catto vs constant speed

[TGRV7]

Thanks ReHeat, nice data.
I have a Catto on the way myself.

[elippse]

Originally Posted by L.Adamson

All depends where you live and fly, I suppose.
Out here in mountain country, we try to convince all that are still "sitting on the fence" to go with a C/S; even with the difference in cost.
Why......... because I don't know any RV owner around here, who wouldn't prefer a C/S over their fixed pitch prop, if money and engine limitations weren't of concern; especially if they've flown both for comparison. With high density altitudes, and quickly rising topography at various airfields, the C/S is amazingly different, performance wise. And that includes getting out of the airport, as well as getting in.
L.Adamson

Not quite a prop item, but I designed a set of wingtips for Jim Smith's RV-6. They increase wing area from 110 sq ft to 115 sq ft and AR from 4.8:1 to 5.8:1. This should make high altitude flight a little faster, and keep the nose attitude further down up high. Let's see how these test out and what performance gains, if any, Jim sees. These might be the answer for you "uppity" people! If you want to contact him about his three-blade ELIPPSE prop, his OAT and TAS testing travails, or just to encourage him in his wingtip experimentation, his e-mail is: Jim Smith, planehouse@hotmail.com

[gmcjetpilot]

Originally Posted by Reheat

Since this data is derived from flight testing, cafe reports, and a Hartzell computer program, and good ole curve fitting, don't pay as much attention to the absolute numbers (they're as close as I could get em) as the comparison of the two types of props. Everyone's aircraft will have a slightly different drag polar and Installed Power number and results will vary accordingly. I should add that the fixed pitch curves are for a GENERIC prop with a max efficiency of .84, but feel it represents my Catto pretty well.....

The C/S curve was generated from a DOS program that Hartzell sent me

The data you got from Hartzell was most likely older F7666 blade and 74" dia data. Top speed wise 74" is a little slower than 72" and the F7666 verses the BA is also a little hit on top speed.

Good info, but how did you mix empirical data (actual flight data) for your Catto and theoretical/derived data for the Hartzell. I'm just asking. I'm guess you derived some empirical aircraft drag data. How did you derived that aircraft drag data? Any way, good effort, love the math and graphs.

A side-by-side with another RV-8 (with like engine, weight, fit and finish) with a Hartzell, to check the data, would be cool. I'm a big fan of the SBS comparisons, and throw a Sensenich and other fixed props in there as well.

[David-aviator]

Originally Posted by gmcjetpilot

The data you got from Hartzell was most likely older F7666 blade and 74" dia data. Top speed wise 74" is a little slower than 72" and the F7666 verses the BA is also a little hit on top speed.

Good info, but how did you mix empirical data (actual flight data) for your Catto and theoretical/derived data for the Hartzell. I'm and just asking. I'm guess you derived some empirical aircraft drag data. How did you derived that aircraft drag data? Any way, good effort, love the math and graphs.

A side-by-side with another RV-8 (with like engine, weight, fit and finish) with a Hartzell, to check the data, would be cool. I'm a big fan of the SBS comparisons, and throw a Sensenich and other fixed props in there as well.

All things considered (weight, complexity, cost), the fixed pitch propeller is an good alternative to the constant speed units at this level of flying without pages of numbers to back it up.

When I came into the RV world, Tom Lynch checked me out in the 6A and later the 6 - one was CS the other FP. I was impressed with the CS operation but the FP was no laggard. It took a bit more runway to get airborne but climb out and cruise was most acceptable. I do not have SBS numbers but the impression from the left seat was good. The overall performance of FP compared to CS was more than just OK.

The item that least impresses me with CS other than cost and weight is glide range. It just does not exist. Unless the system provides for feather or near feather, the airplane will come down much sooner than later.

Missed your ugly dog, George. Welcome back.

[Reheat]

Hi George. Your assumption is correct. This all started with a quest to come up with an estimated Equivalent Flat Plate drag number for my airplane in order to compute the Drag Polar. Sounds easy enough, but… Maybe not! I was intrigued by Jack Norris’ Zero Thrust Glide (ZTG) method. I found the CAFÉ report they did on a RV-6A (FP) using this method. They computed L/D Max using the glide data, but had to admit that as hard as they tried, finding perfectly still air was a problem. There was also the CAFÉ report on the RV-8A, but the ZTG was not used. Still, the RV-6A report gave me a place to start and I knew that their numbers would be closer than anything I could come up with glide-wise. I had a book on Flight Testing of GA acft by Ralph Kimberline and that was also helpful. Neal Willford’s articles in Sport Aviation provided some of the Propeller Data, as well as, performance formulas that allowed me to determine that the ones I learned 35 yrs ago were still valid. Since gliding was out, I decided to use cruise data to determine the drag. My acft consistently hit 179-180 ktas WOT at 7500’ at 75% pwr. Here’s where the hallucinating begins. There are no dyno numbers on my engine, but Mattituck placarded it at 185HP, so that’s what I used. I picked a max prop efficiency of .84 based on some of Neal Willford’s numbers backed up by an Aero textbook. I then juggled with the Installed Power Reduction until .93 seemed to be reasonable. From there it was
185 X .75 X .93 X .84 = PWR AVIAL = PWR REQ’D. Divide by the Velocity and you get DRAG. Next, Calculate the Induced Drag at the data point and separate it out and you are left with the Parasite Drag. This yielded a Flat Plate Area of 2.2. The CAFÉ boys came up with 2.32 for the RV-6A, so I felt I was in the ballpark (yeah, and the left field wall is 400’). So… the data represents SOME acft with a Flat Plate of 2.2 and at least gives a graphic demonstration of FP vs C/S over the Velocity spectrum. This is sorta like standing in front of everybody naked (heaven forbid). I keep expecting Kevin Horton to stand up and shout BULLHOCKY or some other equivalent.

