Prop Modification

[Bob Axsom]

I have looked at the inputs here and received private inputs and I have called a prop shop in Oklahoma. My conclusion is that the prop can be made better for the airplane and its operating speed range but it is risky. Prop shop people are are interested in the idea in theory/science and application/engineering but are extremely reluctant to go beyond that. The guy I talked to in Oklahoma has been in the business for more than a quarter century. He said virtually the same thing as John Huft about twisting the blades - it's dangerous as **** and you shouldn't do it. He did mention adjusting the stop in the hub to allow a greater pitch angle as an alternative. I have seen that my prop contacts the rear bulkhead already and in a google search I ran across a forum where Glassair people were talking about stacking washers to give greater bulkhead clearance. I'm going to let this stew for a while as I work on some other things but eventually it will not be denied more attention. The potential is too great to ignore.

Bob Axsom

[Kahuna]

Originally Posted by Bob Axsom

in a google search I ran across a forum where Glassair people were talking about stacking washers to give greater bulkhead clearance. I'm going to let this stew for a while as I work on some other things but eventually it will not be denied more attention. The potential is too great to ignore.

Bob Axsom

Bob,
I have used washers to stack for the clearance on my Super 8 hartz c/s. Remember this just displacing the rear bulkhead, it is not changing anything as it relates to the prop hub to the crank flange.
So far it has not fallen off or caused me any issues.
Best,

[Kevin Horton]

Originally Posted by Bob Axsom

He did mention adjusting the stop in the hub to allow a greater pitch angle as an alternative. I have seen that my prop contacts the rear bulkhead already and in a google search I ran across a forum where Glassair people were talking about stacking washers to give greater bulkhead clearance.

If the prop is actually hitting the coarse pitch stop in service, the governor would no longer be able to control the rpm, and the rpm would increase higher than the selected rpm and stay there until you either slow down or pull the power back. I would have expected that you would notice it if this happened. If you aren't having a problem with the rpm staying higher than selected, I would expect there is no point to increasing the max coarse pitch.

[Bill Wightman]

I have Hartzell factory supplied (*theoretical*) data for the HC-C2YK/F7666-2 and -4. I don't have time to calculate the F7666's efficiency for Vans flight condition, but its in the 78% or better range.

Very good, George. You're spot-on with that comment, but above about 202 mph, we don't even get 78% out of this prop. And these numbers don't account for fuselage blockage effects.

I got engineering data from Hartzell back in 1999, and distilled it in my own spreadsheet:

F7666A Propulsive Data, 180HP, 75%, 8000MSL


As you can see, the F7666 doesn't even break 80% above 195 mph at typical operating RPM of 2500 and 8000MSL. It gets worse at higher RPM. I was surprised to see such poor numbers - made me wonder if we've been using the wrong prop on the RV's all these years. Its not too hard to find better performance (80-85%) elsewhere. The Hartzell BA section is better, but I don't have data for it (yet).

[elippse]

That statement is really incomplete unless you know all the facts surrounding it and can post them for a complete performance evaluation. That was just a test prop which was to be used on several racers to determine their equivalent parasite drag area from which a race prop matching the plane's drag and power could be designed.

[Bob Axsom]

Originally Posted by elippse

That statement is really incomplete unless you know all the facts surrounding it and can post them for a complete performance evaluation. That was just a test prop which was to be used on several racers to determine their equivalent parasite drag area from which a race prop matching the plane's drag and power could be designed.

Which statement are you talking about Paul? I assume it is the one about your prop on the F1 racer - your clarification is a good one.

[Kevin Horton quote - If the prop is actually hitting the coarse pitch stop in service, the governor would no longer be able to control the rpm, and the rpm would increase higher than the selected rpm and stay there until you either slow down or pull the power back. I would have expected that you would notice it if this happened. If you aren't having a problem with the rpm staying higher than selected, I would expect there is no point to increasing the max coarse pitch.]

I did not mention it but the prop man I was talking to said the governor would have to be adjusted as well. In the telephone conversation I could tell that he was thinking as he was talking and that sometimes lets tecnical details get overlooked but he seemed to have a good grasp of the subject and saw some potential equivalent to changing the twist of the blades without the risk.

