too slow!

I bought an already built RV-6A. It only cruises at 140 knots. It is equipped with an O-320 160 hp w/ a 70cm X 78" prop. This is really a climb prop, but I max out at 2500 rpm at 8,000 ft!
I want to go to a cruise prop, but I'm afraid I won't develop the rpm and power.
Does anyone know what's wrong with my plane?

140 knots... Is that indicated or true airspeed? What altitude? In either case how have you verified the accuracy of your measurements? Do you have a manifold pressure gauge? What does it show? What is your fuel burn rate?

I frequently see 140 knots or less indicated in cruise on my RV-6, but that's at altitude, and the TAS is 155 to 160 knots at 65%.

Too slow

akorchak said:

I bought an already built RV-6A. It only cruises at 140 knots. It is equipped with an O-320 160 hp w/ a 70cm X 78" prop. This is really a climb prop, but I max out at 2500 rpm at 8,000 ft!
I want to go to a cruise prop, but I'm afraid I won't develop the rpm and power.
Does anyone know what's wrong with my plane?

Click to expand...

The first thing I would do is verify the tack, its probably correct but you never know.My 6A with a 0320 150 hp and 70cm x76 would only turn 2500 at 8000', but it was a tired engine with over 2000 hr on it,. indicated airspeed is doesn't mean anything, except for landing.
Doyle Reed RV7A 421 hrs

Kyle Boatright said:

140 knots... Is that indicated or true airspeed? What altitude? In either case how have you verified the accuracy of your measurements? Do you have a manifold pressure gauge? What does it show? What is your fuel burn rate?

I frequently see 140 knots or less indicated in cruise on my RV-6, but that's at altitude, and the TAS is 155 to 160 knots at 65%.

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140 knots groundspeed at 8,000 ft. averaged. I had my tach and timing checked and are good. It's a FP. My static rpm is 2350. I haven't checked my fuel burn yet, will do tomorrow.

casper said:

The first thing I would do is verify the tack, its probably correct but you never know.My 6A with a 0320 150 hp and 70cm x76 would only turn 2500 at 8000', but it was a tired engine with over 2000 hr on it,. indicated airspeed is doesn't mean anything, except for landing.
Doyle Reed RV7A 421 hrs

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I had the tach and timing checked and they're good. What groundspeed did you get with the 150 combo?

Too slow

akorchak said:

I had the tach and timing checked and they're good. What groundspeed did you get with the 150 combo?

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My 6A with the 150 combo would do 177 MPH averaged GPS
Doyle Reed

akorchak said:

140 knots groundspeed at 8,000 ft. averaged. I had my tach and timing checked and are good. It's a FP. My static rpm is 2350. I haven't checked my fuel burn yet, will do tomorrow.

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Well, your static RPM is 250 rpm higher than mine. I guess you're right about having a climb prop. Can you borrow a MP gauge? Combine MP and RPM and you'd have a good idea of your true power setting. It may be that your RPM is fairly high, but your MP is low, giving you far less than 75% at your cruise RPM. The obvious correction would be to switch to more of a cruise prop.

By the way, who made your prop?

KB

Hidden speed let's talk

akorchak said:

I bought an already built RV-6A. It only cruises at 140 knots. It is equipped with an O-320 160 hp w/ a 70cm X 78" prop. This is really a climb prop, but I max out at 2500 rpm at 8,000 ft!
I want to go to a cruise prop, but I'm afraid I won't develop the rpm and power.

Click to expand...

So you are getting 140kts or a bout 161 MPH ground speed. I guess it has to be said you have to average 4 runs approx 90 degrees apart (only accurate within 1-3%). The best is 3 legs approx 120 degrees apart. Note TRACK and SPEED. Using a little spread sheet program, you resolve the ground speed.

You can get the spread sheet at Kevin Horton's RV-8 Project:

Kevins Flight test page - excel spread sheet for GPS to ground speed calculation

Prop: Something is not right. Not sure what "max out at 2500 rpm at 8,000 ft!" means. I am going to assume you at full wide open throttle (WOT)? I am also assuming you are leaned to best power or about 150F rich of peak (ROP). If this is the case you have a cruise prop with way too much pitch. Your rate of climb (ROC) must also be poor? At solo weight you should see at least 1,700-1,800 fpm at sea level.

The prop is the main suspect but your 8,000 WOT cruise TAS should be 199 MPH (75% power). At 55% you should see 178MPH. Your 161 MPH is way off the pace. WHAT IS YOUR manifold pressure (MAP or MP)?

Compression check and oil analysis should be in order also.

After you check your speed again with GPS and get the engine prop sorted out, there are all kind of details that cost speed. I don't know what the fit and finish is, the kind of wheel pants and gear leg fairings you have.

Good Luck George.

Kyle Boatright said:

Well, your static RPM is 250 rpm higher than mine. I guess you're right about having a climb prop. Can you borrow a MP gauge? Combine MP and RPM and you'd have a good idea of your true power setting. It may be that your RPM is fairly high, but your MP is low, giving you far less than 75% at your cruise RPM. The obvious correction would be to switch to more of a cruise prop.

