Starting a 1779 VW with a Zenith Stromberg Carb.

Ian Melville · Post by **Ian Melville** » Mon Jun 06, 2011 10:23 am

Hi Stuart,
The odd thing is that was what I expected to see till I Googled it and realised a "Stinger" may be somthing else to other people. I couldn't quite see a Luton Minor with four megaphone exhausts under the fusalage

I only know a little about exhaust systems, but was keen not to just have four straight pipes on my current build. I would like it to be easy on the ears. "Stingers" have been around for a while on VDubs (Fornier RF4 etc)
What I don't understand is why efficent pipe systems are not used in aircraft. It cannot just be the weight? Since the engine was originally desiged for a muffler, some backpressure must be acceptable.

BTW Minor looks nice

Nigel Ramsay · Post by **Nigel Ramsay** » Mon Jun 06, 2011 3:06 pm

Ian Melville wrote: I don't understand is why efficent pipe systems are not used in aircraft. It cannot just be the weight? Since the engine was originally desiged for a muffler, some backpressure must be acceptable.

Ian, take a look at Great Planes website; there are a couple of 'complex' VW exhaust systems, 4 into 2 into one etc. I suspect it IS a matter of weight. Like you I don't fancy sitting behind a machine gun if it isn't necessary! I'm putting a 1600 into my D9 but I suspect I might end up with a similar system to this. There was an article about silencing in the old PF mag a couple of years ago, similar, but basically the end of each pipe was blanked off so that all the gases escaped via the holes drilled around the last 6" or so of pipe (I think it may have been a Jodel?)

There is a formula for ideal exhaust length, taken from valve seat to exit. As soon as you siamese the pipes, scavanging comes into play and the outgoing exhaust pulse helps vacate the next exhaust pulse and so on...
'bit of a black art!

I have a Supertrapp exhaust on my XT500 (500 single m/cycle). That has a series of baffles which allow for different torque characteristics. It is necessary to rejet the carb to suit the final setup though, much too heavy I suspect for a small aircraft.

G-AWMN · Post by **G-AWMN** » Mon Jun 06, 2011 4:08 pm

Come on now chaps, if we are not careful we will be criticised for making this forum interesting!!!

I would have liked to fit a proper silencer but ruled this out because of weight, space availble under the engine, complication of manufacture and finaly time which it would take, I want to get this plane back in the air this decade! I did think about blocking the end and drilling exit holes in the side or even some sort of fish tail but decided on the baffles as I can easily experiment with these to see what works and what does not.

As well as optimising length the diameter of the pipe has a big effect on gas flow speed. Smaller diameter pipes can some time be better. It is indeed a black art.

Stuart Penfold

Ian Melville · Post by **Ian Melville** » Mon Jun 06, 2011 5:13 pm

Hi Nigel, I didn't realise Great Planes did full systems, I thought it was just the header tubes. I note they also do a "stinger"

Nigel Ramsay · Post by **Nigel Ramsay** » Mon Jun 06, 2011 7:40 pm

I guess you've visited the site but for anyone else, here's a link direct to the exhaust system section http://www.greatplainsas.com/scexhaust.html

Worth checking on the weight though?

Nick Allen · Post by **Nick Allen** » Tue Jun 07, 2011 12:56 am

I believe there is also a formula to determine the optimum diameter. I'll talk to my source...
If weight is an issue (and isn't it always?) how about some titanium? For example: http://www.goodfabs.com

JohnLindsay · Post by **JohnLindsay** » Tue Jun 07, 2011 11:02 pm

According to A. Graham Bell (I thought he invented the telephone) in his book Performance Tuning in Theory and Practice, the formula to work out the optimum length, P, in inches of a single pipe per cylinder, = ((850 x ED)/rpm) - 3, where ED = 180 degrees plus the number of degrees the exhaust valve opens before BDC. For my 1834cc VW, that works out to be 58.5" @ 3,100 rpm.

