Prop Modification

[elippse]

"Slow down. . You're throwing numbers around left and right. 40% increase? Well I'm not buying. From the 221 and 241 mph numbers that would represent in about 29% more efficiency. That is not likely either. The speed I believe came for more than a prop change alone."

Let's see: (241/221)^3 X 3500/3250 = 39.65%; this assumes that HP is directly proportional to rpm. Sorry; I rounded 39.65% to 40%! If you'd like to see pictures of these props, check Sportsman Pilot, www.sportsmanpilot.com and Contact! magazine, contactmagazine.com. The three-blade is in the Fall 2004 on Tom Aberle's "Phantom", then also on "Phantom" and "Miss Gianna" in Fall 2005, then the four-blade can be seen on "Phantom" in the Fall 2006 and 2007 issues. Several people have called into question whether there was a HP increase or drag decrease from 2003 to 2004, including a well-known engine mod shop in CA who claimed that the engine was different; it wasn't! Ask Tom! There was a HP increase from 2005 to 2006, but most of the increase came from the four-blade turning more rpm, the same as the two-blade turned in 2003. His speed increase was actually higher than had been estimated from the rpm increase! Jeff Lo was originally going to get the four-blade design made for "Miss Gianna" for 2007, but was talked out of it and into going with a different prop that would give better performance. It didn't, so they raced the three-blade from 2005.
"There are no magic props, just the best compromise for that airframe, installation and engine."
These props actually are "magic"! Jack Norris in his book says their design principal is better than the best proposed by the masters Betz, Goldstein, and Theodorsen. Some incorrect notions die hard! Past performance differences where two-blade props had better cruise speed than three-blade props was attributed to something they called "tip loss" and then this eventually made it into the "everyone knows" lore as "theory"! Then others attributed this difference to interference, even though the blades in cruise track totally independent helices.

[gmcjetpilot]

Originally Posted by elippse

"Slow down. . You're throwing numbers around left and right. 40% increase? Well I'm not buying. From the 221 and 241 mph numbers that would represent in about 29% more efficiency. That is not likely either. The speed I believe came for more than a prop change alone."

Let's see: (241/221)^3 X 3500/3250 = 39.65%; this assumes that HP is directly proportional to rpm. Sorry; I rounded 39.65% to 40%! If you'd like to see pictures of these props, check Sportsman Pilot, www.sportsmanpilot.com and Contact! magazine, contactmagazine.com. The three-blade is in the Fall 2004 on Tom Aberle's "Phantom", then also on "Phantom" and "Miss Gianna" in Fall 2005, then the four-blade can be seen on "Phantom" in the Fall 2006 and 2007 issues. Several people have called into question whether there was a HP increase or drag decrease from 2003 to 2004, including a well-known engine mod shop in CA who claimed that the engine was different; it wasn't! Ask Tom! There was a HP increase from 2005 to 2006, but most of the increase came from the four-blade turning more rpm, the same as the two-blade turned in 2003. His speed increase was actually higher than had been estimated from the rpm increase! Jeff Lo was originally going to get the four-blade design made for "Miss Gianna" for 2007, but was talked out of it and into going with a different prop that would give better performance. It didn't, so they raced the three-blade from 2005.
"There are no magic props, just the best compromise for that airframe, installation and engine."
These props actually are "magic"! Jack Norris in his book says their design principal is better than the best proposed by the masters Betz, Goldstein, and Theodorsen. Some incorrect notions die hard! Past performance differences where two-blade props had better cruise speed than three-blade props was attributed to something they called "tip loss" and then this eventually made it into the "everyone knows" lore as "theory"! Then others attributed this difference to interference, even though the blades in cruise track totally independent helices.

Thanks I'll research it, but I now see you are talking fixed props on Reno formula Biplane racers.....that explains a lot. I can tell you are excited by all the exclamation points. That is cool stuff. I hope you(?) make props for RV's in the future so we all can go 20 kts faster.

