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6" prop extension?


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Last summer we learned from Rudi Kurth of Switzerland, Ed Hamlin and Bruce Tifft, that a longer prop extension spool would reduce the cockpit noise level and possibly increase performance. We collected accurate baseline data, then removed the standard 3-inch extension and installed longer ones. We have tested 4-inch, 5-inch and 6-inch extensions.

Most of our data was for the 6-inch extension. Results are as follows: As compared to 3-inch, sound level at pilot's station and rear seat was reduced two and a half to three DBA at high speed cruise and climb conditions. Oil temperature on all three airplanes tested with the long extension was increased. Cylinder temperatures on one of the airplanes increased. There was no measurable change in the performance of any of the airplanes due to extension length. We have been reluctant to recommend the long extensions because any change in the propulsion/drive system of an aircraft must be thoroughly tested for long-term durability. We now have a total of 300 hours 6-inch time on Long-EZs with no indication of problems, and since the two 8-inch extensions on the Defiant have run 600 hours it appears that no mechanical problems are indicated. Thus, if you can stand a 20 to 30 degree rise in engine temperatures you can, with a 1.5 lb. Weight penalty, achieve a significant noise reduction by using a 6-inch extension. Note that we have tested this only with Lycoming engines and cannot predict the durability of a Continental application.


So, does this mean in these days of ANR headsets, (and who cares how noisy the cabin is), you could save 1.5 lb and get 20 to 30 degree lower engine temps by fitting a 3" extension instead of a 6" to a Cozy? Has it been tried? Is the relatively (to a Long) porky Mk IV fuselage a factor here?


On another tack, how about the 8" extenstions on the Defiant? If I use a three blade prop I could make a real swoopy cowl.

Mark Spedding - Spodman
Darraweit Guim - Australia
Cozy IV #1331 -  Chapter 09

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It sounds like you're going through the CPs and reading them in order? That's very good! Just keep in mind that 1981 was fairly early in the EZ development cycle. The speed gurus had yet to really work their magic and optimize the design to eek out every knot of speed.


All the speed gurus I know of are running much longer extensions than 3 inches. Anywhere from 6-8 inches. They've solved the higher temps problem, gotten better flow in and around their cowls, and picked up speed here and there.


....Wayne Hicks

Wayne Hicks

Cozy IV Plans #678


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  • 4 years later...

Old posts above. But after reading Waiter's posts on chopping off pieces of his cowl...is it the length of the extension or the distance from cowl to prop?

I don't think that is an option when running a C/S prop.

T Mann - Loooong-EZ/20B Infinity R/G Chpts 18

Velocity/RG N951TM

Mann's Airplane Factory

We add rocket's to everything!

4, 5, 6, 7, 8. 9, 10, 14, 19, 20 Done

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Old posts above. But after reading Waiter's posts on chopping off pieces of his cowl...is it the length of the extension or the distance from cowl to prop?

it is the distance from the cowl that needs to be at least 3" but more is better

on the MT CS prop there is a small extension built into the prop hub. you could use another prop extension with the CS prop but because the CS prop is very heavy it would have an effect on W&B. most of the O-320 ezs use a 6" extension and some up to 8". On the 360s and 540s most use 8" extension. Some like myself and Dick Rutan use a 9" extension.

Evolultion Eze RG -a two place side by side-200 Knots on 200 HP. A&P / pilot for over 30 years

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The MT (electric version) comes with a built in prop extension (about 4 inches). I have my MT mounted to a 6 inch extension. I custom built my cowl for this configuration.


As has been mentioned, you need a minimum of 4 - 5 inches between the prop leading edge and aft edge of any opening in the cowl.


My initial thought would be to mount the prop right up against the starter ring with no extension. The question would be, If your building your own custom cowl, can you design the cowl with good aerodynamic lines, maintain the 4-5 inch clearence, and achieve the proper sized cooling air exit.


Moving the prop forward solves a couple problems, CG (prop weight) and Propellor ground clearence.


However, Moving the prop aft improves propellor performance in two respective areas;


a) the Prop / Cowl clearence. This has been quantified and documented,




b) getting the prop into cleaner, less turbulant air. This hasn't been as well quantified, but is a factor)





F16 performance on a Piper Cub budget

LongEZ, 160hp, MT CS Prop, Downdraft cooling, Full retract

visit: www.iflyez.com

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