[Kahuna quote - I have used washers to stack for the clearance on my Super 8 hartz c/s. Remember this just displacing the rear bulkhead, it is not changing anything as it relates to the prop hub to the crank flange. So far it has not fallen off or caused me any issues.]

I saw the contact marks on my aft spinner bulkhead and I assume this is limiting the high pitch rotation of my prop blades. This change will involve the spinner interface with the forward bulkhead or the rear bulkhead but that is just a task to be dealt with (slots is one option) but I wonder what difference you saw in performance.

Bob Axsom

[elippse]

"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."

The trouble with that is if the prop is designed for rated rpm at 8000' dalt, you obviously can't fly it at WOT and get 2500 rpm unless you're in a climb. Throttling it back to 2500 rpm isn't any good either. I usually do the speed comparison based on the cube-root of the rpm ratio times the speed. This, of course, assumes that over a small percentage of rpm difference that the hp is linear with rpm and that the speed change isn't so great as to do a noticeable change in MAP from dynamic pressure. So going from 2500 rpm to 2700 rpm should result in 8% more power which should give about 2.6% more speed, as from 190 mph to 195 mph. In the case of Jim's 150 HP RV-6, I would estimate that in going from 191.5 mph at 7000' dalt, 2740 rpm, that he would get about 202.5 mph TAS with 180 HP at 2700 rpm with a similar efficiency prop. In my equations I account for total recovery of dynamic pressure added to the outside baro pressure, along with the total rise of induction temperature due to stagnation. Remember, 5.2F rise will cause 1% less power, and at 200 mph the total temperature rise is 7.2F, which would cause a power loss of 1.4%. I am going to try to find the typical induction drop from fuel vaporization so I can account for that also. I only use forecast temperatures aloft since all of the testing I have done with other planes shows significant OAT and IAS errors! I base my engine power estimates from MAP on a sea-level MAP of 28.4" for carbureted engines since the typical sea-level MAP is 28.4" on a 29.92' pressure day, which is what is on the O-320 data sheet from Lycoming for the S-M MA carb. On injected engines I use about 0.3" drop in the induction system as a WAG. BTW, in giving an in-flight weather report to Flight-Watch, I asked if he wanted total or static temperature and he didn't know what I was talking about!

[Bill Wightman]

Originally Posted by elippse

So going from 2500 rpm to 2700 rpm should result in 8% more power which should give about 2.6% more speed, as from 190 mph to 195 mph.

Don't forget you lose propulsive efficiency when increasing from 2500 to 2700 RPM. That's a big factor.

[gmcjetpilot]

Originally Posted by elippse

"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."

The trouble with that is if the prop is designed for rated rpm at 8000' dalt, you obviously can't fly it at WOT and get 2500 rpm unless you're in a climb.

What? If you have a constant speed prop its possible. The best you can do with fixed is throttle back to 2,500 rpm or accept the rpm you get with WOT and make note or the variation. Even Van ran into this with his Prop shoot-out/round-up of 8 props. 7 of the 8 props where c/s, one was a Sensenich fixed. The Sensenich did not cooperate and turned 2,750 rpm WOT @ 8,000 feet Dalt, leaned to max power. So Van did a little math and estimated that the extra 250 rpm was worth about 3-mph (210.9 to 207.9 mph). Yes its hard to compare fixed and c/s props at WOT, but may be that' good, the way it should be. We could just compare fixed prop planes only with fixed prop planes and c/s with c/s. Or we could make a two special benchmarks, one for fixed pitch and c/s prop planes? (Vans prop test)

Throttling it back to 2500 rpm isn't any good either. I usually do the speed comparison based on the cube-root of the rpm ratio times the speed. This, of course, assumes that over a small percentage of rpm difference that the hp is linear with rpm and that the speed change isn't so great as to do a noticeable change in MAP from dynamic pressure. So going from 2500 rpm to 2700 rpm should result in 8% more power which should give about 2.6% more speed, as from 190 mph to 195 mph.

Yea BUT.... You're right but that's why c/s props are more efficient in cruise, you can vary blade angle of attack optimally. I suppose all the c/s props in Van's fly-off could have all ran at 2,700 rpm, so they would have been closer to the fixed prop. However the purposes was not to compare the fixed prop with c/s props. Most folks in the real world don't run 2,700 rpm in cruise, especially when equipped with c/s props (and with gas costing $5/gal). It's more efficient to turn the RPM down and keep the MAP up. A benchmark can be anything. This is just one benchmark that is practical and quasi represents 75% cruise.

