[rulmen]

Forum:

Vans calls for a 70CM7S9-0 (77) for a 150HP RV-6A.

I have a line on a 70CM7S9-0 (78).

Does anyone think it will be a huge deal to use the 78 rather than a 77? I would think it would have better climb but slightly worse cruise. Am I backwards?

 

[Brantel]

Backwards ......

 

[penguin]

Yeah, you have it backwards. Climb will be worse, but cruise may be better. If you fit an electronic ignition you may not notice the difference. It also depends where you live (ie how hot it gets in the summer and the altitude of your airport), and the age of your engine. If you have a new engine at sea level, 78" may not be an issue. If necessary, getting the prop re-pitched is not a big deal and will cost around $250 for re-pitch and re-paint. 1" less of pitch will give you around 25rpm on the runway and 50rpm at max speed. As you will only be getting 2200rpm at the start of the take-off roll 25rpm can be significant. On the other hand at altitude it could make the difference between full throttle cruise comfortably at 2580rpm against a slightly worrying 2630. Does depend what your priorities are. I operated an old 150hp engine with a 77" prop for a couple of years - no problem at all. Its just that 160+hp & 80" is much better!
Pete

 

[rulmen]

Thanks

Penguin;

Thank you very much! I am at about 400 ft field alt in Georgia. So it is hot in the summer, but I have 2 miles of runway at my home field and the -6 will climb better with this prop than any Piper-28 on the field or am I dreaming?

I learned to fly in a PA-28 so.... I am expecting RV-6A performance to be quite a bit better at the same field.

May I please have your thoughts?

Rich

 

[Sam Buchanan]

Forum:

Vans calls for a 70CM7S9-0 (77) for a 150HP RV-6A.

I have a line on a 70CM7S9-0 (78).

Does anyone think it will be a huge deal to use the 78 rather than a 77? I would think it would have better climb but slightly worse cruise. Am I backwards?

I you can get the -78 at a good price go for it. As previously stated, it can be repitched to -80 when you decide to upgrade the 150 to 160. You will definitely have a cruise prop with the 150 and -78 which is fine if you have realistic expectations. Or, you can pull it back to -77 if you decide you want more climb with the 150.

If you do repitch, I highly recommend you use Sensenich. My experiences with repitching my -77 to ~80:

http://thervjournal.com/fairings.htm#repitch

 

[rulmen]

Thanks SAM!

Sam;

Great! I sure appreciate you guys helping out! Also, I really like your site and you should know that I visit it often. All I have to do is go to your site and I can see that there is light at the end of the tunnel.

Rich

 

[penguin]

Penguin;

Thank you very much! I am at about 400 ft field alt in Georgia. So it is hot in the summer, but I have 2 miles of runway at my home field and the -6 will climb better with this prop than any Piper-28 on the field or am I dreaming?

I learned to fly in a PA-28 so.... I am expecting RV-6A performance to be quite a bit better at the same field.

May I please have your thoughts?

Rich

What are you waiting for? Get your check book out!

Climb performance will be better than a PA-28 with just about any prop - I used to operate from a 3500' runway near DFW (alt 600'), no problems at all. With 2 miles of runway you should be fine.

Pete

 

[rulmen]

ALL:

Thanks!