Pipes

Gang --

The village idiot has one more question. Why pipes in stunt? As I recall, the speed guys developed pipes back in the 60s to improve 2 cycle scavenging at top RPMs. Makes sense. Get it tuned right, and the pipe makes the engine hum. A good pipe creates a slight vacuum at the precise moment the engine is trying to do a "phooey" instead of a "suck". (Based on the "Suck, Squeeze, Pop, Phooey" 4 cycle theory, and the more important "Suck/Squeeze, Pop/Phooey" two cycle theory.)

So why pipes in stunt? Most of the good guys have pipes. There is a reason I don't understand. Can anyone explain the reason for pipes in plain English?

Larry, The main reason is that they don't care for the smell of diesel fuel. I think the pipes do increase available HP and also tend to make the engine run at a more steady RPM. Now I'll get out of the way and let someone who knows what they are talking about answer your question.

Jim,
I have a tuned muffler for my MVVS .15 diesel

If I understand pipe concept correctly, the racing guys tune the pipe to make the engine run as fast as it can with the exhaust reflection, and the stunt guys tune to a different part of the wave that causes the engine to regulate its speed.

First of all..He He..to your first comment, Jim..what a
two stroke pipe does or usually does is determine at which rpm
range, maximum torque is generated and it also enables a maximum
rpm point..without getting into the physics involved(as if I knew
..!)...the expansion chamber generates a positive and negative
wave..to extract gasses and to punch the fresh charge back into
the cylinder(due to timing overlap)..I played with two-stroke
pipes a little when I road raced 250cc production several
hundred years ago....there's lots of literature out there on this
technology...some of it's even in English(mostly Japanese)...
As an aside, two-stroke development has just about peaked
in the motorcycle world..GP engine technology is so far ahead of
model engine technology...we'll never catch up...thats probably
a good thing... I guess this tirade is why I fly diesel's too..
Tony

Hi Larry:

I think all of the responses are basically correct. Your basic explanation, I think, is right on. But the way they're used in stunt is not to increase the HP, but to broaden the performance curve. In a "normal" pipe installation, the peaks of the engine HP curve and the pipe dseign point would coincide--thus making the HP curve peakier. For our application, they are offset so the curve essentially gets flatter.

Also, by running the system on the proper side of the "curve" the system is in effect providing negative RPM/Load feedback. If you recall from control system theory, or amplifier design, negative feedback reduces the system "gain" (output) but makes it more stable.

The piped ships dont wind up pointed down, or in the wind.

This is my understanding.

Hopefully one of the pipe guys will validate.

All,

You could get Flying Models May 1991 and read Dean Pappas's with Bob Hunt article Stunt pipes, A new Tune. This give the historical information regarding why they were tried out on PA models in the first plance. As I read the article, it had nothing to do with increasing power, but was to mitigate the power curve on two cycle engines so that they wouldn't rev excessively when pointed straight down. The extra power benefit associated with sound waves packing the chamber and the use of lower pitched props for lap time control where side benefits to the original reason they were used in the first place.

Per Jim T's response a diesel does all of that wonderfully. Good thing I have no sense of smell left for kerosene

PIPES WORK. The speed through the down hill side of the squares and wing over are at the same speed as going into the manouver. Even if I fly at a 5 second lap the Manouver is slower.Paul Walker flys a 5.3 lap. However if you watch the manouver the plane goes though the stunt at a constent speed. Anything down hill seems to slow up. I fly at an 5.1 lap with my PA-51 but the stunts seem to take along time compared to my Fox.35 powered POWWOW that flys at the same lap speed. The B-17 is going at a 4.6-8 lap BUT goes through the patterens manouvers slower and at a constent speed. 4-Strokers are finding this out. Lap times are not as inportant as manouver time or speed. The pipe keep the engine RPM'S steady. To change lap time change pitch of the prop. Scott

Scott,

Why are you flying your PA .51 so fast. I run mine in a moedertely fast four stroke in level and it just barely breaks into a two going uphill in the vertical eights, hourglass and overheard eights. My plane weighs a slightly portly 62 ounces also. My lap times are 5.46 level and 5.39 inverted. In a high wind, I just move my leadouts back about 3/32 of an inch and I don't have any problems penetrating a pretty good wind even at that slow of a speed.