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Why is my RV so slow?

D

dbroo052

Member
I'm hoping some of you experts can help me identify if there is some issue with my plane causing me to lose an excessive amount of knots.

One of the reasons I purchased a Vans RV-6 is the speed that these birds cruise at. For some reason, my plane doesn't seem to be getting anywhere near these speeds. From what I have researched, I should easily be able to hit 160-170 knots. I think I did, once, in a dive, with full power...

Here is my current setup:

RV-6
Lycoming O-320 E2A (160 h.p.) ~1060 SMOH & ~50 IRAN.
Sensenich Fixed Pitch (set at 76")
All fairings installed.

Cruising at 10,500 at 2500 RPM I was seeing 150 TAS, 125 IAS, 144 GS.
Cruising at 10,500 at 2300 RPM I was seeing 134TAS, 112 IAS, 129 GS.

From what I have seen, 76" pitch on the prop is fairly low. I think I have seen most people running around 80-82". I was thinking about putting the twist up to 78" and testing the results, and maybe finally going up to 80 if need be. I am more than happy with my climb rates and would gladly trade some climb for speed.

Looking at the lycoming manual, I need to have a static RPM of at lease 2300 RPM WOT on the ground. As my plane is configured right now, I am just under that (around 2960 rpm WOT). Although I don't think there is much danger in adding 2" of pitch since my climb rates are good and I don't run the risk of sucking up all of the runway before hitting rotation speeds, I still have some hesitations.

Does anyone have any ideas on where some extra knots could be hiding?

Anyone with a similar config care to share your cruise & power settings for comparison?

Thanks all!

(Hoping I can get this figured out so I can chop some time off my flight to Oshkosh from San Diego).
 
Slow cruise

dbroo052 said:
RV-6
Lycoming O-320 E2A (160 h.p.) ~1060 SMOH & ~50 IRAN.
Sensenich Fixed Pitch (set at 76")
All fairings installed.

Cruising at 10,500 at 2500 RPM I was seeing 150 TAS, 125 IAS, 144 GS.
Cruising at 10,500 at 2300 RPM I was seeing 134TAS, 112 IAS, 129 GS.
Click to expand...

Several questions: what is the length of your prop?
what was your manifold pressure at those speeds/settings?
 
Which wheel and gear fairings do you have, old-school or pressure recovery?

76" is definitely a climb prop, 80" is suitable for a 160hp RV-6 with good fairings. To get max cruise speed you need to bump rpm up to 2600. Don't bother giving us ground speed, it is irrelevant. :)

170 kts may be a bit optimistic for 160hp.
 
dbroo052 said:
I'm hoping some of you experts can help me identify if there is some issue with my plane causing me to lose an excessive amount of knots.

One of the reasons I purchased a Vans RV-6 is the speed that these birds cruise at. For some reason, my plane doesn't seem to be getting anywhere near these speeds. From what I have researched, I should easily be able to hit 160-170 knots. I think I did, once, in a dive, with full power...

Here is my current setup:

RV-6
Lycoming O-320 E2A (160 h.p.) ~1060 SMOH & ~50 IRAN.
Sensenich Fixed Pitch (set at 76")
All fairings installed.

Cruising at 10,500 at 2500 RPM I was seeing 150 TAS, 125 IAS, 144 GS.
Cruising at 10,500 at 2300 RPM I was seeing 134TAS, 112 IAS, 129 GS.

From what I have seen, 76" pitch on the prop is fairly low. I think I have seen most people running around 80-82". I was thinking about putting the twist up to 78" and testing the results, and maybe finally going up to 80 if need be. I am more than happy with my climb rates and would gladly trade some climb for speed.

Looking at the lycoming manual, I need to have a static RPM of at lease 2300 RPM WOT on the ground. As my plane is configured right now, I am just under that (around 2960 rpm WOT). Although I don't think there is much danger in adding 2" of pitch since my climb rates are good and I don't run the risk of sucking up all of the runway before hitting rotation speeds, I still have some hesitations.

Does anyone have any ideas on where some extra knots could be hiding?

Anyone with a similar config care to share your cruise & power settings for comparison?

Thanks all!