By the way, who made your prop?

KB

Click to expand...

My prop is a metal Sensenich. If I go to a cruise prop won't I loose to much rpm and power? By the way, there is a MP gauge in the plane, I don't pay attention to it because it's a FP prop. What sort of MP should I get at 2300-2500 rpm at cruise?

casper said:

My 6A with the 150 combo would do 177 MPH averaged GPS
Doyle Reed

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Then there must be something wrong with my plane.

gmcjetpilot said:

So you are getting 140kts or a bout 161 MPH ground speed. I guess it has to me said you have to average 4 runs approx 90 degrees apart (only accurate within 1-3%). The best was is 3 legs approx 120 degrees apart. Note TRACK and SPEED. Using a little spread sheet program, you resolve the ground speed.

YOu can get the spread sheet at Kevin Horton's RV-8 Project:

Kevins Flight test page - excel spread sheet for GPS to grond speed calculation

Click to expand...

I've seen these speeds repeatedly, so I believe that's what I'm getting

Nawwww

akorchak said:

Then there must be something wrong with my plane.

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Fly again and get some MAP and RPM numbers and try the three leg method Kevin suggest above in the link I posted. Also your static RPM is good I think, but your max RPM in cruise is low. DO NOT THROTTLE BACK. Climb to 8,000, leave the throttle WOT, lean to 150F ROP or if you don't have a CHT gauge, lean to rough than push mixture in a 1/2 to 1 inch. Let RPM run up to where it will run up to. DO NOT THROTTLE BACK. This is a RV not a C-172. RPM over 2,500 or even a little over 2,700 is OK.

You mention MAP is not important to a FP. It is and the RPM/FP is the only way to tell what power you have. This applies to any engine. However in GA planes with FP they leave the MP gauge off and make charts that show approx power with RPM only, but this is a estimate not exact.

George

OK so the speed it correct and checked. DO some more flights and check power and max RPM with WOT. You will not hurt the engine unless it is way over say 2,800. THe Sensenich is the best FP you can get for performance. The good news is you can get it repitched but don't do that until you het all the fairings updated if you need to.

gmcjetpilot said:

Fly again and get some MAP and RPM numbers and try the three leg method Kevin suggest above in the link I posted. Also your static RPM is good I think, but your max RPM in cruise is low. DO NOT THROTTLE BACK. Climb to 8,000, leave the throttle WOT, lean to 150F ROP or if you don't have a CHT gauge, lean to rough than push mixture in a 1/2 to 1 inch. Let RPM run up to where it will run up to. DO NOT THROTTLE BACK. This is a RV not a C-172. RPM over 2,500 or even a little over 2,700 is OK.

You mention MAP is not important to a FP. It is and the RPM/FP is the only way to tell what power you have. This applies to any engine. However in GA planes with FP they leave the MP gauge off and make charts that show approx power with RPM only, but this is a estimate not exact.

George

OK so the speed it correct and checked. DO some more flights and check power and max RPM with WOT. You will not hurt the engine unless it is way over say 2,800. THe Sensenich is the best FP you can get for performance. The good news is you can get it repitched but don't do that until you het all the fairings updated if you need to.

Click to expand...

I'll check the MP, but I CANNOT get more than 2500 unless I go into a nose dive! There is nothing left. Ed at Sensenich sais this is impossible, but I had the tach checked. The only known cause that I've heard of is bad timing and I've had that checked too. This 160 hp engine should be able to turn an 81" prop. Could it be wind resitance due to bad rigging or horizontal incidence. Could it be poor fuel flow?

Whats your oil consumption like?

rpm limit with metal Sensenich and O-320

Careful, the metal Sensenich on the O-320 is limited by the manufacturer to 2600 rpm due to issues with potentially destructive harmonics.

Sam Buchanan

Fly again and get some MAP and RPM numbers and try the three leg method Kevin suggest above in the link I posted. Also your static RPM is good I think, but your max RPM in cruise is low. DO NOT THROTTLE BACK. Climb to 8,000, leave the throttle WOT, lean to 150F ROP or if you don't have a CHT gauge, lean to rough than push mixture in a 1/2 to 1 inch. Let RPM run up to where it will run up to. DO NOT THROTTLE BACK. This is a RV not a C-172. RPM over 2,500 or even a little over 2,700 is OK.

You mention MAP is not important to a FP. It is and the RPM/FP is the only way to tell what power you have. This applies to any engine. However in GA planes with FP they leave the MP gauge off and make charts that show approx power with RPM only, but this is a estimate not exact.

George

OK so the speed it correct and checked. DO some more flights and check power and max RPM with WOT. You will not hurt the engine unless it is way over say 2,800. THe Sensenich is the best FP you can get for performance. The good news is you can get it repitched but don't do that until you het all the fairings updated if you need to.

Click to expand...

Limitations

Sam Buchanan said:

Careful, the metal Sensenich on the O-320 is limited by the manufacturer to 2600 rpm due to issues with potentially destructive harmonics.

Sam Buchanan

Click to expand...

