Prop Question

FWF Page 44-02 upper left corner Note states: "Van's Aircraft recommends the use of a C2YR-1BFP/F7497 74 inch Hartzell Propeller on the RV-14a or C2YR-1BFP/F74972 72 inch Hartzell Propeller on the RV-14."

This seems a bit counter-intuitive for me. The longer prop on the Tri-gear RV-14a model with less ground clearance and a shorter prop on the Tail dragger RV-14 with greater ground clearance?

Can anyone, maybe Scott, shed some light, on the rationale? Thanks.

Greg Novotny
RV-14a #140209
empennage and wings built
fuselage nearly complete
1600 hours and one calendar year of construction

GigAir said:

FWF Page 44-02 upper left corner Note states: "Van's Aircraft recommends the use of a C2YR-1BFP/F7497 74 inch Hartzell Propeller on the RV-14a or C2YR-1BFP/F74972 72 inch Hartzell Propeller on the RV-14."
This seems a bit counter-intuitive for me. The longer prop on the Tri-gear RV-14a model with less ground clearance and a shorter prop on the Tail dragger RV-14 with greater ground clearance?
Can anyone, maybe Scott, shed some light, on the rationale? Thanks.
Greg Novotny
RV-14a #140209
empennage and wings built
fuselage nearly complete
1600 hours and one calendar year of construction

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Tailwheel ground clearance is measured with the aircraft in "level" attitude. In this attitude, the taildragger will have less ground clearance and more of a chance of a prop strike.

To look at it another way.....

With a nose up angle of attack and a hard drop to the runway (as in a full stall landing but at a higher altitude than optimal ), the further fwd the main wheels are, the closer the prop will get to the runway when the wheels splay out absorbing the load.
You can visualize this with a ruler aligned with the main tires (to simulate the runway surface) on the profile view of the RV-14 three view drawings.

When I ordered the prop from Vans in a bundle with the engine, they automatically ordered the 72 inch for the 14A. Is the 74 inch preferable, if so what are the compromises? thanks

stevep said:

When I ordered the prop from Vans in a bundle with the engine, they automatically ordered the 72 inch for the 14A. Is the 74 inch preferable, if so what are the compromises? thanks

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Longer blades induce slightly more risk of prop abrasion damage while taxiing (the closer to the ground the prop is, the more sand/gravel and other junk it can suck up).

Longer blades will give a very slight improvement in take-off acceleration and rate of climb.

72 vs 74" cruise performance

rvbuilder2002 said:

Longer blades induce slightly more risk of prop abrasion damage while taxiing (the closer to the ground the prop is, the more sand/gravel and other junk it can suck up).

Longer blades will give a very slight improvement in take-off acceleration and rate of climb.

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Scott, has Vans done any performance testing to see if there's a difference in cruise between the 72" and 74"? In other words is the 74" better all around, or is it a tradeoff between takeoff/climb performance vs cruise performance?

Thanks,

czechsix said:

Scott, has Vans done any performance testing to see if there's a difference in cruise between the 72" and 74"? In other words is the 74" better all around, or is it a tradeoff between takeoff/climb performance vs cruise performance?

Thanks,

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No

Speed difference because of diam. difference would be hard to get valid data on. The difference would be expected to be quite small.

Thanks Scott for you insight.

So to summarize then, the 72 inch OR the 74 inch are both acceptable on the RV-14a, but Van's is specifying the 74 inch, correct?

GigAir said:

Thanks Scott for you insight.

So to summarize then, the 72 inch OR the 74 inch are both acceptable on the RV-14a, but Van's is specifying the 74 inch, correct?

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No, Van's recommends what you posted in your first post.

These prop diameters are chosen for meeting ground clearance requirements of FAR 23

Guys,
I know we are building Experimental aircraft BUT---imho please don't try to outguess what Van has already designed. Especially in the RV14 series. Van's has had eight or more designs to figure out the best combinations of engines and props for the best all around performance. So, unless you are going racing or doing something other than what the planes mission was originally designed for, why bother? As I said, just my two cents worth.

woodmanrog said:

Guys,
, why bother? .

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Because the "why" matters to homebuilders. Keep asking why. Im reminded of the times when the little boy keeps asking "but why daddy?" until there is no further use. Keep asking why. The why matters.
In order to be successful as a homebuilder, knowledge, understanding, reasoning, critical thinking, and critical decisioning all come from being inquisitive to gain knowledge and understanding. Why the 2 different lengths is a very good question, and the responses were quite useful I thought. My 2 cents worth too.

Thank you to every one of you for contributing to my better understanding of prop choices. I too, enjoy the little nuggets of knowledge that surface in a discussion like this. And I agree with Kahuna.

Greg Novotny
Moving forward on 140209

I went with the slightly longer prop because...

First of all, the prop is only 1" different in ground clearance...so it's not highly significant unless clearance is a major issue or is minimal.

When I called Van's about it, they gave me the ground clearance they have with the prototype and the shorter prop. I compared that to my ground clearance that my RV-10 prop has and even with the longer prop, I'd have more clearance than my RV-10 has today.

Then I called Hartzell and talked it out with them. The longer prop should make me marginally happier from a performance standpoint. Either way isn't a big deal. But, they did say that if you ding a tip on the longer prop, you can file away and remove some of the tip to fix it. But, with the shorter prop, you have less margin that you can fix dings in the tip also. At any rate, if ground clearance wasn't critical, they seemed to think that with the added Cu.in. that the IO-390 has, the longer prop would make a great companion for the engine.

So I really don't see it as a big deal either way, if you're building the "A" model of the RV-14.