The Reno guys have ONE condition to run at, WOT, +4000 rpm in little light single seat airframes w/ no drag. The prop parameters don't relate to RV's. A constant speed prop is a different ball game. You are talking about big model planes with a person in it. A prop that works on a C-130 would not work on our tiny RV's. You may be onto something but not sure it can translate to RV @ 2,700 rpm, 160-200 hp, 200 mph.

I don't have a 100% handle on Reno Biplane racer's props & engines, so I'm at a disadvantage. I hunted around the links and found some stuff on the Phantom. I see the little formula biplanes and weird fixed props. That explains a lot. Constant speed props, turning 2,700 rpm is a different deal than fixed pitch props on little engines turning crazy RPM's. More RPM does mean more HP of course. If you under pitch and are willing to run WAY OVER red line, you can go faster. Comparing apples & apples at the same non race RPM's, you'll not find the gains reported for racing.

I'm not familiar with, Miss Gianna (found picture) or Sportsman Pilot magazine. I know a little about Contact magazine. It sounds cool. I've was told the quality went downhill with "Contact", so I never subscribed.

I looked at both magazines web sites. I'm not sure about their journalistic standards. I'm sure "Contact" & "Sportsman Pilot" rags are fine, with a grain of salt. They Contact is Pro anything experimental, which is cool. The skepticism is minimal. If something really works, I hope it makes it to us mortals flying RV's at 210 mph and 2,500 rpm.

40% or 39.65%? I can wrap my mind around that, but fixed props and higher RPM's is a different, and interesting. I would say its not necessarily efficient, just fast. You really lose efficient ops when you turn high RPM, but you can go fast. I don't have the references you quote. I remain confused, impressed and incredulous all at once.

Here is the wild looking "ELIPPSE" Propeller, by Paul Lipps: (are you Paul Lipps?)

(I can see how may be the area and tip issues are addressed, interesting. I found an article
where Paul explains his approach eloquently, Proof is in the eating of the pudding.
http://www.batterson.net/EAA499/Issue77_8-13.pdf)

As far as 2 v 3 blade props and tip loss myth, with RV's its more about constant speed MT verse Hartzell, so blade thickness and design are factors, but number of blades is not just myth and lore or tips.

When you have 160 HP or 260 HP the design of a prop with two blades it's sufficient and efficient. Adding more blades does allow smaller diameter but you usually end up with excess prop area. Its like a mono plane and bi-plane, they both work, but one will always have more drag. Spinning more blades is like having more wings. If you have 500hp, 1000hp or 2000hp, multi blades - 3 or 4 and more start to earn their way, due to other practical needs, like more blade area while keeping the diameter smaller. If you need it you need it. For most GA planes you don't NEED it. May be ELIPPSE found a way around that. May be its just good for racing in small planes turning 4000rpm?

Its not just tip issues or dirty air, its the overall prop design and blade loading (which again has to match the airframe and engine / HP). If 3 blades are good, than 4 is better; than why not use 8 blades? It does not work that way. Up to about 250-300 hp is the tipping point. However the RV-10's go slower with three blades, partly because its also a MT, with the thicker blade than a metal Hartzell. You can go with a metal three blade Hartzell, but weight gets to be an issue.

I would bet money that a Sensenich Fixed prop is faster than a Catto or "ELIPPSE" three bladed prop at 2,700 rpm on a RV. Reno racers and RV need way different props. However its cool stuff and I like to learn more about the little Reno Racers. It's the only class people who are not rich can race in. The Sports Man class is already got too expensive to be competitive.


PS
From Sportsman Pilot Magazine your referenced,
Sample article: Spring 2003 issue, "Cory Bird's Symmetry"

I could only go about 180 knots before it would be over-speeding the
engine. I did some calculations and figured that I needed about 13 more
inches of pitch. I started grinding on the blades and added six inches to
the diameter, then covered it with carbon fiber and clear urethane. Now
at full power and 2,800 rpm, I get about 240 knots. I picked up 60 knots
with just that one modification, but that was another couple of month's
work.