The ID of the pipe = (the square root of (cylinder volume in cc/((P + 3) x 25))) x 2.1. If I have got my brackets in the right places, for my engine that comes out at 1.146" or 29.1mm, although Bell goes on to say that road engines, as opposed to racing engines, usually work well enough if the pipes are the same diameter as the exhaust port.

I won't even touch on what he has to say about what happens when using 4 into 2 into 1 systems!

In the LAA publication on Light Aircraft Noise Reduction, there's an interesting design by Richard Mohlenkamp for a Tuned 'Quiet' Exhaust for VW Engines which meets stringent German noise certification requirements and weighs only 1.7kg. PFA Engineering verified the silencing qualities of the design on a 1600cc Taylor Monoplane and the pilot reported no reduction in power - sounds almost too good to be true!

Congratulations, Stuart, on the Luton Minor - she looks superb.

Ian Melville · Post by **Ian Melville** » Wed Jun 08, 2011 7:13 am

A. Graham Bell may well have invented the stinger exhaust. Just have a look at his first telephone

That is useful info John. Wasn't aware of that book. If I could meet the German regs, without performance loss, I would be more than happy.

I have a video of a German VW powered CX4 being test run which shows a special exhaust. Will dig that out tonight.

Nick Allen · Post by **Nick Allen** » Wed Jun 08, 2011 10:10 am

LAA publication on Light Aircraft Noise Reduction

Couldn't find this...is it a book or one of Engineering's publications?

JohnLindsay · Post by **JohnLindsay** » Wed Jun 08, 2011 10:42 am

Nick,
Light Aircraft Noise Reduction is a ring binder containing a collection of technical papers, design requirements, design examples (3 for the VW), manufacturers' information, references and bibliography gathered together in the days of the PFA. It is not listed in the on-line shop but Dave Dale (direct line 01280 846 783, preferably not Fridays) retains the master and is happy to run off copies as required for a modest sum (I can't remember how much). I certainly found it interesting.
John.

JohnLindsay · Post by **JohnLindsay** » Wed Jun 08, 2011 12:21 pm

Great photo, Ian!

The Mohlenkamp system comprises 34mm mild or stainless steel headers with 2 into 1s on each side of the engine just behind the rocker boxes, and then straight 35mm tailpipes running down each side of the fuselage (and these can be made of aluminium). The ends of the tailpipes are cut at an angle of greater than 45 degrees and flattened, with 24 x 7mm holes along the outside edge for the last 300mm. Length of the complete system front to back is shown as 1923mm but, because the noise reduction is based on resonance and each engine/prop/rpm combination will be different, the optimum length must be determined by experiment. However, the PFA found that small changes in length did not cause any noticeable change in noise output.

I am tempted to give it a go (always remembering the need not to burn one's trouser legs when disembarking).

Nick Allen · Post by **Nick Allen** » Wed Jun 08, 2011 12:49 pm

Thanks for that, John.

Ian Melville · Post by **Ian Melville** » Wed Jun 08, 2011 6:03 pm

I found the video I mentioned above and it was not what I remembered. I cannot see any advantage in this arrangement over that puplished in the CX4 plan unless the plan is to add a 4 into 1 collector and some sort of muffler.

Here it is anyway, not the most sensible place to run an engine.

http://www.youtube.com/watch?v=9kEkThtxMHc

rogcal · Post by **rogcal** » Wed Jun 08, 2011 10:45 pm

John, I experimented with long tubes extending down each side of the fuselage on my Luton Minor and the noise levels decreased significantly, along with the power output which suffered by about 100 to 150 rpm which was no fun in a Minor with a 1600 VW up front.

I removed them after a few weeks and got back to some noisy but uneventful flying!

JohnLindsay · Post by **JohnLindsay** » Thu Jun 09, 2011 8:29 am

Bother!

Many thanks indeed for mentioning that, Roger. I might have wasted a lot of effort repeating the exercise. When I have some spare time, I might instead try and copy the Fournier RF4 design of exhaust which will actually be easier to fabricate.

John.