(Hoping I can get this figured out so I can chop some time off my flight to Oshkosh from San Diego).
Click to expand...

Is this figure correct?? then, you are way under pitched. If you are running a metal prop, it should be a 80" pitch for your installation. Van's doesn't sell a 76" pitch metal prop.... so I guess you have the ground adjustable.

A simple way to test for max performance from your prop is to climb to 8K DA and set rpm to max for prop (metal is 2600) then add first 2 digits of rpm to MP. Your motor should produce 75% at that altitude. And the total of rpm and MP at 75% would be 48. A higher number means you are under pitched. A lower number and you are over pitched. Be sure to LEAN OUT at that altitude for your best performance and wait for speed to build.
 
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Some quick rough calcs show that your performance is about right for a 76" pitch, which means you are not using all of your available power, if your engine is running well. 80" should give you what you are looking for.

Tim
 
gasman said:
Is this figure correct?? then, you are way under pitched. If you are running a metal prop, it should be a 80" pitch for your installation. Van's doesn't sell a 76" pitch metal prop.... so I guess you have the ground adjustable.

A simple way to test for max performance from your prop is to climb to 8K DA and set rpm to max for prop (metal is 2600) then add first 2 digits of rpm to MP. Your motor should produce 75% at that altitude. And the total of rpm and MP at 75% would be 48. A higher number means you are under pitched. A lower number and you are over pitched. Be sure to LEAN OUT at that altitude for your best performance and wait for speed to build.
Click to expand...

Ooops! No, that was my dyslexia kicking in. I meant 2260.
 
tjo said:
Some quick rough calcs show that your performance is about right for a 76" pitch, which means you are not using all of your available power, if your engine is running well. 80" should give you what you are looking for.

Tim
Click to expand...

Thanks for the feedback.
 
Performance numbers form from Van's, but note this was done with a Hartzell 2-blade C/S prop:


dbroo052 said:
Obviously this is worth trying but I don't expect that there are any errors. I just installed Garmin G3X system and everything seems to be in good working order.
Click to expand...

The first thing you need to do is the 3 or 4-leg GPS runs to verify that the TAS numbers you are seeing on your new G3X are accurate. The 3 or 4-leg GPS runs validate that your pitot-static system is giving you accurate IAS, and thus accurate calculated TAS numbers from the G3X system.
 
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In the past, other new RV owners have made similar requests and have learned that they have an intact, non-leaking, functional but somehow aerodynamically inaccurate static source. These tests will detect that if it applies to you.

In any case, it's a fun way to rule out one of the possible error sources.

Dave
 
David Paule said:
In the past, other new RV owners have made similar requests and have learned that they have an intact, non-leaking, functional but somehow aerodynamically inaccurate static source. These tests will detect that if it applies to you.

In any case, it's a fun way to rule out one of the possible error sources.

Dave
Click to expand...

+1. In your post #1 data you show gps lower than true airspeed. Of course it cannot always be that way: half of your flights (tailwind vs headwind) it should be the other way around. Start by verifying your TAS is accurate. As noted above, the most common problems are usually related to static port shape and placement.
 
dbroo052 said:
Obviously this is worth trying but I don't expect that there are any errors. I just installed Garmin G3X system and everything seems to be in good working order.
Click to expand...

Pitot & Static issues will affect any indicator regardless of how new, expensive, or precise.

Best speed is at around 7500' Density Altitude, wide open throttle. If you can't get wide open throttle without running over redline, then that's an under pitched prop.

Best speed is where the engine makes best power. And that isn't at 2400 rpm.
 
I also suspect you are under pitched. Consider a Sensenich ground adjustable fixed pitch carbon fiber prop. These are less expensive new than the metal fixed pitch Sensenich props, so you may be able to sell it for not much difference (I did). Then you can set your pitch for your optimum mission. If you want max cruise performance, set it for 2700 RPM at full throttle at low altitude.
 
I wouldn't be surprised if the E2D engine was still a 150hp engine when the RV6 was built and that is why the 76" prop was installed. Later a new set of cylinders would have brought it to 160hp and consequently not enough pitch.

