Exhaust, ready to cut (or not?)..........but where?...... (with pictures)

[Pilottonny]

Hello,

I have finally finished my FWF installation. Now see what is sticking out of the belly :



After searching the forums, I am more confused than before! I have Vetterman exhausts with mufflers and am wondering if it is worth cutting them off shorter!.

To me it is a very complicated issue:

• Over here (Europe), noise (heard from the ground) is a very important issue, because we have severe noise restrictions. For example, in Germany there is Airfields that charge up to 3 or even 4 times the landing charges for the most noisy aircraft. At some airfields you are not even allowed to take off at certain times of day, if you do not have a noice certificate of a certain (low) grade!
• Extra noise inside the cabin is annoying, but I have the CAD insulated carpets, so I guess shortening the pipes will not be to much of an issue in that respect.
• Weight savings is always nice, so the more I cut, the more weight I save, right?
• Less stuff hanging in the breeze is more speed, so the more I cut the faster I go, right?
• The closer the exhaust is to the belly, the dirtier the belly will get, right?

To obtain my noise certificate, extensive testing will be done. I will have to get it within on year after first flight.

I have seen some planes that have 1/2" holes drilled in the last 6-8" of the exhaust pipe, sort of like you see on automatic guns in cartoons. Do they attribute in noise reduction?

An other thing that I have seen is that the end of the pipes are squeezed close, and holes drilled in the sides and bottom, allowing the exhaust gasses only to escape through the holes. This could well cause noice reduction, but what about additional drag and reduced power due to back pressure?




If you think I should cut it shorter, what do you think is the best CUT in the above picture:

• Yellow
• Blue
• Purple
• Other (please specify)

Any input appreciated.

Regards, Tonny.

[Sam Buchanan]

If this was my plane, I would not trim the pipes. Shorter pipes will add the complications you mentioned without any benefits.

[Frank Smidler]

I believe Vetterman designed his exhaust not to be cut. The biggest issue is if the exhaust is cut shorter the floor will vibrate a lot more which can lead to cracks around stiffener rivets.

[RetiredRacer]

I know this is a modification, but this artical from Vetterman might be of some help.

http://www.vansairforce.net/vetterman/subcowl_2.pdf

[penguin]

Tonny,

I doubt cutting your exhaust will do much for the exhaust note, and may give some other undesirable results. If you must cut the tail-pipe then don't use the yellow line - it will induce severe buffet in the floor, as Frank mentioned above. The tail-pipe must make and angle of (around) 25 degrees with the floor to keep the exhaust from hitting the floor and inducing a really foot numbing vibration. So blue or purple would be OK with purple probably being better. If you get any exhaust staining on the belly it means the exhaust is hitting the belly, which will induce uncomfortable vibration.

I installed tailpipe extensions that were squashed and drilled on my 6A for a few hours. They made very little difference to the exhaust note but did impose stresses on the pipe that caused on of the ball joints to almost fail - it was spewing exhaust gasses over the fuel pipe to the carb - lucky it was firesleeved. So my advice would be to steer clear of any thing other than round or cut off tail-pipes.

Peter

[Pilottonny]

Thanks for the anwers, this convinced me to leave the pipes as they are, for the time being. I will see what speed I will get and what the results of the noice tests are, before cutting anything.

Regards, Tonny.

[Bob Axsom]

I have sound deadening black foam from Becki Orndorf's upholstery materials (I don't know if it is still available from her) and carpet over that. I cut my non-muffled Vetterman crossover exhaust pipes maybe just a little longer than where you show the yellow line. I understand RV-8s have a problem with cracking in this area but I have none, I feel no floor vibration and it was measurably faster (fraction of a knot as I recall). With a bare fuselage skin under your feet the drumming exhaust may drive you crazy according to what I read here. If speed were your major objective you would have built an RV-8. Alan Carroll has recently calculated from almost three years of SARL race result data available at www.sportairrace.org that the RV-8 is about 18 mph faster on average than the next fastest type of RV in cross country air race competition.

Bob Axsom

[Kevin Horton]

Originally Posted by Bob Axsom

I cut my non-muffled Vetterman crossover exhaust pipes maybe just a little longer than where you show the yellow line. I understand RV-8s have a problem with cracking in this area but I have none, I feel no floor vibration and it was measurably faster (fraction of a knot as I recall).

A recent post presented results from two tests in the same configuration where speeds differed by 1.5 kt. A fraction of a knot speed increase is well within the noise of the test results, so it is pretty much impossible to measure. You'd need to do a large number of runs in each configuration, and average lots of data, to measure a speed increase of a fraction of a knot and have confidence in the results.

[Pilottonny]

Although I would like my plane to go as fast as possible, speed is not my main reason to cut the pipes. I was thinking that by cutting the pipes, I would of course reduce the weight, but there would also be less weight (and less length) to vibrate and damage the welding of the exhaust or the mufflers.

One important thing to consider when cutting the exhaust short is that the floor will get hot(ter) Because the fuel lines are running in the center of the floor, to the firewall, there is more chance for Vapor Lock. Since I am planning on running Mogas, that is definitely something I want to avoid.

Regards, Tonny.

[Bob Axsom]

Originally Posted by Kevin Horton

A recent post presented results from two tests in the same configuration where speeds differed by 1.5 kt. A fraction of a knot speed increase is well within the noise of the test results, so it is pretty much impossible to measure. You'd need to do a large number of runs in each configuration, and average lots of data, to measure a speed increase of a fraction of a knot and have confidence in the results.

Well not quite identical - the landing gear fairings were removed and reinstalled to put air in the tires (with my racing subfairings the gaps are extremely small - lightly touching sometimes?) and there was less fuel in the faster run but you are right of course. Small differences in speed are hard to accurately determine with the methods we have available. Still we use the method chosen consistently and record the numbers in order to establish reasonable benchmarks and repetition increases the confidence in those numbers. Chris Murphy is struggling with the the challenge of developing a percise, accurate, reliable and inexpensive method for the speed dash competition he has produced for two SARL events (www.sportairrace.org) in Courtland, Alabama. Once he solves the problems with that we may have something better to work with.

Bob Axsom