Cruise prop vs. climp prop

I have a borer prop on my 150 hp cub (41 pitch) and love it for getting "out of the hole"... but... you sure don't get anywhere fast and it burns more fuel to go a given distance. So, I am considering buying a second prop - a cruise prop so that I can bolt that one on for the longer trips that won't involve short strips. Plus, I'm flying from Alaska down to the Lower 48 this fall and will be there all winter (I don't expect to see too many short strips). So, I figured the cruise prop would be ideal and I would just leave the borer in Alaska for when I return next spring.

I know I know, if I wanted to get some place fast I should have bought a 180.... but right now I get 85 mph, and that is fine for dinking around the Anchorage bowl area, but for trips that involve long flights around the State (going to the Brooks range soon, but no short strips), it sure would be nice to pick up some more range and some speed too.

So I have two questions, is this a dumb idea to think about switching props once in a while for the longer trips (anyone else do this?), and if I get a cruise prop what should I buy to maximize my speed and my total flight distance? (if I get a 74 inch Sensinech, should I have it pitched for maximum speed i.e. 56, or 54?). Or, should I just get up earlier in the morning, have some more coffee, and buy a little bit more gas along the way?

Thanks!

I recently went through the same thought process. Replacing the 42" Borer with a 54" Sensinech, and the Goodyear 26" tires with 6.00X6's. Probably a little extreme, but the gurus tell me it ought to be good for 15 mph. Sold the ranch with the strip (described by a friend as "only a marginal helipad"), and won't get back to Alaska this summer. Before I go the next time, I will repitch the Borer to 41", and ship it with the tires to ANC.

Murph and someone from this site stopped in my shop one day and changed a Borer out for a cruise prop for his trip back up North while I was out playing in a Citabria. He carried the spare with him.

A 7452 will be a big difference to the borer. Just so you don't scare yourself. Your fuel burn will go up because it will take more manifold pressure to turn it the same RPM. You are right that your gas mileage will go up, but be aware that your endurance will go down.

My 170 had a 7653 when I flew it home and it burned 8.7+ per hour. I repitched it when I got home to a 7649 and the fuel burn went down to 7.4, so keep that in mind.

Use this formula,
inches of pitch X RPM X 60 mins / 12 inches / 5280 ft = Airspeed

8241 Borer
41 X 2500 X 60 / 12 / 5280 = 97 MPH in a perfect world

7452 Cruise
52 X 2500 X 60 / 12 / 5280 = 123 MPH

My friend Lonnie just took a prop like this off his recent purchase and I will tell you it did do 120 MPH on 8.00's. But it was burning 10 GPH!! (150 hp)

If you send me a PM or email I can send you his contact information. I believe he'd sell the extra prop. Use the formula to figure out the best prop for you. I think 110 MPH is an acceptable speed for a cub that still gives good off-the-ground performance. This formula has always given accurate numbers to the tune of +/- 2 mph. Don't worry about the length the numbers reflect the difference in diameter.

In case you haven't figured it out by now...
The 41 is not an angle, but rather how many inches forward it will move in one revolution in perfect conditions.

If it were me I would put the last prop the 1994's rolled out of the factory with. My stock cub was 120 MPH at 2550. Piper refered to it as a climb prop in the owners manual. Piper Part No. 450 996. Sensenich Metal
74DM6-0-56. It's not a bad prop.

Thanks folks, great info. Kelly, I appreciate the numbers, I knew about these calculations but never tried it, thanks for walking me through. Just roughly, my borer at 41 gets me 85 mph (ground speed on the GPS), if I switch to a 7454, the equation works out to around 127 mph, using the following equation:

97/85=127/X solving for x (my expected speed on my airplane) I get 111 mph. Granted, this is a simple ratio, but gives me a rough ballpark. If my fuel burn goes from 7.5 cruise now to 8.5 with the new prop (same rpm) then I go from 11.3 mpg to 13.1 mpg, then I might get 65 more miles distance out of each fill-up (36 gals). So the real benefit is speed. Interesting. But if I put on the cruise prop, and throttle back to 85 mph (gps speed) that would probably give me quite a bit more endurance over what I currently have. I'm guessing there is a chart with these numbers in the owners manual...

Props

Funseventy. If I were you I wouldn't quote anything specific on gph vs mp or rpm. During the second war the transports were asked to fly with mp much higher than the rpm. They said you could watch the blades going by the cabin. They later went back to flying the engines squared with about the same efficiency. I experience the opposite as you with climb props. With 2400rpm and 18" mp I can burn 6.0gph. With the cruise prop squared at 2400rpm it burns 4.9gph. Everyone is going to generate different numbers depending on how they fly them. A very coarse rule is that for the same mp a higher pitch will get you farther for the same fuel burn. If you want to go (faster) you can also burn (more) fuel. If you have the space it's sure handy to carry the spare prop. I have for years. Jerry.

I spent a lot of time talking to a prop guy in Iowa a couple years ago when I was playing with prop change ideas on the 85 hp J-3 I had. He said a rough reference for differences was to figure 50 rpm for any change in 1" of pitch or prop length. 82" to 74"- add 400 rpm, on mine 43" to 56" - subtract 650 rpm, or a drop of 250 rpm going to the 7456. Can't remember if we were just talking static or overall change though.
Wilbur

Wilbur
What pitch did you finaly wind up with on the 85
Scott

I ran a metal 7443.
Wilbur

AKtailwheel,
I tried the cruise prop vs. borer (8241) game heading down to the L48 this winter.

I decided on a 7456 sensenich because that was the steepest pitch I could get at the time, and as it turned out the 150 just made the static rpm requirements.

I can't make a head to head comparison in speed because I changed tires, gap seals, and went from wheel pants to wheel skis, but I tended to fly about the same rpm (2300) both directions, and the increase in speed was just about equal to the increase in fuel burn so I didn't see a mpg increase.

The formula above doesn't quite work because the propellers aren't the same diameter, so the longer prop moves more air per rotation at the same pitch.

As for carrying props around and swapping them, it's another 30+ pounds to carry around (and even my 12's extended baggage isn't 82" deep), and unless you're an A&P it's probably more pain than it's worth inside Alaska. But I certainly thought it worthwhile going to the L48.

AKTailWheel

I flew my cub many hours with a 74by56 pitched prop my cruise speed at 2500 RPMs was around 110 mph I had 8:50 tires on at the time. By changing props and going to the smaller tires I increased my cruise by over 20 mph well worth the trouble on longer trips outside of Alaska.

Cub_Driver

It's great to hear people's experience, the equations and numbers rarely live up to what they predict... I'm not thinking about swapping props on any one trip (carrying it on board), I was thinking for certain week long flying trips I would take it off and put the borer in the shed while I sped around with my fancy new cruise prop out front. But, it sounds like it probably isn't worth the extra money of investing in the cruise prop... I think I'll just buy a few new cd's, plug'em in the cd player in the plane, sit back, and watch the scenery slowly go by.

aktailwheel, I refer to my cruise prop and little tires as my "racing kit". Look for me at Reno this year!