That is exactly the situation I had with my RV-6. I had my prop repitched to 80" after the cylinder change and it performs per specs.
 
dbroo052 said:
Cruising at 10,500 at 2500 RPM I was seeing 150 TAS, 125 IAS, 144 GS.
Cruising at 10,500 at 2300 RPM I was seeing 134TAS, 112 IAS, 129 GS.


.
Click to expand...

WHile I agree with others that many other things should be determined first, You will never get the speeds that others get, as you are not turning 2700 RPM which is where it makes the most power. If the above are in knots, it would seem that 2700 RPM would net you around 160 KTAS at 10K' which is pretty much what should be expected for an RV-6 /320 at that altitiude. At 8-10K, I get approximately 160 KTAS at 2730 RPM and 8.1 GPH (LOP) in my RV-6A with IO-320 and EI. Carbs or mags may reduce the LOP speeds but should not affect the ROP speeds.
 
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The reason an O-320 with a metal Sensenich prop needs 80" of pitch is this prop is limited by the manufacturer to 2600rpm due to issues with harmonics at higher rpm.

However, it is still unclear if the original poster has the metal prop or a composite Sensenich adjustable prop.
 
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lr172 said:
WHile I agree with others that many other things should be determined first, You will never get the speeds that others get, as you are not turning 2700 RPM which is where it makes the most power. If the above are in knots, it would seem that 2700 RPM would net you around 160 KTAS at 10K' which is pretty much what should be expected for an RV-6 /320 at that altitiude. At 8-10K, I get approximately 160 KTAS at 2730 RPM and 8.1 GPH (LOP) in my RV-6A with IO-320 and EI. Carbs or mags may reduce the LOP speeds but should not affect the ROP speeds.
Click to expand...

Thanks for sharing your numbers!
 
I had an E2D that was overhauled with 160hp cylinders in a 6A with a Sensenich metal prop. Initially it was at 76" but was re-pitched to 79" after the high compression cylinders were fitted. I could overspeed the prop at low level but at higher altitudes (8K+) it would turn about 2600rpm and true out at around 170kt.

I would suggest your static system may be inaccurate, possibly because your static ports are the wrong shape. It doesn't matter what kind of instrument is in the panel, if you send an inaccurate static pressure it will never indicate an accurate airspeed. It is not possible to test for this error on the ground as it is due to how the air flows over your static ports. The best static ports are those supplied by Van's in the kits - a dome head pop rivet with the centre knocked out.

The GPS squares method can be used to check the accuracy of your airspeed system - search for National Test Pilots School. If your airspeed indicating system is inaccurate I would look leaks in the pipework first and then at the static ports.
 
penguin said:
I had an E2D that was overhauled with 160hp cylinders in a 6A with a Sensenich metal prop. Initially it was at 76" but was re-pitched to 79" after the high compression cylinders were fitted. I could overspeed the prop at low level but at higher altitudes (8K+) it would turn about 2600rpm and true out at around 170kt.

I would suggest your static system may be inaccurate, possibly because your static ports are the wrong shape. It doesn't matter what kind of instrument is in the panel, if you send an inaccurate static pressure it will never indicate an accurate airspeed. It is not possible to test for this error on the ground as it is due to how the air flows over your static ports. The best static ports are those supplied by Van's in the kits - a dome head pop rivet with the centre knocked out.

The GPS squares method can be used to check the accuracy of your airspeed system - search for National Test Pilots School. If your airspeed indicating system is inaccurate I would look leaks in the pipework first and then at the static ports.
Click to expand...

Yeah, the best system is a pop rivet with the mandrel knocked out and tubing pushed onto it and slathered with sealant; sounds great to me!:rolleyes:
 
Why is your RV6 so slow?

Dave, Your RV6 may not be as "slow" as you think. It appears that your propeller pitch of 76 is not optimized for maximum cruise speed.

My built to the original plans RV6 O-320 160 HP with the 70inch diameter Sensenich metal propeller limited to 2600 RPM at 8,000 ft. achieves TAS of 188mph/163kts. Pitch of this propeller is 79 and full throttle engine leaned to best power it will just slightly exceed 2600RPM at 8000ft. So cruising requires reduced throttle to keep under 2600RPM.

