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Old 02-23-2012, 10:54 PM   #1
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Reading this forum I've learned a lot more about fixed pitch props than I knew before (especially following some of the links). I don't have much experience flying FP airplanes, and all my acro has been with CS props, so bear with me.
First off, a question about pitch. I know this number is supposed to represent how far the prop would "screw" through the air with no slippage. Given a pitch of 61 inches and 2700 rpm, that comes out to 156 mph. Does this represent a theoretical upper speed limit for level flight? And if so, how is the prop producing enough thrust to overcome drag if there's no slippage? Does the camber of the prop alone produce this? I guess what I'm asking is whether pitch is measured for a "zero-lift AOA" for the prop blade, or just some convenient geometric reference where the prop is still producing lift at that angle.
The second question is about overspeed. Say you're at redline rpm in level flight at 8,000 ft or so and you push the nose over. Obviously you're going to overspeed the engine/prop as gravity adds to your airspeed. I imagine if you retard the throttle towards idle, your speed won't increase quite as rapidly since the slipstream has to fight against piston compression and frictional losses to windmill the prop. With a fixed pitch prop, is this hard on the engine? I know you can use a CS prop as a kind of air brake, but wasn't sure how this differs with a FP regarding the stress to the engine.
Finally a question about preferences. For an aircraft that spends most of its time doing aerobatics and less time on cross countries or sight seeing, do you lean towards fine or coarse pitch, and do you prefer 2 or 3 blades (let's assume an O-360 is the engine in question)?



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Old 02-23-2012, 11:00 PM   #2
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Redline? Whazzat?

Ask six pilots their opinion on props, you'll get seven answers...

I can't answer to the math portions, I'm still a neophyte there myself.

But after sitting behind a two blade and a three blade in acro, I much prefer the three blade.



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Old 02-23-2012, 11:41 PM   #3
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Is that because it's quieter or for a different reason?
Regarding the difference of opinion, that's why I asked here. Not so much to stir things up but to invite debate and learn some things myself.

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Old 02-24-2012, 12:20 AM   #4
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Ginger or Mary Anne? The Eagles flew some very impressive team acro with fixed pitch props. Cheaper, lighter, less rotating mass, less gyroscopic force means less stress on crank flange and structural parts.
Do we need the aerodynamic braking a CS provides when we are flying a biplane? Back to the Eagles example.
More questions than answers.
Betty or Wilma?

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Old 02-24-2012, 12:56 AM   #5
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Oh I'm already sold on the benefits of a fixed pitch prop for my future needs. Cost and weight issues aside, I'd have to learn a new set of procedures. In turboprops you have two controls: prop and throttle (throttle measured in in-lbs or ft-lbs of torque). In the Herc it was just throttle, since the prop rpm was unchangeable in flight. And even in the free-turbine PT6-engined aircraft we rarely messed with the prop RPM. Flying a CS prop in a piston aircraft would require setting RPM, manifold pressure, and mixture in the proper order for each regime of flight. Simpler procedures for a FP prop, as I recall.
Good point on the aerodynamic braking. Biplanes are pretty draggy as is. How easy or difficult is it to reach or exceed Vne in a Pitts or Eagle? Should that occur, are you more worried about potential damage to the engine/prop or the airframe?

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Old 02-24-2012, 01:38 AM   #6
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It's not that easy to exceed Vne in a PItts. Of course you can do it accidentally by pulling back on the stick to go up when you're inverted but normally it takes some effort. An Eagle is cleaner than an S-2A and will get there quicker but it's not something you need to worry about under normal circumstances. If you exceed Vne with full throttle in a FP Pitts you'll get something over 3300 rpm with a climb prop. That won't really hurt the engine and the airframe would likely tolerate it too as long as you didn't do it frequently. I have spent a lot of time in Pitts airplanes and never exceeded Vne by accident. Or on purpose more than once or twice.

Does a falling tree make a sound if there's no one there to hear it? Does the tach go over redline if you don't look? [img]smileys/smiley1.gif[/img] I'm sure there's a true limit to the rpm a Lycoming engine will tolerate but I don't know what that number is. I literally never look at the tach or any other engine instruments while flying sequences unless the engine gives me a reason to look.


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Old 02-24-2012, 02:27 AM   #7
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Pretty big safety factor eh? Makes sense. So do you pull back the throttle on long downlines or just let it go? Or would it really hold back the revs anyway if you pulled it back while going downhill?

