The International Cessna^® 170 Association

I spoke with the parts department at Kenmore Air the other day. Their STC for the longer prop does not have any tire size requirement for the 170. It seems, according to the person I spoke with, that 6.00 x 6 tires are ok, but I would personally opt for larger. They charge $100 for the STC.

Dave

I have a Hartzell 80" constant speed prop on our '53 B. It has lady leg gear and we are using 850s. I spoke at great length with the holder of the STC for the prop, and he advised me of all of the requirements he had to go through to get the STC. The prop has oodles of clearance and has no problems with clearance with standard tires. One of the requirements for passage was full gross, at take-off attitude, on standard size tires, with one flat tire, then measure ground clearance. IOW, an 80" prop is not a concern. I am looking for an extra set of cleavland wheels so I can put 650s on them for longer cross countries, but because I prefer grass strips I use 850s. Jon

Well, I finally found a prop for 2604D (w/O-300, 180 gear & 800 tires). It is an 8042 prop that I got thru Kenmore Air in Seattle (the STC holder). It will get here before the motor is overhauled. I talked to Western Aircraft Propellers in Troutdale (whom I've dealt with before) and Lauri told me to try it as is rather than repitching it to a 43 as the difference would be very minimal. I appreciated her comments as she could as easily said bring it on down and we'll twick it for $$$$...

I'd be interested to know from some of you that have had both 8042 and 8043 props on your O-300 as to what differences I might see if I did repitch it....

Thanks for all the responses. I neglected to state that I have a DM7651. I am planning on keeping it but am looking for better T/O and climb performance. I am also putting 800s on it. Sounds like the 80 inch prop
is the way to go.

80" prop, 8.50's, 180 gear legs - notice prop clearance -

Nice wheel landing Dave ....that is a good pic to show everyone just how much clearance an 80" prop has with the appropriate gear and tires....Thanks

While it does indeed show what clearance exists with larger tires, taller gear, this looks like a very well executed normal wheel-landing ...it does not show what happens at high rates of descents, hard or bounced landings that spread the gear, or accompanied with nose-down attitudes that are possible with heavy braking and botched landings.
I'm not bad mouthing th 80' prop, I'm only saying the certification regs are developed for a valid purpose., and I wouldn't want to overlook those reasons. It's possible the gear/tire combination exceeds those regulatory requirements even.

had my plane in the hangar last week. Measured the prop clearance at 27.5". That's an 8042 prop, early 180 gear legs, 8.50's at 18 psi, tailwheel at 60.

I run that 8042 100-150rpm higher than my old 7653 and get similar fuel burns and nearly the same cruise speed but way better t/o and climb. Love it.

- paul

Hey Spiro, thanks for getting the measurement. Is that the plane that you were going to rebuild? I snooped around the red one 2 weeks ago, while waiting for a ride out on Delta.
Well I'm starting to wonder whom I'm going to pass that model plane too?
Dave

yo Dave. That measurement was on the red one, 1364D. I saw your note on it after labor day, we'd been in Fbx for the weekend. The project 170B is still sitting at my float spot while I (slowly) gather all the parts. Got an new std/std crank now and a replacement wing should be showing up tommorrow.

I thought you passed that model plane on as a door prize. If not, put it back on Trademart - it's too cool not to be assembled....

- paul

Now that I know my 7651 is a climb prop, I need only one more bit of information. Is there any information, given a "standard" airplane, regarding the difference in cruise speed between a 7651, and a 7653? In other words, I am more concerned with cruise speed on family cross country trips that getting out of that back-country strip. How much speed would I gain with the 7653 or 7655?

Using the equation for prop pitch, the 51 represents 51" forward for every revolution, so punch that number into the equation:

51"/Rev * 2450 Revs/Min * 60 Min/Hr * 1ft/12" * 1 mile/5280ft = 118 MPH.

Substitute "X" for prop pitch and you get a conversion factor of 2.32.

Multiply the 2.32 conversion factor to any prop pitch and get MPH for a cruise RPM of 2450.

Example:
My 8042 climb prop = 42 * 2.32 = 97 MPH (very close to actual)

51 * 2.32 = 118 MPH
53 * 2.32 = 123 MPH
55 * 2.32 = 128 MPH

Of course this is all theoretical and your mileage will vary. I enjoy the safety of getting off the ground sooner with my climb prop and I don't mind the extra time it takes to get from point A to B.

wa4jr wrote:Now that I know my 7651 is a climb prop, I need only one more bit of information. Is there any information, given a "standard" airplane, regarding the difference in cruise speed between a 7651, and a 7653? In other words, I am more concerned with cruise speed on family cross country trips that getting out of that back-country strip. How much speed would I gain with the 7653 or 7655?

Ever notice that when your car shifts from 3rd or 4th gear, into overdrive, that the rpm drops but the speed only increases marginally or remains about the same? Now you'll burn less fuel, with less wear and noise than you would if you'd remained in high gear.
Think of a cruise prop the same way. A 7655 will only add a few mph at the most, but it will improve fuel economy and noise. It's not a sudden "boost" in speed. (I'll bet you've flown constant speed props before too. They really help on takeoff. Then we reduce rpm to a climb setting. (climb prop) Then we level off, let 'er accelerate, and reduce to a cruise setting. (cruise prop) Only a couple of mph gain occurs, but lower noise and less wear.)

A standard prop is best for most.

You're right George. Airplanes are designed to fly at certain speeds. There was an article in a recent EAA Sport Pilot magazine by a fellow from the Luscome Association. (I guess I threw out the old issue), about putting bigger engines in airplanes. In essence, you'll gain a little speed at the expense of fuel and load. You will gain take off performance, or course.
I have a 7651, and love it. alaskan 99669 shows the theroetical speed with different props. Real speed is very close. My 170 indicates 118 mph at 2450. Of course, I don't know exactly how accurate the gages are, so either or both speed and rpm could be off a little.

I just like the take off performance because I like little grass strips. If I was a bigger paved airport guy, I would use a cruise prop to gain 2-4 mph. IMHO

I am also getting right at 118 mph cruise when loaded fairly light. With all the family and baggage on board, I fall to below 115, and may be lucky at some times to maintain 110. Looks like the additional speed, and fuel economy might make a prop change worthwile. Does anyone have a standard 7653, or perhaps a 7655 they are willing to let go of? I'll trade my 7651 if you like. The only fly in the ointment here is static RPM. I already come up 50 rpm short on static RPM. If I mount a 7653 or 7655, my static RPM would be even less, would it not?