P

petersb

Well Known Member
Deciding which to order, the Hartzell 72, or 74 inch prop for my RV7a with 180 HP.
Vans web site suggests the 74 inch unless you are building a tail dragger. However, when I phoned Vans one of the tech reps recommended the 72 inch model for the RV7A.
The additional ground clearance would be good, they do quote the minimum diameter for repair as 72 inch thus there is no margin for error if it is damaged on the tip.
I do have an alternative prop available, a three bladed MT zero timed with new blades for approx $1000 more than the Hartzell

Thoughts please

Peter
 
Tony Bengalis (possibly mis-spelled) used to say "keep your prop as long as possible as long as possible. The longer prop will be more efficient. The three blade prop will climb better, but the top speed will not match the two bladed props. I have had both. Another consideration, especially if you are doing an A model RV is that it is much easier to get the cowl off and on with a two bladed prop.


Steve Ciha
 
I have an -A model with the 74" prop. You are only losing one inch of ground clearance. I have had no issues. And yes, I regularly land on grass. I do have the new fork and the new Matco axle.
 
I prefer to have the extra ground clearance so went with the 72", no regrets :D
 
steveciha said:
Tony Bengalis (possibly mis-spelled) used to say "keep your prop as long as possible as long as possible. The longer prop will be more efficient. The three blade prop will climb better, but the top speed will not match the two bladed props. I have had both. Another consideration, especially if you are doing an A model RV is that it is much easier to get the cowl off and on with a two bladed prop.
Steve Ciha
Click to expand...

A longer prop will only be efficient as long as critical mach is not exceeded at the tip so it depends on RPM. A longer prop in this case may degrade performance.
 
rocketbob said:
A longer prop will only be efficient as long as critical mach is not exceeded at the tip so it depends on RPM. A longer prop in this case may degrade performance.
Click to expand...

Warning!! Thread drift!!

Bob, Do you know how to calculate such a critter to find out the sweet spot RPM for a 74" prop?
 
Its not as simple as radius X rpm X pi. You have to know the airfoil to determine the speed on the top surface to see how close it is to critical mach. As I recall its a function of thickness and sweep.

A question like this one for me meant a phone call to Paul Lipps and I would get an immediate answer. Miss that guy!
 
Last edited:
AltonD said:
I have an -A model with the 74" prop. You are only losing one inch of ground clearance. I have had no issues. And yes, I regularly land on grass. I do have the new fork and the new Matco axle.
Click to expand...

Exact same thing here with the exception of not landing on grass, have not had the need or opportunity yet. No regrets for the choices at all.
 
We've got a 74" on the RV-8 in my avatar. Ground clearance is not an issue. I love this prop, it rocks!
 
I put the 74" on my 7A. No problems, great prop, 204 mph cruise at 70% at 8000msl with 0-360 A1A. Smoothest prop and engine I ever flew behind.

Roberta:)
 
prop length

I have the 74" on my 7, and I land on grass. Its quite amazing how high the tail has to be for the prop to hit. I am not saying it wont, just that it has to be really high. On another note, I know of a 7 with a three blade MT that got very tail high on take off and had a strike. It left the classic fish gill look in the grass for about 60 feet. The prop was repairable and the engine was torn down but did not have any damage.
I feel its propably more of pilot technique that keeps the prop out of the dirt more than anything.
 
 
You must log in or register to reply here.