If you're picking the prop yourself, it really is trial and error (with either lots of luck or lots of error :-) ). Using an alternative engine that turns the prop at a non-typical rpm for the a/c you're building makes it even harder.

However, most of the better known prop makers can come really close if you can give them accurate info on which airframe we're building, the plane's cooling drag, max allowable prop diameter, honest HP estimate, and the prop rpm at that HP. I would think that the hardest thing for us to estimate is cooling drag, compared to a Lyc/Cont on the same airframe. I firmly believe that liquid cooling can have lower drag than air cooling, but I also believe that it rarely happens for us 'shade tree' cooling system designers, trying to graft a liquid system onto a plane designed for air cooling.

The Lyc guys are always asking the same kinds of questions about props, wanting to know what pitch to order. I believe that the best answer for a fixed pitch prop is to ask for the largest diameter that can be safely swung on the plane (usually somewhat larger than the conservative estimate by the designer), and then give the designer all the other info listed above. Then let them pitch and twist the prop as they see fit. The reason for asking for max diameter is for takeoff/low speed performance. With fixed (high) pitch for a high speed airframe, the best way to keep decent takeoff performance is extra diameter, which increases low speed 'mass flow' & helps acceleration. The extra diameter doesn't seem to hurt at high speed until you get up in the >200mph range.

I should say that I'm not an engineer or aero guy by training, or ability, for that matter. But I have played with fast homebuilts for over 20 years, and I've been through quite a few different props on several different planes. What I wrote above has been my experience, and matches the engineering articles I've read on the subject.

Hope this was useful....

Charlie


On 7/25/2014 6:50 PM, Christamarmc wrote:

I understand a bit more about the prop design now, theoretically speaking it would be nice in a perfect world for all to use the same pitch measurement like inches from the hub etc, but I guess it's not a perfect ideah as there are many differing designs of blades and tips etc

Also we are never quite sure where the manufacturer has measured there pitch on all there differing props so I guess it is a starting point for which prop to start with and then trial and error after that.

Thanks all for your input

Cheers

Christian

Sent from my iPad

On 26 Jul 2014, at 5:48 am, "Jeff Whaley" <flyrotary@lancaironline.net> wrote:

We have had this discussion before … I’ll go out on a limb here and agree with both James and Charlie.  That is to say the prop pitch is not the same from one end to the other; therefore the “effective pitch” is an estimation; and that doing better than theoretical is simply an error in the pitch estimation.  It is impossible for any propeller to work at 100% or greater efficiency.  Considering Tracy’s example of 217 mph Vs 212 mph is an error of only 2.5% (not accounting for the takeoff and climb), possibly as much as 5-10% if takeoff and climb considered … is that too hard to believe?  You need a pretty slippery ship to get near the “effective pitch” performance; here’s another example.  My Tri-Pacer has a 57” pitch propeller that cruises at 2300 rpm – theoretically this is 124 mph, exactly what the owner’s manual says; however, I can tell you that the airplane hasn’t seen 124 mph since the last time it was dove in a loop-entry – it cruises at 90 knots (104 mph) or 20% off the theoretical prop pitch – it is not a slippery ship …  Jeff

 

From: "Tracy" <flyrotary@lancaironline.net> Subject: Re: [FlyRotary] Re: Rv7 renises p port Date: Thu, 24 Jul 2014 12:09:11 -0400 To: Rotary motors in aircraft <flyrotary@lancaironline.net> <image001.gif> <image002.gif>

Charlie is right.  Consider that an airplane flying with the wing at 0 deg. AOA does not fall out of the sky :-)   My average speed in the 2004 Sun 100 race was 217.58 mph which includes standing start takeoff and climbout.  Prop was a 74 x 88, RPM was 7250 with a 2.85 : 1 drive.  If you calculate that out it comes to 212 mph with zero 'slippage'.   Draw your own conclusions!   Tracy Sent from my iPad

 

From: "Charlie England" <flyrotary@lancaironline.net> Subject: Re: [FlyRotary] Re: Rv7 renises p port Date: Thu, 24 Jul 2014 17:49:27 -0500 To: Rotary motors in aircraft <flyrotary@lancaironline.net> <image001.gif> <image002.gif>

