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72" Straight Hartzell vs 74" Blended Airfoil

scrollF4

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Well,
2 years ago Danny King gave me a great deal on his used 72" straight-blade Hartzell CS prop off of BEAUTIFUL DOLL. It totally changed my airplane's performance, particularly regarding takeoff and formation flying. However, I took a hit in cruise speeds from the original prop, a Sensi fixed pitch cruise prop. Right now, at 8-10,000' MSL, I'm getting 155KTAS at 2450-2500rpm, peak EGT.

Looking into this, I discovered (silly me :p) that the Mother Ship intended RV-7As to fly with 74" CS Hartzell props. So....

...before I leap into the Mothership's online store to order a 74" Hartzell blended airfoil, can anybody tell me if they have also made the change from 72" straight to 74” BA? And whether it produced tangible improvements in cruise performance?
 
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Thanks Karl,
I talked to the Mothership folks: They said basically what you said but that I might see more than just a 2-3 knot difference. Going from the 72" straight blade to the 74" blended blade might even yield up to 3 or 4 knots difference! :eek: But I would notice a bit more improvement in takeoff and climb performance and possibly a reduction in vibration/increase in smoothness.

I gotta consider this for a while.
 
Check with Hartzell...

Scroll,

You might run your thoughts past Les Doud at Hartzell before you commit to something.

I saw a post on VAF about Engine/Prop/Pmag limitations a while back, long after I had been flying my aircraft a few years without a clue about these kind of things...so I asked the folks at Hartzell if some of this may apply to my setup.

I reached out to Les and sure enough, he recommended a MP/RPM avoidance range of 2350-2550 RPM above 24" MP for my dual Pmag equipped IO-360, 74" BA Hartzell package . "Torsional vibration increases" was his comment due to the more aggressive timing that running Pmags brings.

[email protected]

It's good to know limitations like this (if any) before committing to a course of action...

Cheers,

Aggie
 
I?m kind of interested in this post as well since I?m getting ready to order my Hartzell from the mothership and my question would be , is there any advantage to going with the 74 inch BA over the 72 inch blended air foil when I have 200 pony?s in the engine bay , and good point on the RPM range to avoid when operating above the 24? Mp Aggie
 
Randy Lervold did the back to back comparison of the straight blades versus the BA blades back about the time I was building the Valkyrie. At the time, the price difference between teh two options was about $200, and he got 4-5 knots top end increase with he BA - the “cheapest 4-5 knots you’ll ever find!”

I wouldn’t even consider the straight blades for an RV today - its just almost free performance increase.

I dont know if Randy’s RV-8 web site is still up, but you can reach him here via PM (he’s building a -12) if its not.

Paul
 
I copy all, Aggie.
He had already given me an RPM range to avoid with my engine/PMag/current prop combination. This would be a new phone call.
 
Randy Lervold did the back to back comparison of the straight blades versus the BA blades back about the time I was building the Valkyrie. At the time, the price difference between teh two options was about $200, and he got 4-5 knots top end increase with he BA - the “cheapest 4-5 knots you’ll ever find!”

I wouldn’t even consider the straight blades for an RV today - its just almost free performance increase.

I dont know if Randy’s RV-8 web site is still up, but you can reach him here via PM (he’s building a -12) if its not.

Paul

Roger that, Paul.
I know I can recoup some of the cost by selling the old 72" paddle blade prop, but it'll still cost several new AMUs (aviation monetary units).

I got another recommendation on the side: Keep the hub but replace the blades. Whether or not these parts match, I'm not going to run new blades in an old hub.

Or....

….I could build a jet?! :cool:
 
200nm trip @ 155 kts would take 77.4 minutes.
200nm trip @ 160 kts would take 75.0 minutes.

Ask yourself if that 2.4 minute difference on a 200nm trip is worth a couple AMU's.
 
200nm trip @ 155 kts would take 77.4 minutes.
200nm trip @ 160 kts would take 75.0 minutes.

Ask yourself if that 2.4 minute difference on a 200nm trip is worth a couple AMU's.

Yep,
That?s the basic math I?m having trouble ignoring.

OK, thanks y?all.
 
I have built two RV8s exactly the same except for avionics and prop.
Both had ECI 180 HP engines with fuel injection.
The first one had the 74" BA prop and the second had the 72" BA prop.
I originally got the 74" prop, even though VAN's recommended the 72" for tail draggers, since I felt there was plenty of clearance and in the event I had some tip damage there would be plenty for trimming/repair. The second one got the 72" prop because one became available from a new builder who decided to sell out.
Anyway, although I don't have any actual test numbers, I can say I couldn't tell the difference between the two.
Word on the street is that the longer blade climbs a little better and the shorter blade cruises a little bit faster, but I can't substantiate that.
At any rate, the BA props are definitely faster than the older straight blades. A friend of mine changed from the old straight blade to a BA and gained about 5MPH, as a previous poster also stated.
Good luck.
 
...but the guidelines haven?t changed. Talking to the Mothership tech support verified that the 74? is still recommended and sold for the 7A as you posted:

"The 74" blade diameter propellers are applicable to the "A" or tricycle gear aircraft except for the RV-8, while the 72" blade diameters are recommended for the tailwheel aircraft to ensure adaquate ground clearance."
 
Follow-up Question

OK,
Will the spinner, front plate, and backplate from my existing older 72" Hartzell paddle blade prop work on a new 74" Hartzell blended airfoil prop?
I'm terribly unlucky trying to catch the Van's tech support folks during their all-sacred tech support hours.
 
Yes, but with a caveat.
I just did this exact change for a guy a couple of months ago. Everything worked out except that the front bulkhead nut plates didn?t align with the spinner holes, using the tapped holes on the new prop hub to mount the front bulkhead.
I was able to mark the holes through the spinner and move the nutplates on the front bulkhead for a perfect fit.
No other modifications were required.
In my case, the holes were off enough that I could just drill for new nut plates on the front bulkhead.
Good luck.
 
Well,
...before I leap into the Mothership's online store to order a 74" Hartzell blended airfoil, can anybody tell me if they have also made the change from 72" straight to 74? BA? And whether it produced tangible improvements in cruise performance?

A tail dragger with a 74" prop is more susceptible to a prop strike than on a nosewheel RV. Maybe Van had that in mind when recommending the smaller 72" prop on tail wheel models. We've had four prop strikes (with resulting engine tear down) in our RV group...

I have a 74" blended airfoil prop on my RV-7A and really like the performance.
 
We've moved on

Thanks, Gents.
However, you'll notice in post #19 that we've moved on from the original question.
Now I'm asking if the spinner, front plate, and backplate from my existing older 72" Hartzell paddle blade prop work on a new 74" Hartzell blended airfoil prop?
SF3453, thanks for sharing your experience.
 
Thanks, Gents.
However, you'll notice in post #19 that we've moved on from the original question.
Now I'm asking if the spinner, front plate, and backplate from my existing older 72" Hartzell paddle blade prop work on a new 74" Hartzell blended airfoil prop?
SF3453, thanks for sharing your experience.

Scroll - We?ve got both paddle-blades and BA props in our little fleet, and just looking at them, the root of the blade where it passes through the spinner is pretty similar on both. It possible you might have to open the hole a a touch here or there, but that?s trivial with a Fiberglas spinner.

With regards to the front bulkhead, no two props/prop domes/front bulkheads are going to align the four front holes exactly the same way - and even the same prop, when it comes back from service, will probably not align exactly, as the prop dome will settle into a different spot each time. So at worst, you might ave to remake the front bulkhead, but you?ll have to do that at some point if you have your current prop overhauled anyway.....SOP.

Paul
 
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