That sounds incredible but I believe it: 1) Original prop was lousy, it didn't match airframe or engine. 2) The engine RPM was way high due to way low pitch and area. This is an example where you can say he increased prop efficiency but in context. He did not gain over say a Sensenich; he gained over a homemade prop. I bet a Sensenich (if they make one for an IO360) with proper pitch and go fast. RV's w/ constant speed props will not see gains like this. With fixed pitch, yes you see things like this. You can only optimize for one condition. I suspect your Reno examples is a case where the prop to start with was poor and exaggerated the gains of the new one.

[Bob Axsom]

I have subscribed to Sportsman Pilot for several years. It is published by editor Jack Cox and co-edited by Golda Cox in Asheboro, North Carolina. I just pulled out the fall issue from my magazine rack and inside the front cover it states:

One year subscription rate for U.S. is $12.00; Canada and Mexico $15. Overseas $20.00, payable in U.S. currency only. Address subscriptions and correspondence to: Sportsman Pilot, P.O. Box 400, Asheboro, NC 27204-0400.

All of the articles are written by Jack Cox and many of the photographs are taken by Golda Cox. I would be greatly surprised if you do not know these two classic people. They produced Sport Aviation for EAA for all of it's good years (decades) and he is the only writer I know that goes into the airplane and the builder/restorer/owner/pilot in depth without losing the subject in self love and pretentious diction. I am shutting down subscriptions to all of my other aviation magazines that do not come with membership including The RVator but Sportsman Pilot will remain active as long as Jack and Golda put it out.

In the Fall 2007 issue that I picked up at random there is a photograph of Tom Aberle's four bladed Phantom on page 7 and Jeff Lo's three bladed Miss Gianna on Page 10.

Paul Lipps has been written up in Sportsman Pilot for his Lancair, his ignition and especially his propeller designs.

Bob Axsom

[gmcjetpilot]

Yea sounds great. EAA mag has been somewhat a disappointment and I'll give Sportsman Pilot and Contact a try. Yea it just occurred to me Paul makes props. Cool. I feel dumb but he does not have his name at the bottom. If it does not have Hartzell on it, I don't pay attention. I saw a thread where he will offer these props for RV's. I can't wait. The style looks interesting and addresses my complaint about three blades having too much area for our HP. Structurally it will be a change to make those thin skinny blades work. I'll shut up and let the mad scientist invent the next cool thing.

I was thinking you had a Hartzell prop or Sensenich? May be you need one of the first ELIPPSE props for RV's!

I have to admit some cynicism hearing the next greatest fixed pitch prop announced over the years. They all seem to do the same thing or just fail. One big KEY to better prop performance is better airframe and engine performance. They can make each other look good, especially lower airframe drag. Speaking of lower drag, Bob hows the work going on that new Cowl inlets and Plenum coming?

[elippse]

"Adding more blades does allow smaller diameter but you usually end up with excess prop area. Its like a mono plane and bi-plane, they both work, but one will always have more drag."
When you design for a given CL, the area will be the same no matter how many blades, so more blades will have a smaller chord. But more blades is not like two wings of a biplane. There you have the interference that takes place between the intercepted area of each wing which lowers the overall intercept area and mass flow, which is where the Munk factor comes in. It shows the extra induced drag of the biplane depending on spacing and stagger. This does not happen with multi-blade props. Each blade sweeps out its own helix. Having a four-blade prop of the same diameter as a two-blade results in each blade sweeping out an equal volume of air which will be twice the volume of air of the two-blade, swept area times forward speed. That's more equivalent to having a higher aspect ratio wing where intercepting a greater volume of air, m-dot V, mass flow rate times velocity, results in a lower induced loss. That's why you can reduce the diameter of a four-blade but keep the swept volume as the two-blade for the same induced loss, but have lower tip speed, where most of a prop's losses occur. On my prop, shown in the picture, each blade sweeps out a path 25" ahead of the next blade. My O-235 has 125 hp at 2800 rpm, 28.4" MAP. I climb at 1550ft/min at 1350 lb, 110 mph IAS, 2410 rpm, 1000' dalt. The calculated efficiency at the point is 82%. Peter Garrison, using my data, thought it was more like 84%. So far my estimated efficiency is at least 90% at 10,000' dalt, 201 mph TAS, 2800 rpm. Prop tips produce no lift but have very high parasite drag CD at their high speed which gives lots of drag at the very high dynamic pressure present. That drag is proportional to the chord and is multiplied by the tip radius to give the torque acting against the engine. The greater the tip chord, the greater the hp loss. Big loss, no lift, zero L/D!