[elippse]

Originally Posted by Reheat

Next, Calculate the Induced Drag at the data point and separate it out and you are left with the Parasite Drag. This yielded a Flat Plate Area of 2.2. The CAF? boys came up with 2.32 for the RV-6A, so I felt I was in the ballpark (yeah, and the left field wall is 400?). So? the data represents SOME acft with a Flat Plate of 2.2 and at least gives a graphic demonstration of FP vs C/S over the Velocity spectrum..

In my computer programs I use for prop design, one of the things I get is an estimate of the equivalent parasite drag area (not to be confused with flat-plate drag area) using the performance data I get when someone does TAS testing using one of my props. I got numbers of 2.2 +/- 0.05 for Jim Smith's RV-6 which seems ballpark to the C.A.F.E. number of 2.32. 'Guess the difference might be due to the nosewheel. One of the interesting things that came from my program is that to get a good data match between the computer model and the actual flight data is that I had to tell Jim that his O-320 was only putting out about 150HP rather than my assumed 160HP. He said that's right, it is a 150HP engine! Did you also notice the high induced loss of the -6 at higher altitudes and slower speeds? This throws in an extra term when considering performance, since a higher HP -6 will have more speed and less induced loss, so that you can't estimate speed solely by (HP)^1/3!

[hevansrv7a]

Originally Posted by Reheat

... Since gliding was out, I decided to use cruise data to determine the drag. My acft consistently hit 179-180 ktas WOT at 7500’ at 75% pwr.

IMHO 7500 dalt gives an RV at 200+ mph much more than 75%. Mine gives 75% at about 9500 dalt as explained in my recent posts.

As to finding best L/D speed and drag polar, I stumbled on this and I think that Kevin Horton says it's close to accurate (KH - sorry if I'm misinterpreting you). You can find the minimum sink speed for a given atmospheric condition and loading by finding the lowest power that will sustain level flight. The ratio of minimum sink to best L/D is always 1.316. In other words, if minimum sink is 76 kts, best L/D is 100.

If you know the weight of the aircraft and the minimum sink speed, pull the power, hold that speed and observe your vertical speed. Now compute the horsepower that was required to keep you up there. Ditto for best L/D if you like. It's not quite as precise as the zero-thrust thing, but you can do it inexpensively and come quite close to the truth. Closer, I think than you will get by using 75% assumed at 7500' dalt.

What this does not do is to distinguish engine HP from prop efficiency; it only measures net power as experienced by the aircraft.

Ok, let the flaming begin..

[Reheat]

Heck Evan, no flaming, there are a lot of ways to skin that cat. Checking your website and your performance numbers, it all seems to add up. Your prop would appear to have less effective pitch than mine and you are running higher rpms which should account for the higher pwr%. What did you get for Drag numbers using your method?

Paul, thanks for your numbers. Let us know how your wingtips and increased aspect ratio work out.

[hevansrv7a]

Originally Posted by Reheat

Heck Evan, no flaming, there are a lot of ways to skin that cat. Checking your website and your performance numbers, it all seems to add up. Your prop would appear to have less effective pitch than mine and you are running higher rpms which should account for the higher pwr%. What did you get for Drag numbers using your method?

Paul, thanks for your numbers. Let us know how your wingtips and increased aspect ratio work out.

I get 75-80 kts IAS for minimum sink, but I go by the Lift Reserve Indicator, not airspeed. If I'm turning 2800 instead of 2700 then my extra HP would be approx. 3.7%. More of my "extra" power is coming from ram air and a good induction system. Next time I fly, I'll hold it to 2700 and see what I get. I never did the rest of the numbers on this airplane, but have used that method on cars and a Cessna. When I get my Catto prop back it should be able to test this better because it will have more pitch.

When I used this idea on the Cessna I was trying to find total HP by adding the sink rate and the climb rate. I think I ended up with about 30 HP available for climbing and the rest went for drag.

If an RV with 180 HP at 1800 pounds climbs 1500 fpm for example, then it is using 82 net HP to climb and the rest to maintain. If it sinks at 900 fpm, for example, it needs 49 HP for minimum sink. The problem here is that the minum sink is at 75 and climb is at 110, more or less. So there's 82+49= 131 leaving 49 for the speed difference and the prop losses and losses for density altitude, etc. It would be good in these computations to test the sink rate at the same speed as the climb but I haven't done that. This is a simple framework which limits our errors to the basic performance parameters that can be measured. While it's not perfect, it is designed to contain the errors within a known zone. So, it's a good cross check.

[gmcjetpilot]

Originally Posted by David-aviator

All things considered (weight, complexity, cost), the fixed pitch propeller is an good alternative to the constant speed units at this level of flying without pages of numbers to back it up.4.

Roger that, I agree 100%, no put down on fixed props at all, Pos and Cons. Regarding weight, the RV7 does not mind weight on the nose; it almost needs it, but a light RV4 or RV6, with a light weight fixed wood prop and 320....is a pure joy.

Originally Posted by Reheat

This all started with a quest to come up with an estimated Equivalent Flat Plate drag number for my airplane in order to compute the Drag Polar. Sounds easy enough, but… Maybe not!

Roger that, very nice number crunch-in. Thanks, that explains your methodology; I was just curious. You explained it very well, and I followed your logic. I see nothing wrong. I know still air is hard to find for flight test. That's why I like side-by-side "normalized" flt tests between two like planes simultaneously; it's easier and takes some variables out; all those numbers makes my head hurt, but your numbers are interesting and a useful starting point. Cheers Dudes.