I suggest that for maximum cruise speed at 8000ft+ the O-320 Sensenich metal propeller with 80 or 81inch pitch will let you cruise at altitude with full throttle and less than the 2600RPM limit.

Vans order form currently recommends 80inch pitch for 160HP RV6 but 79 or 81inch pitch are available if you want a little better climb 79 or full throttle cruise with margin below 2600 RPM 81. With fixed pitch compromise is necessary between better take off and climb performance or maximum cruise speeds.

Vans early factory RV6 with O-320 was reported top speed at 8,000 ft. of 195mph/170kts.
 
Any progress?

I am wondering if you were able to achieve the performance you hoped for? Or did you find an error in the system or data?

Interesting thread.
 
Tach accurate?

I know a lot of the older mechanical tach’s were not accurate. I an not sure about newer systems, anyone think his tach may be off a little?
 
ShawnR said:
Any progress?

I am wondering if you were able to achieve the performance you hoped for? Or did you find an error in the system or data?

Interesting thread.
Click to expand...
Sadly yes. While flying out to to Twenty Nine Palms airport to watch the meteor showers and get some awesome pics, I noticed my fuel consumption was ridiculously high (13-14 gph) and my performance above 3000 feet was crap compared to what it normally is. After seeing this issue, I ran a compression test and the results were...well...not good. My #1 cylinder was about 25/80 and I only had about 50/80 on the #4. Other two cylinders were mid 70s. I have my concerns as to what the heck happened. The engine was about 60 hours since IRAN from Western Skyways.

The good news is that I have 4 O-320 jugs sitting around that I can swap. I'll use a new set of rings of course but my question is whether I need new pistons. I will take a look at them once I get the jugs off, I just don't know if I should go ahead an order them now or just use the ones installed.

Overall I want to send the current (crap) cylinders off for overhaul so maybe during my next annual I can do a full top rebuild. Since that is my plan I don't know if it is worth the expense to get new pistons. Hoping I get some much better numbers once I get the new top end bolted on.
 

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Before you yank the jugs, figure out where the leak is going -- into the crankcase or into the exhaust. If it's the exhaust, lap the exhaust valve in place -- lot's of documentation on how to do this. If it's the case, do a ring flush/seafoam treatment -- again, lot's of documentation online here and elsewhere on the procedure.
 
dbroo052 said:
A compression test and the results were...well...not good. My #1 cylinder was about 25/80 and I only had about 50/80 on the #4. Other two cylinders were mid 70s. I have my concerns as to what the heck happened. The engine was about 60 hours since IRAN from Western Skyways.

The good news is that I have 4 O-320 jugs sitting around that I can swap. I'll use a new set of rings of course but my question is whether I need new pistons. I will take a look at them once I get the jugs off, I just don't know if I should go ahead an order them now or just use the ones installed.

Overall I want to send the current (crap) cylinders off for overhaul so maybe during my next annual I can do a full top rebuild. Since that is my plan I don't know if it is worth the expense to get new pistons. Hoping I get some much better numbers once I get the new top end bolted on.
Click to expand...
Is the leaking air during the compression test coming from the exhaust (exhaust valve leak), carburetor (intake valve leak) or the oil fill tube (ring leak)? I'd get the borescope in there before I pulled the cylinders. What do those valves that might be leaking looking like? That may give you more information than the compression test. Are the valves sticking? A wobble test might yield some good information.

Most new cylinders and many overhauled cylinders come with new pistons. If you are going to swap the cylinders out, I would encourage new pistons. Why not? Everything else in there will be 'new' including the rings.....that you should be putting on new pistons. AND you will be making sure the pistons match the cylinders. Ask me about a friend who had 300+ hours on his standard pistons in his ten over cylinders....... :oops: :oops: :oops: IMHO....

GREAT pictures, by the way! How cool is THAT??
 
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bjdecker said:
Before you yank the jugs, figure out where the leak is going -- into the crankcase or into the exhaust. If it's the exhaust, lap the exhaust valve in place -- lot's of documentation on how to do this. If it's the case, do a ring flush/seafoam treatment -- again, lot's of documentation online here and elsewhere on the procedure.
Click to expand...
Unfortunately I wasn't really able to tell where the air was going. The air compressor I was using was left behind from the previous hangar owner. The only thing that seems to leak more than my cylinder was his compressor. I bought a new coupler so I can run the test again and identify where the air is going.

