RV-7A Prop CS Hartzell 72" vs 74"

[Willem Botha]

Hi there experienced prop gurus! Can anyone please give some advice - I have decided on a metal Hartzell CS prop for my RV-7A under construction, but do not know which would be the best option either 72" or 74" I would appreciate some knowledge/guidelines.

[robertahegy]

Go for the 74" on a 7A. That's what I used.

Roberta

[Pat Hatch]

Willem,

I was hoping this would draw more comments than it has. I am curious about the same thing. All that I know is Van's official statement that they are not aware of any performance differences between the 72 and the 74" props. So I guess no one has tested one against the other. My guess is that there's probably not an appreciable difference, but it would be good to have confirmation.

Van appears to recommend that if you have a nose-wheel aircraft go with the 74" and if tail wheel, go with the 72", and this seems to imply that there's a slight advantage to the larger diameter. Don't know.

[Walt]

I used a 72", I wanted the extra ground clearance along with less chance of picking up FOD causing dings/nicks. So far so good with only a few extremely minor nicks. Considering some of the unimproved taxiways/runways I've been on I feel like it's done very well.

[rvator51]

Hi Pat,

If I recall correctly from prior postings, the 72" gives more ground clearance on the tail draggers so less chance of a prop strike.

The 74" props advantage is that it can be cut down to a 72" if the ends get nicked up/damaged.

[Pat Hatch]

Originally Posted by rvator51

Hi Pat,

If I recall correctly from prior postings, the 72" gives more ground clearance on the tail draggers so less chance of a prop strike.

The 74" props advantage is that it can be cut down to a 72" if the ends get nicked up/damaged.

I think you're right, but I'm wondering if there isn't a theoretical performance advantage for the larger diameter as well. I was hoping the prop gurus would weigh in. Bob Axsom, Bob Mills, are you racing guys out there?

[Willem Botha]

Thanks everyone for your feed back etc. Van's replied to the same question by merely stating performance difference is negligible and they recommend 72 for taildraggers & 74 for nosewheel. My feeling is that if there is no diffs in performance to definately go for 72 cause everyone would like more ground clearance if there is no down side. However, it is hard to believe that performance difference is negligible. I would have thought that a larger diameter prop would generally produce the same thrust at lower rpm (obviously this could be neutralised by blade design) but I am in the dark here. Maybe someone out there has had real life experience with these two props on a RV7 or 7A - I am specifically interested to know whether rpm does come down (for same horsepower) if larger prop is installed and also whether it is desirable to run lower rpm etc.
It is also important to know that I am fitting a Fuel Injected Factory New Experimental 180hp Lycoming