Cessna COAX

[flateric]

Unfortunately all the photos from the source are gone with ImageShack...

http://www.airlinepilotforums.com/hangar-talk/78300-interesting-hangar-mate.html#post1522432

http://www.airlinepilotforums.com/hangar-talk/78300-interesting-hangar-mate-2.html#post1523918


11-20-2013 05:34 AM by Cubdriver
Quote:
Originally Posted by UAL T38 Phlyer
The one I was curious about was in the last two pics in part 7 (my mistake, above). Single-engine turboprop, contra-rotating prop. N4571L.

I've never seen that before.
I dug this up about it.

Model: COAX
Date: 1986-1987
Specs: 335 / 340 prototype conversion
(2) Allison 250-C20S 375 shp
Engine-to-prop gear box with coaxial output shaft.
Forward and aft three bladed propellers driven in opposite directions.
Engine nacelles cleaned from wings.
In both multi and single engine climb, COAX beat 340A by
200fpm but there were some who thought the data was suspect.

There is one photo of it on thread 8 of the series too. With two turbines mounted in series it would have to be a pretty expensive airplane. This was a time when GA manufacturers were trying to get rid of low end prop models for liability reasons. Maybe the idea with this one was to make the customer pay for their own insurance policy rather than let the insurance company take away their choices with the other twins, which they did around that time. At any rate I wonder how smooth the setup ran because the second prop gets mostly dirty air.

 

[Stargazer]

Quite an amazing discovery! I'd never heard of the COAX before. Well done.

 

[Stargazer]

I've attempted to do a three-view of the COAX using a modified Cessna 340 one.

Unfortunately, I haven't been able to locate a plan showing this particular variant of the Allison 250 in front and top views.
Could anyone suggest other aircraft that might have used it, so I can use their plans as a starting point?

Also, I'm not sure if the wing tips are correctly sized and shaped after removing the tanks.

Thanks in advance to anyone who can help!

 

[Basil]

This is the only western civil coaxial propeller aircraft (besides Reno racers) I have seen so far. Did the powerplant consist of two Allison 250 turbines each driving one propeller?

 

[Apophenia]

This is the only western civil coaxial propeller aircraft (besides Reno racers) I have seen so far. Did the powerplant consist of two Allison 250 turbines each driving one propeller?

Yes, independent engines sit side-by-side and drive concentric prop shafts. From a retired Cessna guy on BeechTalk:

This was strictly a concept test bed.

The photo is of the final flight where the tip tanks were removed to test the effects on speed. With only the wing aux tanks (30 gal each?) it was practically in a fuel emergency at takeoff.

I don't have any numbers, but I believe that performance was not quite as good as expected.

Props and prop control system were highly non-standard and a bit awkward. It was said that this project paid the McCauley engineering budget for a year. The drive path for the front and rear propeller were independent and isolated from each other except for the necessary intershaft bearing or in the case of major structural failure. Gearbox was from a small specialty shop with minimal testing beforehand (it was spun, but not under power or prop load.

https://www.beechtalk.com/forums/viewtopic.php?f=49&t=156177

BTW, the second prototype Cessna 340 (s/n 673) was registered N4571L in August 1970. I presume that same prototype formed the basis for the COAX?

 

[famvburg]

Regarding the fuel capacity, C340s had either standard 20 gallon aux tanks per side or optional 36 gallon tanks per side. Some also had nacelle tanks of about 20 gallons capacity. Sometimes in 1 nacelle, sometimes both nacelles. Without knowing the exact set-up, the test airplane could have had a minimum of 40 gallons to as much as around 100+ gallons.

 

[Basil]

Apophenia, thx for the info via an original source. So the propulsion layout was comparable to that of the Fairey Gannet.

 

[Apophenia]

So the propulsion layout was comparable to that of the Fairey Gannet.

Yes, very similar to the Double Mamba layout.