[gmcjetpilot]

Originally Posted by elippse

"Adding more blades does allow smaller diameter but you usually end up with excess prop area. Its like a mono plane and bi-plane, they both work, but one will always have more drag."
When you design for a given CL, the area will be the same no matter how many blades, so more blades will have a smaller chord. But more blades is not like two wings of a biplane. There you have the interference that takes place between the intercepted area of each wing which lowers the overall intercept area and mass flow, which is where the Munk factor comes in. It shows the extra induced drag of the biplane depending on spacing and stagger. This does not happen with multi-blade props. Each blade sweeps out its own helix.

Well why did you not post your article Paul? It explains it all. It makes sense. It addresses several areas that never seemed right about props, like the root area. I'm also a proof is in the pudding eating kind of guy. The Biplane Reno results where dramatic. The plane with your Ellipse prop was way faster. So when are you going to make props for RV's?

I agree not a great analogy, but if you don't design a biplane properly it can be way worse. Having one bad wing is better than having two bad wings, that was my point. Adding a third so-so blade does not make it better, it just makes it even more so-so. Also a mono plane has no worries about wing spacing, stagger or relative incidence/dihedral. Adding another blade takes more engineering and things like tip chord become more critical. Adding another blade can hurt more than help, but if you optimize for three blades and the engine and airframe like you have, three blades can be a positive. The prop theory says so. However 3 or 4 blades is never automatically goodness even if theory says so, especially if material/structural/physical limits don't allow you to optimize the blade.

Classic prop theory says more blades = more efficiency not less, so the myth is wrong, but the real world results show multi blade props often give up top/cruise speed performance (for other advantages like climb). It's not a myth just the fact despite theory. Unfortunately it seems things get lost in translation when adding blades time & time again, at least in our class of plane. Why?

I think you've addressed it with your unique prop (dia., chord, planform, airfoil, twist, tip). Cool, I agree with what you say. I appreciate how you got more efficiency at the root; you did not ignore it. I appreciate having a super-thin-narrow-chord tips with a laminar airfoil. Why not. I guess some might say your ultra thin tip might not be durable or damage resistant?

The case of the MT three blade props being significantly slower than two blade Hartzell's on a RV is the real world example. What's going on? Its not the 3 blades as much as the blades them self. For one they're wood composite blades, which are thicker than Hartzell metal blades. This is where practical limits and compromises come in. Your prop does look delicate, no offense. Structurally it's fine I'm sure, but if a rock hits the tip, it looks like it might distroy it, just from inspection of the picture. I realize its a trade off to get more aerodynamic efficiency. You say the tip can deflect easily to the touch on the ground but rigid when spun. That's interesting, what material did you use?

Having a four-blade prop of the same diameter as a two-blade results in each blade sweeping out an equal volume of air which will be twice the volume of air of the two-blade, swept area times forward speed. That's more equivalent to having a higher aspect ratio wing where intercepting a greater volume of air, m-dot V, mass flow rate times velocity, results in a lower induced loss. That's why you can reduce the diameter of a four-blade but keep the swept volume as the two-blade for the same induced loss, but have lower tip speed, where most of a prop's losses occur.