The 'negative slippage' was a tongue in cheek verbal concoction intended to hint at the fact that 'slippage' is an artifact of old hangar tails based on lack of understanding, even among 'experts' of old, of how a prop actually works. (insert pause here to parse that awful sentence....) 'Pitch' implies that a prop is a screw. A prop isn't a screw. It's an airfoil (properly made, a *twisted* airfoil) that rotates. If you put the flat blades from a ceiling fan on the nose of an airplane, then 'pitch' might have some actual meaning (but I don't think so). Consider that 'pitch' is usually measured somewhere between 2/3 & 3/4 out from the center to the tip of a blade. If you measure 'pitch' near the root, you'd probably get something like 120-140 inches of pitch on a prop for RV's or EZ's, etc, and at the tip it would probably be something like 60", begging the question, which part of the prop is actually screwing through the air at a particular 'pitch'. It doesn't mean that almost half the prop is 'slipping' and almost to half the prop is actually dragging, with a couple of inches moving the plane. It just means that each 'station' of the prop blade must be pitched to generate thrust at the relative speed it moves through the air. The airfoil (BTW, I've never seen a symmetrical prop airfoil) is, or at least *can*, make the air move aft faster than the 'pitch' of the imagined screw is moving through the air. Just like a sailing vessel, it's not moving directly with the wind; it's moving at an angle. The relative wind for the prop is not aligned with the path of the plane. Depending on your religion, either Newton or Bernoulli makes the air go back when the prop spins. Polytheists like me believe in both. I love talking religion. Can we talk about politics next? Charlie ;-) On 7/24/2014 12:18 PM, James Osborn wrote: Charlie said:  Ice sails, desert sails, and now, even unlimited class sailboats can sail faster than the wind. 'Negative slippage'. :-)   I don't think the analogy quite applies here.  For those types of crafts it is the wind that is doing the powering.  While it is true these types of craft can sail faster than the wind, but not while pointing straight into it!  By definition to cruise in an airplane, the prop has to be generating some kind of thrust and therefore could never go faster than "the wind" - the only wind it sees is the relative wind that is generated due to its own thrust (in cruise).  In a descent, sure negative slippage is a fact.  And slippage has to increase greatly in a climb.  I guess I am arguing that the only way you could see zero or negative slippage in cruise is if either your blade cross section is asymmetrical (it usually is right?) or if the pitch number used in the calculation is not really right based on the kinds of factors you outlined Charlie.   Supposing that we have a typical non-symmetric blade cross section, an accurate pitch based on the chord line of the cross section, and I suppose a twist that is correct for the cruise RPM.  What then would be considered a good or reasonable slippage in cruise?  I saw 3% thrown out there.  And if your prop selection is good for all those conditions (in other words as efficient as possible), is this the slippage you expect?  I am just wondering if you can use a slippage calculation to judge efficiency (roughly).   -- James On Wed, Jul 23, 2014 at 7:59 PM, Charlie England <flyrotary@lancaironline.net> wrote: Ice sails, desert sails, and now, even unlimited class sailboats can sail faster than the wind. 'Negative slippage'. :-) A more significant point might be that pitch numbers are virtually meaningless, unless you're comparing two props from the same prop maker with the same blade plan form. Even then, it just tells you which has a finer pitch than the other. Variables can be: whether the pitch is measured on the back side of the blade or through the chord line, where along the diameter the pitch angle is measured (due to blade twist), and no doubt others I'm not smart enough to think of at the moment. Bottom line is that unless there's an identical airframe flying an identical prop, the pitch number isn't a reliable indicator of speed. FWIW, Charlie On 7/23/2014 7:45 PM, James Osborn wrote: I don't know jack about slippage, but I think it is the percentage difference between actual distance traveled and theoretical distance traveled if your propeller corkscrewed through the air with no thrust.  I found a prop slip calculator online and for 86 inch pitch, 2.85 gear ratio, 7000 rpm, 180 mph, I get 10% slip.  Granted the calculator was for boat propellers, but I don't think it matters as long as the units are correct.  There has to be some slip because there would be no thrust otherwise.  So what is considered a reasonable or good amount of slip?  Using Bill's numbers 86 inch pitch, 2.85 gear ratio, 7000 rpm, 200 mph, I get 0% slip.  That can't be right!   On Fri, Jul 4, 2014 at 7:19 AM, Bill Bradburry <flyrotary@lancaironline.net> wrote: Christian, It seems that you have a lot of prop slippage at cruise.  I think that at that prop rpm you should be getting 200mph if you had no slippage. Bill -----Original Message----- From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] Sent: Thursday, July 03, 2014 4:01 PM To: Rotary motors in aircraft Subject: [FlyRotary] Rv7 renises p port Well hi all Just thought I'd throw out there the mods I,ve done to the renises in an rv7'. Well the p ported engine is now back in the plane and running well, over the standard short manifold that was originally in the plane I have gained around 400 static rpm, same prop and gearbox combo, 2.85 ratio, this equates to allot more hp at takeoff, just shy of 2300 prop rpm, I'm running a prince p tip prop at 68" x 86" pitch, At 8000 ft it is turning 7000 at 180 mph tas which is an improvement of 25 mph on previous tests, . So next plan is bigger prop and less pitch to let it rev to 7500 in strait and level. Cheers Christian Rv7 renises Aus Sent from my iPad -- Homepage:  http://www.flyrotary.com/ Archive and UnSub: http://mail.lancaironline.net:81/lists/flyrotary/List.html -- Homepage:  http://www.flyrotary.com/ Archive and UnSub:   http://mail.lancaironline.net:81/lists/flyrotary/List.html

 

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