A-75--Why the variations in prop selection?

[PhilP]

Hey Everyone...

I was wondering about the A-75 conversion and prop selection. From what I understand the prop length and pitch to have in order to turn up the extra RPM is 70/38. However, after searching the forum threads regarding the 65/75 change many folks are running similar props to the A-65 standard but with less pitch on their A75's. Lengths from 71 through 74...but say a pitch of 40-41. Why go through the trouble of the conversion if you keep a longer prop??

Is the reason for not cutting down the prop all the way to 70 the desire to have the best of both worlds? Enough of an increase in RPM for extra horsepower but enough length for thrust in cruise flight?

Lastly, could some of you please share your A75 cruise speed/rpm/Prop selection?? Most notably those who have the 70/38 prop or something very similar in dimension...wondering how much you'll give up in cruise for the extra climb...

Thanks a ton!

Philip

[Guest]

Re: A-75--Why the variations in prop selection?

hi Philip,


Here's my opinion.

I would not cut it down because there are usually more of the short props around at lower cost because they have been trimmed due to repairs (and are no longer legal on the original plane) . The tend to be less $$.

However the longer props are less in number and often new(er), ie more $$.

Cutting it is like throwing $$ away.

Repitch or buy a short prop and sell yours, make $$.

Dave

[akndrifter]

Re: A-75--Why the variations in prop selection?

I don't remember what the RPMs are for A75s, but with my 74/38 I can get to 25rpms (if I'm not paying attention) no problem. I've not wanted or tried to go past, mines an A65. It also makes for an awesome climb prop, kind of like a mini borer.

[lfries]

Re: A-75--Why the variations in prop selection?

Dos anyone have the paperwork foran A-75 installation. Forest indicated there was a pre whatever the year was 337 but I've never seen the 337 or found it on many "N" number searches

[astjp2]

Re: A-75--Why the variations in prop selection?

Longer props are more effecient, climb better and the main difference between the 65 and 75 is the timing and RPM. The pistons, rods, cam, carb venturi and cylinders are the same between all of the "A" engines except for the A-80-8, the pistons are different and the rods are drilled but most of the 65's now have drilled rods too. Tim

some of the info stated above is in the type certificates and parts manuals....

[PhilP]

Re: A-75--Why the variations in prop selection?

Jim what I'm wondering is why, if a longer prop is better for climb, do most folks that change over to an A-75 cut the prop down 4 inches and repitch to 38??

Why not leave it 74 but repitch to 38??

Also, anyone with the A75 care to share their indicated Cruise speeds and RPM settings??

Thanks!

[astjp2]

Re: A-75--Why the variations in prop selection?

Well look at the Borer props, long and flat, Cessna 150 seaplane props are 75-38....Tim