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Fred
Here is a letter I recieved from Ed about a year ago on this very subject, I hope it helps....
Chuck
N94191
Chuck asked:
> I was wondering where I can find what the static rpm should be for
> a 46 coupe upgraded to a c85 with a 69-50 prop? any ideas? I just
> had a pre buy done on one. the compressions where 78,78,78,74,
> which is pretty good but the static RPM was 2250.......I would
> think this might be a little low.......
Chuck,
My first worry is that a 6950 prop, with a length of only 69 inches, wont convert the engines power to motion properly its too short. Thats reflected in your 2250 static rpm which is actually too high for a C-85.
On Ercoupes, the only short props allowed by the type certificates are on the C-90 (minimum 69.5) or O-200 engines.
On the C-85 engines, the type certificate calls for static r.p.m. of not over 2225, not under 2025.
With my C-85 and a 7146 prop, I was right on the line at 2225 rpm static (on the ground zero wind full power).
7146 extreme climb prop
7148 normal climb prop
7150 normal prop
7152 cruise prop (which may cruise horribly because of inability for the engine to spin the prop to speed resulting in little climb and way-slow cruise speed) If youre flying at a light weight, you may cruise fast.
Presumably, the 7152 would give a static rpm of barely up to the minimum of 2025.
The STC for the O-200 engine calls for a prop something like the 6950 because that was standard on Cessna 150s so there
are a lot of them out there on wrecked aircraft so thats what the original STC applicant specified. The higher power engine is allowed to spin faster by the smaller prop so it can develop rated horse power. Still, that engine/prop on a Coupe gives good climb but not so good cruise.
To put that prop on a C-85 will have substandard results.
The link to the type certificates is on my Coupe web page, at the bottom. See Note 4 (at the bottom of A-787) and propeller paragraph 2.
Please do submit questions like this to the Coupe lists ( join via http://www.ercoupers.com ) so you can get lots of answers including answers from A&Ps, AIs and at least one person with some really good ERCO and FAA records.
Hope this helps.
Fred Lamkey <[EMAIL PROTECTED]> wrote:
From: "Fred Lamkey" <[EMAIL PROTECTED]>
To: "tech" <[email protected]>
Subject: [COUPERS-TECH] Prop
Date: Fri, 2 Jun 2006 10:26:13 -0500----[Please read http://ercoupers.com/disclaimer.htm before following any advice in this forum.]----I have a McCauley 7351 metal prop on my 415C C75 Ercoupe. As I understand, that is considered a standard to cruise prop. I purchased the plane in September and I find as temp and humidity rises, two up takeoff is deteriorating. I would like to have better climb and takeoff performance. Would you recommend having the prop changed, is 73 too much for the C75, what pitch (and length would be recommended), how much would it cost to have it changed, long would it take and finally, who does this? Thank you for your assistance (getting my money's worth on this post).FredErcoupe 415C 93994============================================================================== To leave this forum go to: http://ercoupers.com/lists.htm
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