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wrongway john

Well Known Member
I've seen enough feedback from RVer's to know most do meet or come very close to meeting Van's numbers. While looking at various planes, one came in at about 20-25 mph under advertised cruise speed, and lower for top speed. I suspected a climb prop in place of a cruise, but this particular owner said it has a cruise prop on it. I?d like to get the length and pitch of the prop, to double check, but he doesn?t know the numbers, and I?ll have to personally make a trip to get the particulars.

I can?t imagine a plane being so out of rig that would cause it. He says engine is in mid-seventies on compression. It?s an RV-4 with a 150 hp, and Van?s site says 189 mph cruise solo @ 8,000 feet with that particular set-up. This one he says is 165-170 mph cruise. What else should I check?
 
Off the top of my head

Accurate tachometer?

Accurate airspeed indicator?

True airspeed versus indicated?

mcb
 
I believe Van's specs airspeed numbers are in TAS.
IAS airspeeds will of course show much less when you're at altitude.

Wheelpants and wheelpant alignment can have a significant affect of airspeed in an RV, so we've found out ;)

Of course, so can a fixed pitch "climb" prop zap your top end cruise speed.

A 150hp O-320 can run smooth as silk, show great compression readings, and put out a lot less than 150hp if the cam lobes are worn down. Even make good static RPM (I've seen this happen in a friend's Cherokee 140 before he overhauled his engine). But of course it will only run that way for a little while longer ;)

EDIT: forgot to mention, even something simple as incorrect spark timing can rob a fair bit of power from an otherwise healthy Lyc, and that's easy to check... and remedy.
 
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He has gps, so I suppose he's able to get some fairly accurate numbers, and wouldn't be relying on indicated, but I wondered the same thing.

Never thought of a bad cam. Engine has less than 700 hours since new; not overhauled. Would an A&P be able to measure cam lobes with dial calipers by removing the valve covers?

Assuming it turned out to be a bad cam. Would you or anyone know what costs I could expect to have someone do this?
 
You surely wouldn't expect a 700 hour engine to have the cam lobes worn... unless those hours were spread over like 20 years and the engine sat unflown for a long time and the cam/followers rusted, or something like that, but yeah, that's how you'd normally verify the cam's lift -- with a dial indicator on the rockers.
 
wrongway john said:
He has gps, so I suppose he's able to get some fairly accurate numbers, and wouldn't be relying on indicated, but I wondered the same thing.
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Never assume. The data strongly suggests a measurement error. Has he tried comparing against another RV, side by side?
 
Most pilots don't know what 75% power means.

With a FP prop, go to 8000' DA, push the black knob all the way to the stop and then lean it for best power.

With the correct pitch, the RPMs should settle in around redline.

Toss in that some engine FP prop combos can't run at 2700 RPM and you are giving up some speed.

With my O-360 powered RV-9, I like to save $$$ and still cruise at 175 MPH by dialing back the power a good bit.

With the old O-290, I would cruise it all day at 140 kts / 165 mph while burning around 7 GPH.

In other words, the planes are probably fine.
 
N941WR said:
Most pilots don't know what 75% power means.

With a FP prop, go to 8000' DA, push the black knob all the way to the stop and then lean it for best power.
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I hope you?re right. I had heard that's how that's how Van's did it in quite a few threads where this has been discussed.

Sprucemoose, I called and asked him, and he said he?s never had another 4 up beside it.

He?s owned it since mid-nineties; he's the original builder and owner. I think he would be more than happy to tell me about the numbers being higher. I suppose they are about where he says they are after having it this many years. He says he doesn?t have any trouble pegging out the roc well past 2,000 fpm though. I?m going to take a road trip, and take a good look at that prop since he?s not been able to tell me a thing about it including the manufacturer other than it's wood. I?ve read some amazing speed increases by just getting the right prop matched up to the engine. I?m crossing my fingers that is all I need to do, but again, I just can?t help but feel like he knew what kind of prop to put on it, being an A&P and all.

