What prop pitch do I use???

[cubnut93]

I am working on a L21B Super Cub that will have a Lycoming 0-320 A2B Modified from 150 hp to 160 hp. The paper work that we have says that we should use a Sensenich M74DM6-0-(52 to 56 pitch) prop. So what pitch would we use for a Standard prop???

[8GCBC]

Prop giving the most RPM's needed for full HP in climb Vy (not cruise) at gross/sealevel. The 52 inch pitch is where I would start...

[8GCBC]

BTW What RPM are you using for redline? Did the mod change RPM or just compression? Or both?

[skywagon8a]

A stock 150 Cub from Piper had a 74" diameter and 56" pitch.

[MainlandCub]

I run a 52" pitch Sensenich on my Super Cub with O-320-A2B with flow balanced cylinders. It pulls 2550 rpm static. Same prop on a non-flow balanced engine pulled 2500 static. For a 160 hp you might want a little more pitch than 52".

The 56" pitch on 150 Cub is a bit dozy I think, but it does help it get along in the cruise.

Andrew.

[Steve Pierce]

You can figure an extra 50 rpm per 1" of prop pitch. If you test someone else's prop remember that the pitch flattens out over time. Sent many a prop to the prop shop and they were way flatter than marked.

[cubnut93]

Red line is 2700 rpm.

BTW What RPM are you using for redline? Did the mod change RPM or just compression? Or both?

[cpthazard]

Steve, you say that pitch flattens out over time? I'm curious how that happens, not trying to start a fight or anything else, just curious how that happens. In my case I'm sure it's the sage brush and tall grass whittling away at the leading edge but I've always assumed that the pitch would pretty much stay where it was set in the beginning of it's service life.

[skywagon8a]

If you test someone else's prop remember that the pitch flattens out over time. Sent many a prop to the prop shop and they were way flatter than marked.

Steve, you say that pitch flattens out over time? I'm curious how that happens, not trying to start a fight or anything else, just curious how that happens. In my case I'm sure it's the sage brush and tall grass whittling away at the leading edge but I've always assumed that the pitch would pretty much stay where it was set in the beginning of it's service life.

Yes Steve I have the same question??????????? Do the molecules move around that much? Was it the same prop shop with each prop?? I would question the prop shop's technique of measuring the angle. Have you had the props checked at a different shop? This is disturbing since this basically tells me that none of the props are accurately marked except the new ones. Are all of the various prop repairmen following the same procedures?

[Steve Pierce]

Very repeatable prop guy that has been doing props since WWII. He said the hardest thing on props is static run ups. the prop is flexing while it is sitting where it is pulling the airplane it is not flexing so much. After being aware of this for many years and taking in many props and seeing what difference it makes I do believe it to be true. Granted I am talking about props with unknown times since new or rework.

[8GCBC]

I think an aircraft propeller develops less pull (aka efficiency) from time in service due to common errosion and use (abuse). But, "pitch flattening over time", I have never heard of. I am struggling to imagine how that happens?

[Carey Gray]

Northwest Propeller is on my home field. They say that a prop will actually develop more pitch after a few years. That really made no sense to me, but my Pitts prop that was stamped 60, measured 62. A 41 pitch borer with a few years on it measured 43.
Maybe some blanks are pitched really high and after vibrating for years, work back that direction.
Got me.

[cpthazard]

I actually helped repitch my prop from a 60 to 54, he had me sitting on the end of a 12 foot long cheater to twist the metal, boggles the mind that a twirling motion could equal the force of 12'x300 fa# (fat ass torque o meters) I guess it's why we send our fixed pitch props in at engine overhaul....to be measured along with removing the fatigue layer.
I'm excited to be going to Valdez this year, top on my list is seeing which prop the guys are using to get their airplanes off the ground in their own length.

[Longwinglover]

Propeller blades flattening in pitch over time all comes down to keeping the sun out of your eyes. As nobody likes to have the sun shining in their eyes and the need to wear sunglasses that hinder our pilots "egale eye" vision, we tend, by sheer habit, to fly unconsiously with the sun at our backs.

With the sun at our backs there is the ever present risk of the sun reflecting off the back of the propeller and again causing difficulties with our seeing. This is why we paint the face (back) of the propeller blades flat black. It obviously reduces the reflection when flying with the sun at our backs. The unintended consequence of painting the face of the blades is a gradual flattening of the pitch of the propeller blades.

As capthazard pointed out it takes an incredible amount of force to change the blade pitch. To lessen the amount of force needed if one could apply heat to the blades to soften the aluminum. In the case of the working propeller mounted on the plane, centripedal force from the spinning of the propeller provides almost, but not quite enough of the force ncecssary to flatten the pitch of the blades.

Heating of the propeller blades provides just enough additional assistance over time to complete the job. The heating of the blades, of course, comes from natural absorptive process of the flat black paint when exposed to the rays of the sun when we fly with the sun at our backs. The heating of the blades is mitigated to some extent by flying with the sun at an angel, flying at night, and the cooling of the blades from the spinning motion of the propeller. This is why is takes a long time to affect a noticable change.

Fly into the sun and squint - it'll make you look like a real pilot!

John Scott

[8GCBC]

Do you have other references? I can't see that happening yet.

[OLDCROWE]

Do you have other references? I can't see that happening yet.

