' Hi Jim!
Not writing a lot at all but still reading often!
So, our UC-61K is now in the hands of Jean-Pierre Gandon, 40minutes flight South from our base, and he reads now this forum too!
Apparently all stacks "looks like" beeing the same length, but if you look carefully at the cowlings shape knowing that Inside them the engine is horizontally straight you'll obviously notice that all stacks couldn't really bee identical.
To the front cowlings are narrowing the nose, to the firewall sides they are enlarging sideways AND horizontally (a bit more at the bottom than on the top).
The counter pressure phenomenom is not that easy to explain:
-In one side you have the atmosphere...1013.25 hPa (mbar), or 1 atm on older units. (sorry I'm not familiar with inch Hg). ("low" temperature and quite "high" viscosity"...
-On the other side, you have the flow+pressure front starting from the exhaust valve seat at each valve opening with very hot, high pressure, and fast gas velocity (warm gas are more fluid than cold intake gas, the viscosity of exhaust gas is very low).
Then you have the 4-cycle "otto cycle" of the engine, and you now the cruise and/or full rpm you want to optimize.
Thus you got the "frequency" of exhaust shock waves.
With some empiric knowledges and some other measurements on test bench, specialists could then determine the optimum length of a pipe depending of EGT and gas velocity along the pipe to get all the gas in the "cold" atmosphere at the best rate.
Then, basic principle shows that all the pipes should have the same length, even when merging some of them together, or merging them at precise length being multiple of a basic length to enhance gas flow.
More over, longer are the pipes, more the gas temperature diminish along the travel...and figures of calculations changed. Density/viscosity evolve along the pipe (that why some race cars use thermal screens and wraps to keep the pipes as warm as possible to keep the exhaust heat Inside the pipe).
So, I do not know precisely the optimum values for Ranger engines, and it is noticeable that on low rpm running engines the "power" gain is low, and that merely only the thermal homogeneity is to be considered first, getting the heat outside with the best conditions to keep the valves and their seats at the same temp' on each cylinders. (let say keeping all heads at the same temperature too).
Beside those "engineering" considérations, the exhausts sound could be changed quite a lot (even quieter) playing with pipes length!
Have a nice day!
And fly safe!
cheers,
Airy
To:
fairchildclub@yahoogroups.com
From:
fairchildclub@yahoogroups.com
Date: Wed, 28 May 2014 10:37:42 -0700
Subject: Re: [fairchildclub] Single Stacks on Ranger
Hi Airy!It's been a while since we have spoken. Do you still own you UC-61K?The picture you linked of your plane shows the stacks all look to be the same length. I have a set of Stainless Steel pipes and the are all different lengths. the shortest at 6 inches (15.24 cm) up to 1 foot long (30.48 cm).
How does one measure the counter pressure?
Take CareCheers!JIM
On Wednesday, May 28, 2014 10:35 AM, "'Airy-H. Millet'
flyingairy@hotmail.com [fairchildclub]" wrote:
Only one advantage...it is easier than 3-in-1 to weld and to screw on engine!
But it is not really better for the engine as they are too short to keep the counter pressure from the atmosphere.
Most of the time front stack is shorter than firewall-side stack...
http://tagazous.free.fr/affichage2.php?img=4158
Moreover, you may experience loosing one during flight... (six more items to check at pre-flight, hitting each stack with your screwdriver* to ear the clear sound of a well tight stack or feel one looseness.....)
*the one you need to open/close oil door, battery door, etc...
Airy
To:
fairchildclub@yahoogroups.com
From:
fairchildclub@yahoogroups.com
Date: Wed, 28 May 2014 06:36:27 -0700
Subject: [fairchildclub] Single Stacks on Ranger
Hi All Just a quick question for Ranger Operators. What is the drawback from using single stack exhaust pipes on the 200 HP Ranger? If they are used, how long should/ can they be?
Jim C Midwest Aeronautique UC-61K USAAF# 43-14964 RAF # HB-690
still looking for: UC-61/ PT-19 Pitot Tube UC-61K / F24R with Throttle/Mixture pushrod Engine Mount.
'