I did pickup a cheap borescope from amazon for about $20 and took a peak inside the cylinder. I am no expert but that doesn't look normal... Not sure if a ring came apart and scared the wall or what. Didn't see anything of concern in the other cylinders at least.
 

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FredMagare said:
New cylinders should come with a new pistons and rings. Have you checked the assemblies?
Click to expand...
Not new cylinder. My uncle who seems to have more airplane parts than he knows what to do with gave me a set of 4 use O-320 jugs with about 250 hours on each. He did also kick in oil rings but wasn't sure where he left the compression rings or pistons. Since I have the jugs, I figured it would be a decent idea to swap all 4 and send off the currently installed ones for repair or overhaul.
 
Mconner7 said:
Tach accurate?

I know a lot of the older mechanical tach’s were not accurate. I an not sure about newer systems, anyone think his tach may be off a little?
Click to expand...
I don't see any reason to think the tach is off. The new electronic systems are pretty accurate as far as I know.
 
David Paule said:
And it needs to be located correctly; the location is shown in the airplane plans.

Dave
Click to expand...
Funny you mention this. The current static ports are the Vans factory part (pop-rivet) located on the fuselage side. I did buy some shiny new ones from Spruce but never got around to installing them. Once I get the new panel installed (had one waterjet cut from Xometry for less than $100 WITH SHIPPING!!!!), tackle the compression issues (already have the new carb installed) I will run the airspeed calibration tests and go from there.
 
dbroo052 said:
I'm hoping some of you experts can help me identify if there is some issue with my plane causing me to lose an excessive amount of knots.

One of the reasons I purchased a Vans RV-6 is the speed that these birds cruise at. For some reason, my plane doesn't seem to be getting anywhere near these speeds. From what I have researched, I should easily be able to hit 160-170 knots. I think I did, once, in a dive, with full power...

Here is my current setup:

RV-6
Lycoming O-320 E2A (160 h.p.) ~1060 SMOH & ~50 IRAN.
Sensenich Fixed Pitch (set at 76")
All fairings installed.

Cruising at 10,500 at 2500 RPM I was seeing 150 TAS, 125 IAS, 144 GS.
Cruising at 10,500 at 2300 RPM I was seeing 134TAS, 112 IAS, 129 GS.

From what I have seen, 76" pitch on the prop is fairly low. I think I have seen most people running around 80-82". I was thinking about putting the twist up to 78" and testing the results, and maybe finally going up to 80 if need be. I am more than happy with my climb rates and would gladly trade some climb for speed.

Looking at the lycoming manual, I need to have a static RPM of at lease 2300 RPM WOT on the ground. As my plane is configured right now, I am just under that (around 2960 rpm WOT). Although I don't think there is much danger in adding 2" of pitch since my climb rates are good and I don't run the risk of sucking up all of the runway before hitting rotation speeds, I still have some hesitations.

Does anyone have any ideas on where some extra knots could be hiding?

Anyone with a similar config care to share your cruise & power settings for comparison?

Thanks all!

(Hoping I can get this figured out so I can chop some time off my flight to Oshkosh from San Diego).
Click to expand...
Sounds like you have more of a climb prop than cruise. I re-pitched a Sensenich from an 85 to 87 and showed over 200mph. But I kept the build lite (987lbs)
 
dbroo052 said:
Not new cylinder. My uncle who seems to have more airplane parts than he knows what to do with gave me a set of 4 use O-320 jugs with about 250 hours on each. He did also kick in oil rings but wasn't sure where he left the compression rings or pistons. Since I have the jugs, I figured it would be a decent idea to swap all 4 and send off the currently installed ones for repair or overhaul.
Click to expand...
Hmmmmmmmm............... :unsure: You have a flying airplane with maybe some funky cylinders, or maybe one funky cylinder. The better compression test listening for where the air goes will be informative. What you are thinking is to take cylinders off a flying airplane and putting used cylinders on that are in unknown condition. With your airplane flying (and going to some cool places like to watch a meteor shower, for example😊) I would consider shipping the Uncle Cylinders off and having them gone over, THEN swapping those reconditioned cylinders for the ones on your airplane now. The cylinder shop will probably return them with new pistons, rings and yellow tagged cylinders you will now know way more about. Then send your current cylinders off for overhaul and have a spare set you know something about. That way maybe you can avoid cylinder musical chairs: cycling cylinders off and on as they start to not do well. IMHO; YMMV
 