Paul: We (RV'ers) have decide on a BENCH MARK so we can more easily compare our planes. It's not perfect but it works and you can vary slightly from altitude or rpm if you want, there is no law against it. Its a fair way to do it. It may not be ideal for a fixed pitch prop; it's just a convention not a MUST.

In general 8,500' is a good cruise altitude, so it's a benchmark. Well actually Van uses 8,000' for his performance spec or benchmark. May be 8,500' or even 9,500' would be better, especially for cruise evaluation (leaned to best power 100F-150F rop). RV's have very good induction & typically pull more than 75% power at 8k or even 8.5k dalt. It's common for RV's at 8,500' to pull 21.5" to 22.5" MAP or more. So @ 8,000', 22"/2,500 power is about 77%, a tad too much to lean but close enough (depends on temp). So may be 9,500' would be a better altitude. If a RV'er wants to record and report at 8.5 or 9.5 than fine.

Most guys & gals flying c/s props select 2,400-2,600 rpm, so 2,500 is a good average. If everyone follows the benchmark or tries to come close, its an easier apples to apples comparision. one density altitude, one throttle setting, one power 75% or less, one mixture - best power and yes one RPM - 2,500 rpm. Van publishes speeds specs for all models at 8,000 feet and 55% and 75% power. A standard to compare all RV's by. It's not perfect or will make all happy.

We could use std sea level conditions, full 100% power, max RPM, but that is not practical for everyone. Its hard to do and unsafe to fly fast low to the ground.

When a guy flight test his new RV and reports their speed at odd powers, altitudes and does not give temp and baro, it is harder to compare. This has nothing to do with optimal prop conditions. In fact, if its to the detriment of the fix pitch so be it. Most guys do not want to cruise at 2,750 all day. The fix pitch pilot has a choice, throttle back or get a big bending bar out and increase their prop's pitch (if its metal). If their prop is wood/composite its harder to change pitch.

In the case of Jim's 150 HP RV-6, I would estimate that in going from 191.5 mph at 7000' dalt, 2740 rpm, that he would get about 202.5 mph TAS with 180 HP at 2700 rpm with a similar efficiency prop. In my equations I account for total recovery of dynamic pressure added to the outside baro pressure, along with the total rise of induction temperature due to stagnation. Remember, 5.2F rise will cause 1% less power, and at 200 mph the total temperature rise is 7.2F, which would cause a power loss of 1.4%. I am going to try to find the typical induction drop from fuel vaporization so I can account for that also. I only use forecast temperatures aloft since all of the testing I have done with other planes shows significant OAT and IAS errors! I base my engine power estimates from MAP on a sea-level MAP of 28.4" for carbureted engines since the typical sea-level MAP is 28.4" on a 29.92' pressure day, which is what is on the O-320 data sheet from Lycoming for the S-M MA carb. On injected engines I use about 0.3" drop in the induction system as a WAG. BTW, in giving an in-flight weather report to Flight-Watch, I asked if he wanted total or static temperature and he didn't know what I was talking about!

I'll have to think about all that, and it is sounds interesting. I did not think about all the factors. I'm just talking about RV'ers comparing their plane to another plane by reporting a benchmark speed in REAL WORLD conditions (or as close as you can get it).

The benchmark is suppose to practical and achievable for most planes. It would be better to prescribe a set MAP and RPM, but again not all RV's have a MAP gauge. Even so fixed and c/s prop comparisons are problematic. The BEST we can do in this case is like what VAN did, allow the the RPM to exceed the benchmark or just make a note of it. This is not theory.

[elippse]

I looked up evaporation fuel temperature drop in CF Taylor and it said that with gasoline it could be 44F, but in practice it is more like 20F to 24F. However, we then have the temperature rise in the induction tubes in the oil pan which heat up the flow. So, it's anybodys guess! My fixed-pitch prop designs show a fairly flat peak efficiency curve vs TAS and rpm over range of about +/- 10%-15%, so it's fairly easy and accurate to estimate performance at other rpm-speed. The true test of a fixed-pitch prop is for you to measure the effective pitch of the prop in level flight at several different TAS and rpm. EP = TAS,mph x 1056/rpm. It should come out to a very constant value within 1% or less.