Thanks, good point Sam. I was aware there was a max rpm limit but forgot. So as always observe all limitations. G

VAN's provides the following table for prop selection, specifically for Sensenich props:
............Aircraft...........Horsepower.....Required Pitch


RV-3.....................150/160..............(81)
RV-4.......................150...................(79)
RV-4.......................160...................(81)
RV-6/7/8..................150..................(78)
RV-6/7/8..................160..................(80)
RV-6A/7A/8A/9A......150..................(77)
RV-6A/7A/8A/9A......160..................(79)
RV-4.......................180..................(85)
RV-6/7/8..................180..................(85)
RV-6A/7A/8A...........180..................(85)

You say you have a 78" pitch, which may not be the optimum selection for your engine/plane combo, according to VAN's.
I had the 150 HP RV6 with the 78" pitch and my top speed WOT was about 165 knots with all of the fairings tight and the newer style wheel pants. (They do make a difference. I know, because I switched from the older style after about 500 hrs on the -6.)
One thing that happened to me, I had a stuck exhaust valve and didn't know it. I did notice a drop in performance but the engine seemed to run fine, etc. When I finally figured it out and fixed the bad cylinder, my performance went back up to normal.
My point is, it could be any one of a number of things, or a combination of them, that is stealing your power. I agree with all of the previous posts in that any one or several of them could be a problem.
Start with the engine. Check out everything you can: clean air filter, good plugs, timing, harnesses, compression in all cylinders, verify fuel flow...check that the fuel pump is pumping properly, verify mixture and throttle controls are all working to their limits, if you have a primer system, make sure it isn't leaking, look for induction leaks, etc.
Then, start looking at the fairings...are they all snug and tight or do they have gaps and open areas that create drag? Make sure the intersection fairings at the gear legs and fuselage are staying tightly up agasinst the fuselage. I have seen them open up in flight, but close back up on the ground. The RVs are fast anyway, but fit and finish does make a difference.
Try to verify that all of your instrument readings are accurate. Looks like you are already doing that. One easy check, in doing a series of stalls, see ifyour indicated airspeed is around 55 MPH...if so, that's at least one point of reference for the IAS; however, that doesn't necessarily mean it is correct at other higher speeds. You can check out you pitot and static system to make sure they are open and don't have leaks, Verify that the pitot tube is straight into the relative wind. Make sure the static ports are not clogged.
So, there are a lot of things you can do to check and tweak your plane before you go to the extreme of trying a new prop. If possible, borrow one from someone with an RV who is willing to help. You might even try the prop manufacturers. Sometines they will loan you one to try, if they have one available.
Good luck...I think you will eventually get it all straightened out and have a good time doing it, even though it is probably a little frustrating for you now. In the end, you will learn a great deal about your plane that you probably didn't know before.

Andrew,

FWIW your email doesn't work. My 150hp 6A with a 77" prop (the same one Casper is talking about) would cruise all day at 8000' WOT at aound 150kt GPS speed, usually slightly below, say 165 to 170 mph (never did scientifically measure it). The rpm would be around 2500, usually slightly below, but the engine did have 2500 hours on it.

It now has 160hp engine, and a 79" prop, cruise rpm is at least 2550 with not much leaning (CHTs getting too hot), and goes over 2600 with a very small nose down attitude (= you have to be careful not to over rev the prop). Cruise is now generally over 150kt, say 175 to 180mph. The otehr day at 9500' I was getting 140 KIAS, 160 KTAS (can't remember how that was calculated, might have been GPS ground speed so not really KTAS). MP is always dependent on altitude, in above example it was 21.4". At 6500' 2450 rpm it was 22.0" (that gave 140 KIAS also).

I would try to get an optical tachometer to check your tach airborne, a 50 rpm error would be significant. If that doesn't help maybe a top overhaul or new cylinders are in order? I would not put much more pitch on the prop as your climb rate will really suffer.

Pete

penguin said:

Andrew,

FWIW your email doesn't work. My 150hp 6A with a 77" prop (the same one Casper is talking about) would cruise all day at 8000' WOT at aound 150kt GPS speed, usually slightly below, say 165 to 170 mph (never did scientifically measure it). The rpm would be around 2500, usually slightly below, but the engine did have 2500 hours on it.

It now has 160hp engine, and a 79" prop, cruise rpm is at least 2550 with not much leaning (CHTs getting too hot), and goes over 2600 with a very small nose down attitude (= you have to be careful not to over rev the prop). Cruise is now generally over 150kt, say 175 to 180mph. The otehr day at 9500' I was getting 140 KIAS, 160 KTAS (can't remember how that was calculated, might have been GPS ground speed so not really KTAS). MP is always dependent on altitude, in above example it was 21.4". At 6500' 2450 rpm it was 22.0" (that gave 140 KIAS also).

I would try to get an optical tachometer to check your tach airborne, a 50 rpm error would be significant. If that doesn't help maybe a top overhaul or new cylinders are in order? I would not put much more pitch on the prop as your climb rate will really suffer.
Pete

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Well, thanks to everyone for the helpful advice, I'm going out flying now and I'll get more data.