Citabria Speed, Fuel Flow w/Climb Propeller

C

Cowboys Air Ranch 84ME

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Forest Twp, Maine
Howdy,....

I have a 1C90CLM7246 Propeller on my 72 Citabria 7ECA w/ Lyc O-235-C1. Static is only 2225 RPM (Measured w/ Strobe Tachometer), and level flight 2600 RPM, which matches to dash tachometer. Performs OK, but looking for a little better climb.

I did some research, and found that Buzz Wagner Conversions state that repitching down to 42 inchs should give me the 2425 Static RPM's which is in the top range for this aircraft (2225 to 2425 static range as per ACA). Should give me 2800 RPM full throttle in level flight.

So, I operate 99% of the time from grass runway, and speed is NOT a factor, but performance is. I level out at 3000'MSL, and lean and get 115 MPH (100 KTS) True airspeed now.

Question #1, If I were to put on a finer 42 inch propeller pitch, what would my speed be?

Question #2, How much fuel would I burn cruising at 2800 RPM (My Operators Manual says 2800 RPM constant). Manuals for later years say 2600 RPM constant, and 2800 RPM max.

Question #3, Anyone have a propeller of the 42 pitch I can borrow and try, or one for sale, or know of anyone who does,

Lastly, I Love this airplane, and I will be buying a performance Citabria w/ 150hp or larger engine in a few years (Just not right now). This is only a "Stepping Stone" Citabria for me.

Comments? Input? Anyone flying this pitch?

Ken "COWBOY" Winiarski
84ME Cowboys Air Ranch
 
I had the same prop. I had my 46 repitched to 43 by Sensenich in GA. Before I decided to do it I called and talked with them. They told me I could expect ~30 rpm/degree.

I had it repitched 3 degrees and it gave me an additional 100 rpm static.

Before repitch my static was 2225, After repitch 2325 rpm static.

I lost about 8 mph cruise for the same rpm.

I think expecting a gain of 200 rpm with a 4 degree pitch change is about 75 rpm optimistic.
 
Howdy Ron86654,

Thanks for that info. At least I have some baseline to go on. I am told that every aircraft acts a little different. Some may loose 8 miles in speed while others loose only 5 mph, and so forth. Also RPM may differ slightly from one aircraft to another due to drag, cowling, Etc.

This is the reason I am trying to get as much info as possible. I do not want to have to re-pitch a propeller more than once.

Do you remember what your Cruise RPM was at wide open, level flight?

Ken "COWBOY" Winiarski
84ME Cowboys Air Ranch
 
Cowboys Air Ranch 84ME said:
Howdy Ron86654,

Thanks for that info. At least I have some baseline to go on. I am told that every aircraft acts a little different. Some may loose 8 miles in speed while others loose only 5 mph, and so forth. Also RPM may differ slightly from one aircraft to another due to drag, cowling, Etc.

This is the reason I am trying to get as much info as possible. I do not want to have to re-pitch a propeller more than once.

Do you remember what your Cruise RPM was at wide open, level flight?

Ken "COWBOY" Winiarski
84ME Cowboys Air Ranch
Click to expand...
No, sorry I don't.
When I talked to the guys at Sensenich, they were pretty certain of the rpm gain/degree of twist, did not matter what airplane it was on. They proved to be right on.
 
You can look at it from an advance rate perspective. With a 46” pitch it’s 3.83 feet per revolution. At 2600 rpm that’s 9,967 ft per minute or 1.89 statute miles per minute, which is 113.4 mph.

With a 42” pitch it would be 3.5 ft per revolution and 9,100 ft per minute at 2600 rpm. That’s 1.72 sm per minute and 103.4 mph. So…a 10 mph loss in cruise at 2600 rpm.

Using Bart’s example of 43” pitch that’s 3.58 ft per revolution and at 2600 rpm 9,317 ft per minute and 1.76 miles per minute, which is 106 mph - a 7.5 mph loss in cruise speed - very close to his reported 8mph loss.n

That‘s essentially the math Sensenich is using to estimate the change in performance.

Rated takeoff power for the O-235-C1C is 115 hp at 2800 rpm, but max continuous is 108 hp at 2600 rpm. With a cruise prop 75% power will be 86 hp at 2350 rpm burning 7.3 gph. 65% power will be 75 hp at 2250 rpm burning 5.8 gph.

In terms of fuel burn, at the same rpm less pitch will result in lower manifold pressure, less power, and less fuel burn when leaned to the same “best economy” (peak EGT) or “best power” (approx 150 degrees rich of peak) Mixture setting. (Max CHT and max engine stress occurs around 50 degrees ROP.) with a 42” pitch you will develop less power and burn less fuel at a given rpm.
 
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Howdy BB57,

So, a lower pitch will give me more HP/RPM at take off, due to the fact that RPM builds more Horsepower, but when throttled back to a cruise power of 2600 RPM, I would loose about 10 +/- MPH due to the inefficency of the engine producing power due to less load/manifold pressure.

Seems right to me. Your math also seems correct from what I remember from 40 years ago.

With everything being the same between the C1 and C1C engines, HP rating differs slightly due to variables like weight of rotating mass (Crankshaft), accys that are driven, and configuration of internals (Piston Weight, Etc).

I guess with a flatter propeller I can expect to get out of a grass field a little shorter (Maybe 100 ft shorter), climb a little better (Maybe 100 FPM), and burn slightly less fuel/or burn the same amount at cruise, but sacrafice about 10 MPH in cruise.

I know "I SHOULD NOT FIDDLE WITH THIS PROPELLER!" and just buy a O-320 version. Like I stated in a previous post, this aircraft is just a stepping stone till I get the Citabria that has everything I want in a plane.

I am Old and Stubborn, Just ask my wife, so I will still experiment slightly for now. Looking for a propeller to try first before I commit.

Got a phone call from a "Cub Flyer" who suggested that the propeller I want to try also fits some Pipers? 1C90CLM7242 is what I want to try. I am still looking and getting input. No Hurry !

Thanks for the input.

Ken "COWBOY" Winiarski
84ME Cowboys Air Ranch
 
Howdy RON86654,

Yup, Sensenich was correct. I do appreciate the advice. Have sent a Email out to a fella in New Hampshire that flys an experimental O-235 Bi-Plane with a fixed propeller turning at 3200 RPM and says he Dyno Tested at 140hp, But that is experimental, so no use to me on my certified aircraft. He shuts off his charging system when he flys aerobatics to get 5hp more. It is a little single seat Bi-Plane and really screams. Sounds like a "High Pitched" seaplane taking off, only with a lot more RPM. Saw the plane perform many years ago.

Waiting to hear back from him if he is still around. I sent message thru a fella I met at a local STOL Competition I went to in the middle of June in Dover-Foxcroft, Maine. Them STOL guys with their "Modified" experimental's really know how to do this stuff.

One Kool thing, I saw some really nice newer Citabria's and a lot of Cubs. Nice to have an extra $300K to play with. Check it out. Northeast STOL Series.

Thanks for the input again.

Ken "COWBOY" Winiarski
84ME Cowboys Air Ranch
 
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