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Old 02-24-2012, 02:31 AM   #8
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For me, pulling the throttle back some on downlines is the same as coordinating stick and rudder. Just comes with the territory.

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Old 02-24-2012, 03:00 AM   #9
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Gotcha. In my case retarding the throttle was to avoid Vne or overstressing on pullout, not due to worries about overspeed. Didn't know how it works with a biplane and FP prop.

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Old 02-24-2012, 03:10 AM   #10
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I have a simple way of looking at what to do with the throttle -- if you need to go fast, use full throttle, if you need to slow down go to idle throttle. The exceptions are turns, particularly rolling turns -- staying in the box can make partial throttle a good choice and for rolling turns you can add that the airplane is more willing to turn at 120mph than 140mph. The only other time I can think of to use a non-digital throttle is to place a spin or other low speed maneuver closer to the edge of the box.

Using full power results in less altitude loss than using a longer dive at reduced power. Altitude is money in the bank. You use that money to buy speed for other maneuvers and to keep the sequence as low as possible to make the box feel larger and give the judges a better look at your flight.

I know of no evidence that reducing rpm saves the engine discounting gyroscopic loads from heavy props. Composite props have solved that issue.

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Old 02-24-2012, 03:15 AM   #11
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I should mention, I'm not doing competition, so the thought process/perspective is a little different.

I do mostly gentleman's aero. 3 1/2 Gs, 4 if I mess something up.

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Old 02-24-2012, 03:36 AM   #12
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Thanks for the clarification, Doug.

Quote:
Originally Posted by cwilliamrose
I have a simple way of looking at what to do with the throttle -- if you need to go fast, use full throttle, if you need to slow down go to idle throttle. The exceptions are turns, particularly rolling turns -- staying in the box can make partial throttle a good choice and for rolling turns you can add that the airplane is more willing to turn at 120mph than 140mph. The only other time I can think of to use a non-digital throttle is to place a spin or other low speed maneuver closer to the edge of the box.

Using full power results in less altitude loss than using a longer dive at reduced power. Altitude is money in the bank. You use that money to buy speed for other maneuvers and to keep the sequence as low as possible to make the box feel larger and give the judges a better look at your flight.

I know of no evidence that reducing rpm saves the engine discounting gyroscopic loads from heavy props. Composite props have solved that issue.
Thanks Bill, that clears up a lot for me. Any preferences for a particular pitch in the S1 for pure aerobatics?
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Old 02-24-2012, 03:50 AM   #13
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My info on props is too dated to be of value. We used 76 x 56 metal props on stock O-360's and 76 x 60 on IO-360's (200hp). Composite props weren't used except on Hoffman CS props which had a lot of wood content in the blades.

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Old 02-24-2012, 03:53 AM   #14
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Does that imply one might want to use a different pitch, all other things being equal, when the material of the prop is changed? Sorry for nickel-and-diming you with questions, I just ask them as they pop into my head.

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Old 02-24-2012, 03:59 AM   #15
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The composite props have (sometimes) less diameter, more pitch and often three blades instead of two. I guess it's an apples/oranges thing. There are folks on this board with experience using composite props, hopefully they'll contribute to your thread.

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Old 02-24-2012, 07:47 AM   #16
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As for the 2/3 blades thing, there are already been some posts discussing toroughly the differences on the forum, but the main things to me are :
- 2 blades imply far more stress on the engine during gyroscopic manoeuvres (alternative stress : max when prop parallel to precession, none when perpendicular)
- I've always been told that 2 blades give more usable torque at a given RPM than 3 blades, hence better effeciency for aerobatics on a draggy/pretty slow biplane (compared to 300cv monoplanes)... although I think I can "feel" it, I never could find a mathematical explanation for this (but if you find one, I'll be happy to hear it!)
So for me : 2 blades, but composite to limit stress for gyro...
Xav

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Old 02-24-2012, 01:57 PM   #17
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You've already gotten good info from several folks. Prop pitch between different propellor makes is not necessarily comparable. I'm currently running a 2-blade metal Sensenich 76-59 and will be replacing it with a 3-blade Catto 72-56 that should arrive in a couple weeks.The performance that Craig (Catto) described for his 56 prop seems pretty close to what I'm currently running.I'll post comparison notes.Can'twait to get it on.


