72" versus 74" inch Prop

[Davea320]

Looking to buy a used prop for my -7 QB kit and see it is 74". Vans recommends a 72" prop for the 7. Is the extra prop length that big of a deal? If that one inch creates a significant prop strike risk I prob shouldn't be building an airplane. Opinions anybody?
Thanks,

Dave

[jMINTj]

I'd like to know the answer to this also. I remember reading somewhere that the longer 74" was ok for a tailwheel, but 72" should go with a nosewheel. Then I was browsing props on the mothership website and one company (I believe Hartzell) had just the opposite, where they recommended a 74" for nosewheel and the shorter for a tailwheel, which defies the logical (to me) reason for one vs the other regarding prop strike potential. I'll be following.

[rv7charlie]

There are some rotary powered -6's & -7's (both nose & tail wheel) flying with 74" & 76" props. Given a choice, I'd pick the 74" over the 72". No downside on top speed, and better mass flow on the low end for improved climb performance.

Charlie

[maniago]

Originally Posted by rv7charlie

There are some rotary powered -6's & -7's (both nose & tail wheel) flying with 74" & 76" props. Given a choice, I'd pick the 74" over the 72". No downside on top speed, and better mass flow on the low end for improved climb performance.

Charlie

I could go either way on the M2, but I bought a 74 over a 72. I was told and also read that there is no difference in performance as the hub in the shorter prop has its course stop set slightly higher so that it takes a bigger bite of the air than the longer prop - engine speed wont change in any regime because the hub is set up to compensate appropriately. So the only reason I went with the 74 is it was available when I had cash ready and it gives me a small margin of rework over its life - the 72 is at its min size already. Price was the same. I also have bigger tires on the front than the standard 5x5s. Maybe consider what your prop clearance will be with one flat tire.

FWIW, my hub came off a Mooney w/72" blades. Also makes sense to me that in general a 3wheeler would have a shorter prop than a taildragger.

[ronschreck]

My Hartzell is 78-inches. I have no issues with ground clearance. I have flown RV-8s with 80 inch props. Those with a nosewheel may have issues with a larger prop.

[TylerII]

There is very little difference in performance. If your mission is aerobatics then the longer blades will give a bit of improvement in those instances. BUT, there is a large difference in noise. The longer the prop the greater the tip speed at a given RPM. As the tip gets closer to transsonic speed the noise and vibration increases a lot. If you want to sound like a Bonanza on take off roll then go with the longer one but be prepared for an earlier prop overhaul.
Tyler
A&P, EAA tech

[Davea320]

Thanks for all the input guys. Love this site for stuff/questions like this.

Dave

[CharlieL]

I have a 74" whirlwind on my 7A, still have over 9" ground clearance. No issues.