McCauley 7651 vs. 7653

[Robert Eilers]

I know we have had this dicussion off and on before. However, I am planning a trip to Cavanaugh Bay, Johnson Creek, Smiley Creek and back from the SFO Bay Area, and I am considering having my 7653 repitched to 7651. I am considering repitching the existing prop rather than buying a climb prop to save a few bucks. My current average cruise ground speed with the 7653 is 100 to 110 MPH. Can a member flying the 7651 tell me what their average cruise ground pseed is?

[rupertjl]

I did that exact thing with my prop, I cruise at 2450 RPM and show 120MPH/105KIAS on a no wind day, my portable GPS confirms the 105 KTS regularly...

[Bruce Fenstermacher]

Robert,

Because of the differences in aircraft and instruments it is unlikely you will get real usable information to your question for your aircraft and prop. For example my airplane only goes about 115 MPH at 2500 RPM and I have my prop pitched at 54 inches.

I've created a spread sheet attached that you can enter the information you see on your aircraft. It figures out a percentage of efficiency which is not really perfect but close enough for our purpose here. You can then enter a different RPM and see the change or an RPM and Prop Pitch and see the change you are likely to see for your aircraft.

You will see that adding or reducing RPM by 100 RPM adds or reduces speed by about 5 MPH. Also changing prop pitch by 2 inches has about the same effect.

So if your airplane does 100-110 with the 53 inch pitch prop at a given rpm then reducing the pitch by 2 inches means your airplane will fly at 95-105 for the same RPM. Again this is not perfect but close enough for our purpose.

Prop Pitch RPM calc.xls

(13.5 KiB) Downloaded 557 times

(I think I might have posted a similar file before)

[GAHorn]

The DM 7651 IS considered a "climb" prop. The standard prop is 7653 and cruise prop is 7655. A cruise prop does not give greater speed...it gives the same speed with less fuel consumption. (Speed increase is a difficult thing to achieve unless MORE horsepower is developed...and a higher-pitched prop produces LESS rpm and therefore less hp...consequently less fuel-consumption.)

The MDM prop, and the EM series prop (6-bolt used on O-300-C/D engines) is slightly more efficient than the DM series due to thinner blades. Therefore approx. 2-inches more pitch is roughly equivalent to whatever the DM prop has, ... i.e., a DM7653 is approximated by a MDM or EM 7655.) The EM prop is further refined over the MDM in that it has a lighter hub and saves weight, another reason Cessna moved to that prop.

[blueldr]

Robert, you really don't need to have your prop repitched for the places you're planning to visit. I've flown a stock engined C-170 all over the Idaho back country with a 53 inch prop without any problems. It seems like a rather serious and expensive change unless you want a thinner prop anyway.

[blueldr]

George, are you telling me that you can get the same speed with less horsepower by changing the pitch on a fixed pitch prop? Come on, now!

[buzzlatka]

Robert. Drop me a line or pm. We bought a 7651 to replace the 53 and will never go back. You can do a search for my old posts where I give my opinions. I'm posting from a iPhone so I can't do all the fancy search and pasting of previous posts. Basically we lost 3-4 knots. With the 51. We now cruise at 104 knots vice 108 knots. The climb performance was definitley noticable as was the increased rpm/hp on takeoff. I would reccomend it.

[buzzlatka]

http://www.cessna170.org/forums/viewtop ... 17&start=0
let's hope this works. This should be the link to my previous post on the prop. If you want we can meet this weekend and do some side by side comparisons with the two props.

[Abe]

If any member is interested in a McCauley 7651 in I have one that I can part with....It is in great shape and I replaced it with a 8043 for more my type of flying that I'm doing here in NE Oregon...But you know that even with the "43" on my stock O300, I'm still cruising at 100-110 mph ground speed at 2450 RPM's, but sure like the authority when I'm coming out of a strip in Hell's Canyon at 85-degrees....PM me and I'll get you the details...It might me a bit before I get back to you as I'm off to Nevada today for a buck hunt at 8500' in the Ruby's.... I tried hard to stay on the subject...

[1SeventyZ]

But you know that even with the "43" on my stock O300, I'm still cruising at 100-110 mph ground speed at 2450 RPM's

Can the 8043 be run legally with standard 170 gear on 8.00x6? Just curious, as I'd love to find one.

[blueldr]

1seventy2---Why don't you just go out and measure your ground clearance and apply the difference?

[Robert Eilers]

Thank you for al the excellent replys - I am going to retwist the 7653 to 7651 and not look back.

[1SeventyZ]

1seventy2---Why don't you just go out and measure your ground clearance and apply the difference?

8lu31dr,

My question was not whether reducing prop clearance by 2" would present a problem, it was whether it was legal or not. I do not see the 1A175 on the TCDS, so I assume it's an STC. I guess I should not ask questions before looking for the answer. So, after a brief search, it appears there is an STC by Kenmore Air for the MacCauley 1A175 8042 prop.

From this:

Check the hub dimensions and bolt pattern and----- if the mukluk fits---
what the heck!
I spent four years in Fairbanks with my airplane on Weeks Field (Now long gone) and those guys used any prop that would bolt on the shaft. The FUZZ was very understanding up in that part of the world.

I understand your opinion on the matter. So let me revise my question :

For those who've purchased the Kenmore Air STC for the 8042, did it require changes the undercarriage, or was the stock 170 landplane undercarriage approved?

[buzzlatka]

I almost got the kenmore stc and from what I remember when I reviewed it the stock 170 was good.

[GAHorn]

George, are you telling me that you can get the same speed with less horsepower by changing the pitch on a fixed pitch prop? Come on, now!

In theory, ...yes. Proving it might be a bit more difficult.

But consider this: Our engines, using fixed-pitch propellers, rate their hp output purely on RPM. (That's why with a std prop they actually never produce 145 hp on takeoff, and is why the higher-pitched prop requires greater runway.) If all things remain identical except for prop-pitch...(and of course, that's the difficult thing to prove)... then the same wide-open throttle will result in lower RPM for the higher-pitched propeller...which results in less hp output despite similar speeds. A parallel example might be a constant-speed propeller wherein RPM is reduced in cruise-flight. Slower RPM means less engine-wear and fuel consumption (due primarily to reduced-piston-travel-per-mile but also affected when MP is reduced commensurately) resulting in less hp output at the same TAS.
It's undoubtedly one reason Cessna switched to the EM series,...in order to achieve greater fuel-specifics with the higher-pitch the EM allows for similar performance.