One nice thing about a fixed pitch 3-blade (other than the reduced gyro forces already noted) is that it's much quieter for folks on the ground if you're turning your engine in the 3000 rpm range due to the slower tip speed with the smaller diameter. Makes it less likely for folks to complain about where you're practicing acro. Also, I wonder if there is any efficiency advantage to the 3-blade when turned at very high RPM due to the fact thatthe tips of a2-blade would be supersonic which reduces the aerodynamic efficiency of the blades.Not sure I've ever seen any data on this.


When I first started flying my Pitts, I made unnecessary effort to manage RPM. Igot over it and just let it turn up so that in general, you're flying a two position power setting - full and idle (for spins). As Bill mentioned, rollers work a little better, and look better from the ground if entered from reduced speed/throttle. But you might just need to add a little power after getting it going to maintain that speed.


In competition, throttle is also frequently reduced a bit for line length and energyissues when there is a snap roll on a line. For example, if you have a snap roll on a 45 degree or verticaldownline, your line before and after the snap should be equal, but you'll still want a decent amount of speed on exit for the next figure. Since snap rolls have fairly low (and specific) entry speed limitations, you might want to pull off some power before the snap so that speed doesn't built as quickly, allowing you to draw a longer line before reaching your snap speed. You can then draw a longer line after the snap and build more speed without the line after the snap being too much longer than the line before.

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Old 02-24-2012, 03:15 PM   #18
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Found on
http://eaaforums.org/showthread.php?903-IVO-prop-installationI thought you may find it interesting :
"This season there has been quite a bit of discussion in the aerobatic community about installing very light weight 2 blade props on 4 cylinder Lyco engines. The short version is that the engine needs mass to absorb the power pulses. This mass can be in the form of 6th order crankshaft counterweights, or a propeller that is heavy enough, or made of the right materials, to absorb the energy pulses. I know of at least one owner who discovered that a carbon fiber 2 blade propeller was bad for his un-counterweighted Lyco. The engine was discovered to be internally destructing. Gears coming apart and other very expensive problems.<br style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif; font-size: 13px; : rgb(250, 250, 250); "><br style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif; font-size: 13px; : rgb(250, 250, 250); ">The bolt loosening described in the previous post is a symptom of the prop (and the engine) getting beat up by the lack of dampening of the engine power pulses.<br style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif; font-size: 13px; : rgb(250, 250, 250); "><br style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif; font-size: 13px; : rgb(250, 250, 250); ">Interestingly, 6 cylinder engines have much lower amplitude power pulses so they don't have this problem. 3 blade propellers absorbe the energy differently than 2 blade props (different phase relationships) also, so the 3 blade MT's (wood core blades) seem to work well.<br style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif; font-size: 13px; : rgb(250, 250, 250); "><br style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif; font-size: 13px; : rgb(250, 250, 250); ">Hope this info helps. My guess is that you should not do that installation.<br style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif; font-size: 13px; : rgb(250, 250, 250); "><br style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif; font-size: 13px; : rgb(250, 250, 250); ">Best of luck,<br style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif; font-size: 13px; : rgb(250, 250, 250); "><br style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif; font-size: 13px; : rgb(250, 250, 250); ">Wes<br style="color: rgb(51, 51, 51); font-family: Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif; font-size: 13px; : rgb(250, 250, 250); ">N78PS"

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Old 02-24-2012, 07:55 PM   #19
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Thanks very much everyone, this has taught me a great deal.

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Old 02-24-2012, 08:38 PM   #20
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My Pitts S-1S numbers, O-360 Lycoming, 2x2 crossover exhaust, runs good. Prop is a 76-62 Sensenich.

2700 rpm gives me 138 knots - 159 mph, cross-checked with GPS groundspeed on a two-way run.

Full throttle, let the nose drop a little and at 200mph it is turning 3100 rpm.

Hangs on to altitude real well in the Unknown, so has decent acceleration out of the hole.

I fly it pretty much the way cwilliamrose and grassroots describe.Edited by: Guido Lepore

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Old 02-25-2012, 12:08 AM   #21
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Is there still a place for wood props on S-1's.
Steve

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Old 02-28-2012, 01:56 PM   #22
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Steve,


Performance Propeller, Caddo, and Sensenich just to name a few. I have had two Performance Properller (Warnke) and a third one on the way for my Single seat 300hp. Will be a three blade custom. 76"Dia 58 Pitch.


JST

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Old 02-28-2012, 03:46 PM   #23
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Catto aerobatic propsare wood core, with glass/carbon layups, similar to an MT. Much more rigid and stable than an all-wood prop. IMO,the only advantage of an all-wood prop over a composite wood core propis price. Edited by: grassroots



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