I've come to the conclusion many 3 & 4 blade props are not designed properly. Two blade props have poor blade design as well, but having less blades is an advantage if your blades are poor. The Hartzell BA was optimized for the RV and gets the best top speed for RV's. The Sensenich is almost as fast or even faster if you are willing to turn a tad over red line. A fixed pitch prop can be more efficient of course, at least in one flight condition, like high speed cruise or top speed at a given altitude. It will be fun to get some of your props on RV's and do fly-off's against other props.

As far as diameter / chord, you can reduce it only so much. Looking at your prop you can't get much thinner or narrow chord, right? At some point you can have too many blades, because you can't practically shrink the blade dia/chord any more. Where a C-130 and 4,300 hp is going to have 4 or 6 blades (because it needs them to absorb the HP). Metal blades weight is a factor on number of blades. With composite the trend has been to more blades. The case in point is the C-130, the old ones had three and four blades and now they have 6 sweeping composite blades.

On my prop, shown in the picture, each blade sweeps out a path 25" ahead of the next blade. My O-235 has 125 hp at 2800 rpm, 28.4" MAP. I climb at 1550ft/min at 1350 lb, 110 mph IAS, 2410 rpm, 1000' dalt. The calculated efficiency at the point is 82%. Peter Garrison, using my data, thought it was more like 84%. So far my estimated efficiency is at least 90% at 10,000' dalt, 201 mph TAS, 2800 rpm. Prop tips produce no lift but have very high parasite drag CD at their high speed which gives lots of drag at the very high dynamic pressure present. That drag is proportional to the chord and is multiplied by the tip radius to give the torque acting against the engine. The greater the tip chord, the greater the hp loss. Big loss, no lift, zero L/D!

Makes sense. I believe it, going to get the file the tips on my Hartzell pointy! Those tips on your prop are so thin and the chord is narrow. It does look radical but the Reno Racer hauled tail. Clearly the prop contributed to the performance. Lets get some on RV's.

[mannanj]

George:

Does this mean that you're off Hartzell's payroll now?

(Sorry; couldn't help myself!)

[elippse]

I use a constant 15% thickness all the way to the tip in my design; 12% to 15% thickness is ideal both from the standpoint of L/D and strength. There is an RV-6 flying with a three-blade prop I designed, and it was made by Fred Felix. It is flying on the 150 HP, auto-fuel RV-6 of Jim Smith of Wichita. Jim has made four separate runs with this prop at baro altitudes of 4000', 6000', 8000' and 10,000'. The best performance speed-wise he has seen is an average of 191.5 mph TAS at 7000' dalt, 2740 rpm. We found that there was a lot of improvement to be made to his pitot/static system, which showed IAS errors of about 9-11 mph. He finally corrected this by placing a 1/16" thick washer, that had been cut in half, immediately behind the rivet static-port, cut end forward. At 1440 lb he was getting rates-of-climb of about 1300-1400 ft/min at 95 mph IAS at 2000' baro alt, and averaged 1032 ft/min from 2000' to 10,000' at 95 mph IAS, with rpm at 2300 at 2000', decreasing to 2175 rpm at 10,000'. With his previous two-blade prop he did a run from TO to 10,000' where he averaged 700 ft/min. One thing of note: with his two-blade prop his OAT showed the normal drop of temperature with altitude increase. The first two data runs with the three-blade showed an almost constant temperature with altitude increase! I had him move the OAT sensor from his cabin-air inlet to inside the tail-cone and the temperature went back to the normal drop with altitude on the next two runs. I saw the same effect on an RV-9A I had tests run with my two-blade prop design, and I think what is happening is that this design has very little blockage of the airflow in the root region due to the continuation of the airfoil and the true helix angle all the way to the spinner, and so the flow of air into the cooling inlets is so much greater that air flows in, gets warmed, and then flows out and down the side of the fuselage where it enters the cabin air inlet and heats the OAT sensor. It would be interesting to get some side-by-side tests of Jim's plane with others of the same stock 150 HP, both for speed and climb, and also try this prop on an RV-6 of 150 HP equipped with the Sensenich. I wouldn't want to hear of other's speeds with their props if they base it solely on IAS as opposed to TAS from average GPS ground-speed tests. Also, I only trust density altitude data using forecast temperatures since I have also seen way too much error in OAT sensor mounting that exhibits stagnation temperature rise with speed, as much as 7.5 F at 200 mph TAS! That's 63' dalt/deg F. My dual OAT sensors on my Lancair, one behind the rear wing-spar and ahead of the flap, and one in the tail-cone, both agree in flight and agree with forecast temperatures at altitude. If your density altitude is incorrect, then your conversion from IAS to TAS will also be in error.

