Anybody running a smaller diameter prop?

[Dandvt]

We had a prop strike on our RV7 with an MT composite, constant speed 3-blade prop.

We're considering having the prop cut down for increased ground clearance going forward. We would wind up with a 67" diameter.

Is anyone running a prop avout that size? We're wondering what the performance impact would be going from 72" to 67".

 

[RV6_flyer]

Is there a TCDS (Type Certificate Data Sheet) for the MT prop? That should tell you how short you can go and still be safe. If it is not a type certificated product, you would need to talk to an MT prop shop to see what they recommend.

The TCDS for my Hartzell prop says that it cannot safely be shortened below 72". Yes I know of RVs flying with the same prop that are less than the required 72".

 

[Dandvt]

We have spoken with MT and they said that they can cut it down to 67 inches. What we’re wondering is if anybody has any performance data for a prop approximately that diameter on an RV?

 

[blaplante]

You got my curiosity up, so I did a little research

From http://www.epi-eng.com/propeller_technology/selecting_a_propeller.htm
In general, the larger the propeller diameter, the more efficient it will be. The following three equations (ref-4:9:219) provide an estimate of the recommended propeller diameters (inches) as a function of the horsepower available to the prop. ("Fourth root" is the square root of the square root.)


Two-blade: d = 22 x fourth root of (HP)
Three-blade: d = 18 x fourth root of (HP)
Three-blade (agricultural application): d = 20 x fourth root of (HP)

However, the maximum useful prop diameter will be limited by the speed of the propeller tip and by the airframe-provided ground clearance.

Assuming a 200 HP engine (3 bladed), 67" is actually the result of the formula above.

 

[Dandvt]

Thank you for the formula.

Interestingly if I do the two-blade formula it comes out recommending an 83-inch diameter prop. The agricultural formula comes out recommending a 76-inch prop.

I see that Catto offers a 66-inch diameter three-blade prop. Does anyone have experience with that?

 

[brad walton]

I've got a Catto 66 x 75 on Superior io360 on an RV 8. Right size per the above formula. I am very happy with the performance. I asked Craig Catto for a prop that would give me Van’s claimed performance. He nailed it exactly.
I've also observed that true air speed and rpm are correlated at varying altitudes.