V

vonjet

Well Known Member
Has anybody had their prop blades cut down? If so how much does it typically cost to have done?
 
cut Down Props

Dont do it , conventional wisdom and accident reports have shown the (fatal folly) to this type of modifcation...
 
Larry is right

Never cut prop blades to less than the manufacturer's recommendations.
 
It's very dangerous...

...to consider that because there are vibration modes/nodes? that define RPM ranges to avoid, etc. I had an experimental cut down and re-twisted prop on my Cassutt racer back in the early 70's and we were cautioned to never ferry with the race prop but use the wooden one.

A guy left Indiana after the races, ferrying to Texas with the race prop, when one blade let go..about half of it. He managed a dead-stck to a dirt road and was OK but shaken. The engine was dangling by the safety cable we had to install for just that reason.

Regards,
 
Yes. 78" to 76" hartz.
I bought mine already done that way.
It was a speed mod.
Havent died.......YET!:eek:
Best,
 
I guess the real question is why you want to cut them down. For some experimentals, cutting and repitching the blades was the only solution because there was no commercially available prop but, as noted above, it's a dangerous process. For the RV, there are a lot of commercially available props and little need to 'roll-yer-own'. Hartzell's 72" blended prop is actually a 74" prop cut down by the manufacturer; they've done all the testing and experimentation for you, so no need to take it farther.

Now, if you want to cut down a blade for repair, then you should probably take it to a shop for a couple of reasons. First, to evaluate the damage: if your tips are damaged, was it a prop strike and was there non-apparent damage as well. Second, the prop will need to be re-balanced. As for the cost, it'll probably depend on how much work is involved and what shop you choose. Apparently, the early responders like myself have focused on the safety issue; maybe other posters can make some shop recommendations.
 
Hayward California

I don't have the name available but there is a prop shop at airport in Hayward that does or did cut & twist work on props for the biplane racers at Reno. It shouldn't be to hard to look up if you are interested. It is not a simple mail in process as I understand it so be prepared for extensive testing and cost.

Bob Axsom
 
Bob Axsom said:
I don't have the name available but there is a prop shop at airport in Hayward that does or did cut & twist work on props for the biplane racers at Reno. It shouldn't be to hard to look up if you are interested. It is not a simple mail in process as I understand it so be prepared for extensive testing and cost.

Bob Axsom
Click to expand...

Sullivan Props. They've also done a few Sport Class airplanes. They're good but its still black magic.
tm
 
Metal prop? If so, the advice to not cut applies. Might be OK if done by a competent shop that can do some kind of vibration analysis.

Wood props are OK to shorten.
 
That is the one

tin man said:
Sullivan Props. They've also done a few Sport Class airplanes. They're good but its still black magic.
tm
Click to expand...

That is the one alright. I talked to Cris Ferguson about the company last year and he confirmed they are good as well. I called and talked to them but living in Arkansas makes it difficult to take the plane out there and go through all they have to do to make sure it is right. I still think about it from time to time. If I were to do it they are the company I would go with. I talked to Les Doud at Hartzell and he of course advised against it. He said what seems reasonable to me - if you are only interested in optimizing speed a custom fixed pitch prop is the way to go. Cato is often mentioned.

Bob Axsom
 
vonjet said:
Has anybody had their prop blades cut down? If so how much does it typically cost to have done?
Click to expand...

If you're asking about a fixed pitch prop, and you would like to increase the rpm and efficiency, there's a fairly simple mod you can do, assuming your prop has a square tip. Starting about three to four inches in from the tip on the leading edge, cut the prop back to the trailing edge at the tip with a nice, gentle curve that looks something like a portion of an ellipse, then shape the airfoil of this section similar to the inboard portion. The tip at the trailing edge will be a point! I've done several like this and have improved the efficiency from 6% to 10% along with the desired rpm increase; I'm doing one now for a Quickie owner. The tip is a big source of HP loss, since the parasite drag coefficient there is about eight times as high as further inboard, and the resulting drag there is multiplied by the extreme radius to give a much higher torque and HP loss! A one inch span-wise portion of a four inch wide chord tip of a 72" two-blade prop turning 2700 rpm at sea-level, with a CD of 0.05, will have 3.7 to 4 HP loss, but it generates almost no thrust! That's 2.4% of a 160 HP engine!
 
You must log in or register to reply here.