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Prop Choice 72" vs 74" data point

T

TimO

Well Known Member
While working on the nose fairing I remembered that I hadn't actually measured my as-installed prop clearance. When picking out my prop, I talked to Hartzell and pondered what to do about choosing the 72" vs 74". I figured that with the IO-390 making a little more power than the IO-360, the better choice may be the longer prop. Hartzell felt that way too, when I called them. They also said that the longer length gives more repair you can do to the tips should you need to shorten or file it for nicks. Van's tended to lean from what I gathered from talking to them to the 72", primarily for added clearance. The price is the same either way.

It was a tough choice but I went with the 74" after they gave me the factory demo prop clearance and I found it was more than the clearance I have on my 80" RV-10 prop.

I just wanted to give the data point for my IO-390+74" C2YR-1BFP/F7497 prop, installed on the "A" model RV-14. As installed, it is just over 11.5" of ground clearance...close to 11-5/8".

I can't say that it's the perfect prop for everyone. Taildraggers may want a different one, perhaps someone wants to comment on that. But for the RV14A, it sure seems like the 74" has no real down side.

I just wanted to post that in case there were others who sweat the details before signing the order form, like I did.
 
TimO said:
But for the RV14A, it sure seems like the 74" has no real down side.
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I suspect that the 74" prop has quite a few downsides when you think about it. The 72" vs 74" debate has been going on for years. Working on the assumption that more must be better many builders opt for the 74" prop (hey, 2 extra inches of prop for free...surely it can't be a bad thing).

But here are some considerations:

1. The 74" prop will be slower in cruise (with a slight improvement in climb as Scott has pointed out). I'd reckon that increased cruise speed would be better than increased climb given that there's a lot of time spent in cruise and not nearly so much in climb.

2. Two inches of extra diameter increases the chance of having a prop strike. You can never have too much clearance. I think that's Vans' point.

3. If you have a prop strike it will probably be an insurance claim. If there's an engine strip down involved then it will certainly be an insurance claim. If you have a 74" prop then there's always a chance that the insurance company might decide to repair your mangled prop to save some money. With a 72" prop you're getting a new prop guaranteed.
 
Maybe...maybe not

Captain Avgas said:
The 74" prop will be slower in cruise (with a slight improvement in climb as Scott has pointed out). I'd reckon that increased cruise speed would be better than increased climb given that there's a lot of time spent in cruise and not nearly so much in climb.
Click to expand...

Each engine and airframe combination will have an optimal length/blade profile for each flight condition (takeoff, climb, cruise, etc). It's true that the optimal blade length for takeoff and climb will be longer compared to the optimal length for cruise, but that does not necessarily mean that a 74" prop will be slower in cruise than a 72" prop for the RV-14 with an IO-390, because those lengths may not be optimal for this engine/airframe combination to begin with. The Hartzell may be more optimized for the 180 hp O-360 on an RV-8 and the optimal prop for cruise on a 390-powered RV-14 might actually be 76" for all we know. The RV-10 is a larger airframe with more power and uses an 80" diameter prop. A C-130 uses a 162" diameter prop. If it were as simple as saying that smaller diameter = better cruise performance, then a 5" diameter prop from my R/C model airplane would make my RV-14 go faster in cruise (albeit with terrible climb performance...but the point here is that cruise performance would also suffer with such a small propeller).

The only way to know for sure how the 72" Hartzell compares to a 74" is to test both on the same RV-14 and take very careful performance data. I would not be surprised if the difference was so small that it would be lost in the noise of flight test data points...
 
Having no clue regarding prop performance.....why would a shorter prop by 2" give a possible higher cruise, which would equate to a greater range.
Looking to better understand the theory , if in principle only.
Very interesting.
I ordered a 72" because the order sheet said so and kick myself for doing so. But now, hmmmm looking for smoke and mirrors to make me feel better.
H
 
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frghtdg said:
Having no clue regarding prop performance.....why would a shorter prop by 2" give a possible higher cruise, which would equate to a greater range.
Looking to better understand the theory , if in principle only.
Very interesting.
I ordered a 72" because the order sheet said so and kick myself for doing so. But now, hmmmm looking for smoke and mirrors to make me feel better.
H
Click to expand...

In theory, because it is operating at a higher mach number. The Hartzell guys ran some comparison points for me at cruise and commented that the higher mach for the 74" affected the efficiency slightly. I went 72" by the book, but had not thought about the tip mach effects.
 
Totally an apples to engines sort of comparison, but some years back a wingspan increase was offered for my Cessna. It added more wing area, more span and a higher gross weight.

I ran the numbers and decided that it would give me better climb and slower cruise. Sound familiar? Then I compared typical trip profiles and found that the net fuel burn and time spend traveling would be practically the same. The time saved in climb would be lost in cruise. Since I didn't need the gross weight increase, I didn't buy it.

For this prop, I'd go with the shorter one. The overall performance on a trip might be about the same as with a longer one and the prop will give you more ground clearance and possibly lower noise on take off.

Dave
 
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