Roadjunkie1 said:
Hmmmmmmmm............... :unsure: You have a flying airplane with maybe some funky cylinders, or maybe one funky cylinder. The better compression test listening for where the air goes will be informative. What you are thinking is to take cylinders off a flying airplane and putting used cylinders on that are in unknown condition. With your airplane flying (and going to some cool places like to watch a meteor shower, for example😊) I would consider shipping the Uncle Cylinders off and having them gone over, THEN swapping those reconditioned cylinders for the ones on your airplane now. The cylinder shop will probably return them with new pistons, rings and yellow tagged cylinders you will now know way more about. Then send your current cylinders off for overhaul and have a spare set you know something about. That way maybe you can avoid cylinder musical chairs: cycling cylinders off and on as they start to not do well. IMHO; YMMV
Click to expand...
Did the compression test again today. Sadly no simple valve lapping for me, air was coming out of the case. Guess its time to drive over to the engine shop.
 
dbroo052 said:
Did the compression test again today. Sadly no simple valve lapping for me, air was coming out of the case. Guess its time to drive over to the engine shop.
Click to expand...
If inspection of the cyl bores shows no signs of broken rings (scoring, scrapes, etc) it might be worth the effort to solvent flush the rings ( process discussions elsewhere in the forums).
 
I have not seen any post reporting a GPS box being flown and verified. All I saw was "its all new G3X so it should be right".

PLEASE GO BACK AND READ THAT POST. Doing a GPS box accurately comparing your TAS as shown on the screen compared to the actual TAS calculated by doing a GPS box might yield the real problem. Static error.

Post up a picture of your Static port. If it looks anything like this below that is highly likely your problem.
1710055980270.png

I had the worlds slowest RV10 15 years ago, until Kevin Horton put me onto the GPS box test. Once I fixed the static source, and we are talking an inflight dynamic static, not on the hangar floor, all was good in the world. You want to have no more than 2 knots of TAS error. Also say a 9 knot error could be around a 140' altimetry error.

The static ports supplied by Dynon I have used many times and always test perfectly on any of the RV models I have tested over the years.

As for the leak downs affecting your speed greatly if at all, not likely. Unrelated issue. TCM ran an engine with 0/80 (removed compression ring) and it still made rated power.
 
RV10inOz said:
I have not seen any post reporting a GPS box being flown and verified. All I saw was "its all new G3X so it should be right".

PLEASE GO BACK AND READ THAT POST. Doing a GPS box accurately comparing your TAS as shown on the screen compared to the actual TAS calculated by doing a GPS box might yield the real problem. Static error.

Post up a picture of your Static port. If it looks anything like this below that is highly likely your problem.
View attachment 57984

I had the worlds slowest RV10 15 years ago, until Kevin Horton put me onto the GPS box test. Once I fixed the static source, and we are talking an inflight dynamic static, not on the hangar floor, all was good in the world. You want to have no more than 2 knots of TAS error. Also say a 9 knot error could be around a 140' altimetry error.

The static ports supplied by Dynon I have used many times and always test perfectly on any of the RV models I have tested over the years.

As for the leak downs affecting your speed greatly if at all, not likely. Unrelated issue. TCM ran an engine with 0/80 (removed compression ring) and it still made rated power.
Click to expand...
^^ This ^^

World's slowest RV-7 until I flew the GPS box and found a +7kt error caused by the static port shape.. In my case, the home-machined static port was too thin and had a (relatively) large flat area around the orifice. Changed to Showplanes/MSP-42 shaped port and .032" spacing from fuselage and got the error down to ~2kts.
 
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