If I like it, I?ll give smokeyray a call, and have him give me a pre-buy inspection. I bet he?ll get to the bottom of it. I?ll let you know what I/we uncover.

Vanheads, thanks again for all of the help. Need all I can get.
 
Are you sure you two guys are talking about the same measure of speed?

The difference in these speed numbers are exactly knots x 1.13 = mph.

Maybe your talking mph and he's talking knots?:confused:
 
update

Left early this morning, and been on the road nearly all day. Just got back.

Rhino, I had actually double-checked that, we both were using mph. Woodman, thanks for the speed mods, I hope this one wouldn?t require me having to redo the HS pitch. That?s a bit beyond my scope at this time. For now, wouldn?t even know how to check for that.

Anyway, he was a lot more informative, and had all the paper work on everything including the prop. Really nice guy, and one of the few that doesn?t mind pointing out the bad along with the good, even certain things that I wouldn?t know about until I had flown it a bit. E.g., he mentioned this one also burns more gas than most, up to about 9 gallons an hour, and I believe that was at 160 indicated in mph if I understood correctly. He?s rebuilt the carb twice, but that didn?t solve the high fuel consumption.

The prop papers show it to be a Aymar-Demuth prop with 68? length and 71? pitch. He does have a manifold gauge in it, and was familiar with the 24/48 rule that you guys talk about, so he?s probably got the mph down right.

Some time ago, when I researched some threads on Smokeyray?s props, he went through seven on his old 150 HP RV-4, and he thought the best numbers to go with for this engine set-up was 69? length and 72? pitch. Some props made quite a bit difference in speed gains. Don?t know how much this was a difference to pitch and length, or just different manufacturers. Anyway, it looks like his prop is close enough. Even so, does anybody think switching to a Hertzel, Cattoo, or MT prop might still make that much of a speed difference, particularly if I only changed the length and pitch slightly?

I think I can see some of the areas that are costing him some mph as well as maybe burning more gas. Some are just simple fixes. Like prop spinner isn?t flush around the prop, probably about an inch and a half gap all away around. Also maybe another three-quarter of an inch gap between the back of the prop spinner and cowling. Many years ago, he had a different prop on it, and just used the same prop spinner. The cowl is kind of beat up, but it seems like that is the case with just about every old RV I see. I?m not sure what I would do with that.

The fiberglass fairings where the legs meet up at the fuselage weren?t very flush, in particular the one on the left side which had a huge gap. But again, not really a big deal at all, and seems like an easy fix for that. The fiberglass tail fairings weren?t bad at all.

One thing that concerns me a bit, and might not be so easy a fix, but I didn?t like the big rudder trim. I guess it to be about six inches long, sticking out another two inches or so, with a fairly good bend in it. Seems like I read a thread where somebody undertook to correct theirs but haven?t been able to find that particular thread again. Others were also giving him numbers on how many degrees he would need to correct it. I?ll keep looking for it.

Overall, it?s not a bad looking plane at all, and the metal and paint looked good. It?s also a very lightly built plane which I liked. Part of me wants to get this plane, just for the educational aspects, and to document each change to see what mods would do for the speed. Watch me do all of the ones I suggested, and the speed still be the same. :)

I think this plane might work for me. It was actually the first time I even sat in a 4. Man, I could have stayed in there all day. Before I make up my mind on it, I'll get Smokeyray or find a very good A&P to give it a good look over.
 
Get a good pre-buy

Have an experienced RV'er/AP do a thorough pre buy for you. It's well worth what it may cost. I spent many hours cleaning up my first RV6 (the one the article is about) but the efforts were worth it. If you are running with a FP prop at around 2400+ rpm, 9 gals. an hour isn't too unusual. With our "A" we run about 2350-2400 and average 7.5-8 gph, 150KPH. Its a really clean plane with a Sensenich metal FP prop.
Woodman
 
You're going in the right direction.