You're not squinting hard enough... Think differential heating and differential loading of a spinning piece of aluminum that was twisted into shape. It's going to change (creep) and since flattening out in pitch would appear the logical direction of least resistance, it works for me.

[8GCBC]

You're not squinting hard enough... Think differential heating and differential loading of a spinning piece of aluminum that was twisted into shape. It's going to change (creep) and since flattening out in pitch would appear the logical direction of least resistance, it works for me.

Aluminum is very endothermic, it distributes heat fast and evenly thru it's structure. A thin piece will not have enough differential heat at that temperature to affect the yield strength? Still squinting...

[ron cope]

If you what takeof cub go with 8241 bore if motor is good it will turn 2700 on take off if not that mutch go whith a 8242 bore if you don't want take off you can go with a 8244 bore fore about 90 mph @2450. my 160 hp #1050#.ron

[NimpoCub]

Ron, now you got me scratching my "you know what".

In two separate flights, one on floats (warm air) and one on skis (cold air) spinning a Borer 82X41 w/my 160+ Bart 0320, I get 90mph @ 2300rpm, and 95mph @ 2500rpm. WFO @ 2800rpm she's indicating 105mph.
Does this wash? I have a new tach, and the ASI matches pretty good with GPS testing.

When I rebuilt/recovered, I was fairly anal about draggy thingies on my fuselage/wings. (Mine weighs 1080#)

[OLDCROWE]

And springs don't relax with time...Keep squinting. No metal that is force-shaped and left self-supporting to resist a load (not to mention differential heating and yes aluminum distributes it well but not without loss which generates more stress) will hold its shape forever, even when "stress relieved."

Some will leap and some will creep but the movement towards towards a stress relaxed shape happens.

[sierra bravo]

I've never heard of props changing pitch over time. I've heard of factory props not meeting pitch specs. That's common with Borers. Good prop shops repair that when you buy a new prop. My props have never required re-pitching unless I directed a change. I've had several props overhauled and in every case the prop is re-profiled to return the airfoil shape to specs but there's a limit to how many times the shop can remove material before the blades don't meet dimensional minimums. The problem is most pronounced at the tips where the most erosion and corrosion occur. To the idea that props relieve stress and flatten? Interesting. I guess the concept of the blades being airfoils and the prop pulling you forward as opposed to pushing you forward must be incorrect? I wonder why its so important to profile the blades properly? And I wonder why I can recognize long props bending forward when rpms are increased (when viewed from the side). And the NTSB determines whether an accident airplane was making power by how the prop blades are tilted progressively forward toward the tips from a power load. If these props are seeking to relieve stress as suggested here all our prop tips would be pointing north when our tails were oriented south. Hmmm.

[OLDCROWE]

And tail wheel springs don't flatten out with time?

[aktango58]

New props (unless checked by the shop as said by SB), can be way off. Check them before you install.

Nimpo, I had a 180 cub that only lost 5 mph putting on the floats in cruise. However, my fuel burn went up 2 gph. With flat props there is a speed in which they turn so fast that they lose efficiency... like being in first gear on the flats. But with a load you still achieve almost the same forward speed, just use more fuel to do that...

speed and boer props are not good discussions.

Find some props to use and check out your performance. If you are going to fly with Cliff into his 230' bar, then get the flat prop. If you are flying on big long turf strips and it is not a concern if you can get off in 100', but want to get to the burger joint quicker, get the larger pitch...

Try some out if you can though.

[sierra bravo]

top on my list is seeing which prop the guys are using to get their airplanes off the ground in their own length.

I expect history will repeat itself and most of the top competitors will be using Catto props. Mr. Catto seems to be very active in maximizing the performance of these airplanes. I wish we certified guys had that kind of support.

[8GCBC]

I have beat fixed pitch props (and constants speed) in the tropics for 20 years and never once did the pitch change. They get smaller and die. First time I heard of it is on this thread, I think.

[ron cope]

Nimpocub I have not had my 160 HP cub cense 1993 as I biult the 180HP that I now have that weighs in at about 1100#with 90" 33pitch has more on takeoff but not as much braking but as speed gose about 85MPH GPS but it pulles hard best I have ever had but back to the 8241 is better brakeing and Quicker responce if that rings your bell with less fuel burn. For a leight cub for max proforance you can bet a leight 160 HP cub with a 8241 bore prop my two cents. I don't now about the speed. Ron

[NimpoCub]

With flat props there is a speed in which they turn so fast that they lose efficiency...
Find some props to use and check out your performance.

Yeah, I noticed that when I get close to 100 indicated that I don't get 5mph per 100rpms any more.
I wish I had some flying buds around here w/spare parts! 2 Cubs sold out, one Scout moved away, another pilot died trying to save his Hardley. I hate (afraid) to change anything 'cuz I get incredible mileage, so think I'll stick to my 41" & fly "slower".

For a leight cub for max proforance you can bet a leight 160 HP cub with a 8241

That's what I'm thinkin' but another bud tells me I'll cover more ground/hour with a coarser prop, so even if my gal/hr goes up, I'll still get better mileage. Makes sense too, but am afraid to change anything, on one 3 hr flight I burned 15 Canuck gals. (5 gal/hr Canuck, a bit over 6 gal/hr 'Merican.)

I was hoping to get a temporary fuel flow thingie to get myself educated on what MY plane does, (then get rid of it) but they're more spendy than my curiosity requires.
Thanks guys.