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  #11  
Old 12-29-2010, 11:11 AM
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smokyray smokyray is offline
 
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Default Practicality

Wow, if you think that was a cool write-up, you should see Paul's latest post in "Contact" magazine! Having never had the experience or knowledge Paul has, I learned everything he stated the hard way. Longer props give better takeoff performance! That's why the best Super Cub prop is the "Borer" prop or seaplane prop, the longest available. Ever seen a certified FP prop that the spinner doesn't fit closely around?

I built my short gear RV4 in the 80's when Van's was in North Plains, Reagan was President, RV's were cheap and props were predominately wood. The best length for my operations over time in my short gear 0-320 RV4 I found was 69" masterfully made for me by Ed Sterba. Later, I worked very hard with Craig Catto to develop an acceptable pitch/length number for a 2 blade prop. The final result (1999) was a 69" Catto 2 blade with 71" pitch. This gave me the best compromise in performance with ground clearance on unimproved strips. 80% of my landings are off pavement which for me is an important factor. Not too many of us left in the RV world from back in the days of "the RV4 being the highest number". The ones that are will tell you Paul is right on the money.

A properly rigged RV3/4/6 or any of the newer RV's with equal HP and the right prop will match or exceed any C/S out there. For the current generation "pour the clecos in the kit box, shake it and out pops and airplane crowd" (RV7/8/9/10/12) you don't have to spend a fortune on panel/engine/prop to have a nice, efficient airplane.

Nuff said...
Smokey
RVX

Last edited by smokyray : 12-29-2010 at 11:18 AM.
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  #12  
Old 12-29-2010, 11:23 AM
John Courte John Courte is offline
 
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Default

This is a good thread. Just to make sure, I checked my prop specs, because I'm too much of a noob to be flying with a 74" prop on a taildragger -7. 72 inches. I haven't thought about the prop in a while, good to get familiar with all those parts in the shop waiting to go on an airplane.

But the argument still stands that if you nose over far enough to hit the prop, you got bigger fish to fry. Didn't take into consideration the spring flex on the -7 mains though. Good point, especially for rough field operations.
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  #13  
Old 12-29-2010, 12:50 PM
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tomwebster tomwebster is offline
 
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Default Not to muddy the waters, but

I don't know about the 74" prop, but
I think my '72' Sensenich is actually
71" long.
Tom
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  #14  
Old 12-29-2010, 01:21 PM
Bill Dicus Bill Dicus is offline
 
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Default Prop length

If you're talking the Hartzell cs props one factor is that the 74 inch can be shortened for tip damage whereas the 72 inch cannot. Len Kaufman has the 74 on his beautiful RV-8 for that reason IIRC.
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  #15  
Old 12-29-2010, 01:44 PM
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F1Boss F1Boss is offline
 
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Default not so fast there...

Paul sez:
"All of these factors enter into a prop's efficiency, so comparing two blades of different diameters, planforms, and streamlining is futile!"

The initial question was should the user purchase the longer or shorter version of the Hartzell BA 2 blade CS prop. This was not a CS/FP question; a search of this site will reveal the many opinions to that particular question.

My guess is that other factors (streamlining etc) are not entered into this equation -- it is simply an A or B question.

My answer is B: go for the longer version. This will require a higher level of skill during any wheel landing activities, and on rough strips.

Carry on!
Mark
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  #16  
Old 12-29-2010, 01:56 PM
Daver Daver is offline
 
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Default simple, cheap, light....

Quote:
you don't have to spend a fortune on panel/engine/prop to have a nice, efficient airplane.
Kinda what I built. I'm very happy with the result (and the price)

Dave
-9A flying
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  #17  
Old 12-29-2010, 04:38 PM
jimgreen jimgreen is offline
 
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I don't claim to be an expert, but I can't see any advantage to going with a 74" prop. The 72" props we use on most RVs are plenty big enough to absorb 180hp. I'd go for the extra ground clearance. Of course you and I would never do it, but a certain combination of PIO with a hard touch down would bring your prop closer to the ground than you could imagine.
Also, on our airport with sloping taxiways (needing a shot of power in places) rock chips are a major annoyance.
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  #18  
Old 12-29-2010, 05:05 PM
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hevansrv7a hevansrv7a is offline
 
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Default How much efficiency change?

I used the program written by Andy Bauer to test some typical numbers:

RPM=2700
Altitude=8000'
BHP=135 (75% of 180)
TAS=200 mph
2 blades

If prop diameter is 72" then the program can produce a prop with a design efficiency of 89.95%,

If prop diameter is 74" then the program can produce a prop with a design efficiency of 90.35%.

The program does show clearly that the smaller prop must accelerate the air more to produce the same TAS with the same BHP. Paul is, of course, correct on that.

The efficiency delta is a little less than 1/2%, prop to prop.

In real life, the prop efficiency would be less. I would expect the delta % to be relatively constant, though. Just my guess.

Just thought you might want to know. Given such a small delta, I'd go for the 72" prop which would be lighter and stronger and less liable to be damaged.
You may see it differently.
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  #19  
Old 12-30-2010, 01:22 AM
Captain Avgas Captain Avgas is offline
 
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Default Shooting yourself in the foot.

Quote:
Originally Posted by Bill Dicus View Post
If you're talking the Hartzell cs props one factor is that the 74 inch can be shortened for tip damage whereas the 72 inch cannot. Len Kaufman has the 74 on his beautiful RV-8 for that reason IIRC.
Yes true, but with the 74" prop you're much more likely to have a prop strike in the first place. In this particular case maybe an old adage might be applicable: Prevention is better than cure.

Also consider this....in most cases a constant speed prop strike is an insurance claim. If you have a 72" prop you are guaranteed to get a nice sparkly new prop from the insurance company (it can't be repaired if there's any tip damage). If you have a 74" prop you might just end up getting the damaged prop repaired if that's the cheapest option for the insurance company. In other words you might just be shooting yourself in the foot.

There has been talk on this thread of prop strikes and landing techniques but most of my friends who have had prop strikes did so while taxiing across unmade areas. One went into a dirt mound. another went into a dip, and yet another hit a tie down. You just can't have too much prop clearance. It's easier to have a prop strike than you think.

I don't think builders should get obsessed about absolutely piddling theoretical efficiency gains from a 74" prop (that may or may not exist). The only thing that is certain about a 74" prop is that it will significantly increase the chances of having a prop strike....and a prop strike will REALLY ruin your day and your prop (and perhaps your engine as well).
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Last edited by Captain Avgas : 12-30-2010 at 08:48 AM.
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  #20  
Old 12-30-2010, 09:25 AM
Bill Dicus Bill Dicus is offline
 
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Default Prop length

Good points Bob. I hadn't thought through the insurance scenario you describe. When I had a prop strike due to my own stupidity, being 72 inches long wouldn't have saved the prop. Insurance did pay, less deductable and depreciation, for a shiny new prop. The prop on my -8 (still in garage) is a 72 inch BA Hartzell and I fervently hope never to damage it!
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