ? Any data or comments could save me a great deal
of time. Thanks in advance,
Nick

I think you are asking to much of the .91. Just put a YS 1.10S or 1.15S in it and enjoy. They will easily spin your APC 15-8 at close to 10,000. If running 30%, a 15-10 is in order.

I believe 4S is a great solution but it is too expensive while I have all the things
I listed (OS.91+Pipe+Header+APC12.5x12.5) and also an airplane where OS is
mounted already. I just wander what in fact .91 could deliver if pushed to high
speed ?

Let's not hijack the OS thread. He is asking about his OS .91FX.

Nick - what does the FX .91 turn that prop at with just a stock muffler or an open header (no tuned pipe)? I have not had any experience with the 12.5 x 12.5 - closest I've used are 12 x 12 (estimate - about 10K rpm with open header) and the 13 x 13 Narrowblade (estimate, about 9K rpm).

Use the tach reading you get without the pipe as a baseline then aim for 1000 - 2000 rpm increase with a tuned pipe.

Knowing what type of airframe you're running the engine in will help too. At rpm levels in excess of 10K, a 12.5 inch pitch prop needs a pretty slick airframe to take advantage of it.

Thanks for the hints so far. Here are the measurements so
far at: 27degC, 40% humidity, including the numbers from the
Pe Reivers calculator (note: k prop is always 1.2 but might be not OK for some):


Diam Pitch Type RPM HP Kg Speed k prop.

12 7 APC-acro 13,200 2.10 5.2 141 1.2
12 8 APC-acro 12,200 1.88 4.9 149 1.2
12.5 5 Graupner 13,500 2.00 5.1 103 1.2
12.5 12.5 APC-acro 9,900 1.80 4.9 188 1.2
13 6 APC-acro 12,500 2.15 5.6 114 1.2
13 7 Sonic-G 12,300 2.35 6.0 131 1.2
13 7 Maro-wood 11,000 1.68 4.8 118 1.2
13 10 APC-acro 10,200 1.87 5.1 155 1.2
14 6 APC-acro 11,000 1.98 5.6 100 1.2

All is with 10% nitro and 18% castor, and OS-8 glow plug.

By the way, if looking at MACS tuning info, it seems to me more than strange
that those guys have for more than 2 years pending info on OS.91FX; why
is that ? Could it be that OS.91FX has not good gas-daigram from reso-pipes ?

Regards,
Nick