[plehrke]

George Andre, who finished second in the gold Formula 1 at Reno this year, tested an elippse prop and it was slower than his conventional prop. He did not use it in the race.

[gmcjetpilot]

Originally Posted by mannanj

George: Does this mean that you're off Hartzell's payroll now? (Sorry; couldn't help myself!)

Don't tell them, shhhhhhhhh, ha ha, excellent Smithers.

Originally Posted by elippse

I wouldn't want to hear of other's speeds with their props if they base it solely on IAS as opposed to TAS from average GPS ground-speed tests.

Great points on OAT and accurate flight test data gathering. You make good points about flight test data. The RV'ers are pretty good about data. As a group have pretty good discipline in measuring speed. When a someone post data that is not compete we jump on them. Many have Airdata recorders through their EFIS and Eng Monitors.

We RV's Van-O-holics have a pseudo standard benchmark we have kind of agreed on to compare speeds: WOT @ 8,000' dalt & 2,500 rpm. We tend to also use GPS three leg averaging and a spread sheet to factor for wind (a couple versions floating around). We also record(or should record: IAS, Baro and OAT at minimum.

For temp, besides the OAT gauge, I always get local ground temp and subtract the 2C per thousand and/or check the FT (winds and temp aloft forecast). There has been much discussion in VAF forums about measuring OAT and probe location. Do you account for RAM rise or just indicated OAT? Because RV's are going well over Mach 0.20 its something to think about. The problem is the temp recovery factor for RAM rise is unknown for our temp probes usually. Most guys have EFIS with an electical probe, not the typical Cessna or Piper mechanical "meat thermometer" poking out the windscreen or air-vent. Of course most spam cans are going less than 0.20M, so RAM rise or total air temp (TAT) is not critical.

Do you have any RV data with a Lyc with before & after switching to your prop, from a know configuration, like a Sensenich, Catto or Hartzell?

Are you making props for sale?

"It would be interesting to get some side-by-side tests of Jim's plane with others
of the same stock 150 HP, both for speed and climb, and also try this prop on an
RV-6 of 150 HP equipped with the Sensenich."

That would be cool. I'd like to see that.

Two things about prop claims I have seen independent of our discussion:

1) It's hard to nail down true difference between props and prop manufacture claims unless you
fly different props on the same plane or two equal planes side-by-side, which is the best way.

2) Most manufactures don't do any comparison to competition and probably don't want to,
so all the fly-off data comes from Van or RV folks like Randy and John.

Manufactures are happy to let their relative prop performance remain unquantified. Most recent tests and prop fly-offs where around constant speed props. It's time we have a fixed pitch prop fly-off / round-up. I have to admit my prejuduce for the Sensenich for many reasons, not the least of which is performance. The metal Sensenich allows re-pitching and the thinner blade is more efficient. Also metal are less maintenance than wood or even wood composite in my opinion. Wood props require rechecking torque from time to time. As far as prop weight, Van's latest models the RV-7 and RV-8, need or are designed for more nose weight. A light engine / prop combos tends to shift the CG too far aft for full utility of the aft baggage compartment. My point is the Sensenich is the Standard by which others should/could be judged. However for RV-3/4/6 there's nothing like a light 320/wood prop combo.