John, just because a prop is stamped 68 X 72" or whatever, says nothing about that prop's efficiency. The typical wood prop has very thick shanks near the spinner, whereas Craig Catto's prop are composite, glass/carbon fiber over wood and a very efficient design.

A good friend of mine picked up over 5 MPH by going from a pure wood prop to a Catto in his -4, also with a 160 Lyc.

Paul Lips (sp?) on here has the strangest-looking but fastest composite props to be found! Blade design makes all the difference in the world....something that not too many run of the mill A@P's know much about.

Best,
 
wrongway john said:
I've seen enough feedback from RVer's to know most do meet or come very close to meeting Van's numbers. While looking at various planes, one came in at about 20-25 mph under advertised cruise speed, and lower for top speed. I suspected a climb prop in place of a cruise, but this particular owner said it has a cruise prop on it. I?d like to get the length and pitch of the prop, to double check, but he doesn?t know the numbers, and I?ll have to personally make a trip to get the particulars.

I can?t imagine a plane being so out of rig that would cause it. He says engine is in mid-seventies on compression. It?s an RV-4 with a 150 hp, and Van?s site says 189 mph cruise solo @ 8,000 feet with that particular set-up. This one he says is 165-170 mph cruise. What else should I check?
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I'd be very interested in the engine. To be that far out on speed indicates a lot of HP is missing.

Been there done that with the Subby H6 vrs IO360. For 5 years I was convinced that I had built a dog of an airplane and felt awful about it, but the engine swap proved otherwise.

While my airplane is no princess beautiful, it does go as fast or just a bit faster than Van's numbers with an engine that is up to snuff.

It's all about HP. Speed mod tweaking will net a few more knots but the starting point has to be a good engine.
 
Pierre,
That's not a fair statement about wood propellers. There are many wood props on RVs that give Vans advertized performance numbers. And, I know of other RVs that have switched from Catto to a Wood prop and gained speed.It's not always what the prop is made of, it's the mission the propeller was made for. You are correct in saying "Blade design makes all the difference in the world".


pierre smith said:
John, just because a prop is stamped 68 X 72" or whatever, says nothing about that prop's efficiency. The typical wood prop has very thick shanks near the spinner, whereas Craig Catto's prop are composite, glass/carbon fiber over wood and a very efficient design.

A good friend of mine picked up over 5 MPH by going from a pure wood prop to a Catto in his -4, also with a 160 Lyc.

Paul Lips (sp?) on here has the strangest-looking but fastest composite props to be found! Blade design makes all the difference in the world....something that not too many run of the mill A@P's know much about.

Best,
Click to expand...
 
I apologize Frank, if your props are faster.

However, after 5 years and over 500 hours in my -6A, I've yet to see a wood prop beat or equal my -6A's performance in cruise.

Show me some numbers for your props compared to a Catto and I'll become a believer. Use an O-360 Lyc with a cruise-optimized prop and turn it 2750 RPM at 8,000' and go at least 201 MPH TAS, as we did.

Thanks,
 
WW John - The propeller was not the thing that jumped out at me. I run an AD 68x72 on a 150hp O320 (on an RV-8) and use the 24/48 rule. So, I think there are enough similarities to comment. ...

I flight plan at 160kts at 7500 and above and burn about 8gph (maybe a little less but I plan for 8).

The thing that jumped out for me in your description was the rudder trim. Could the need for a lot of rudder trim indicate alignment issues elsewhere and the combination of those things cause a lot of drag? Enough to count for the performance reduction? <- this is really a question to the VAF community that knows about these things
 
John,

Your statement about the owner using GPS to infer that his numbers are accurate really doesn't change the question about accuracy. Remember, we're discussing indicated airspeed and true airspeed difference. GPS will only give you groundspeed. What matters is if the owner has done a good series of calibration checks while airborne (triangle or 4-way pattern) to see how his pitot/static and indicator systems true out. Might be good indication, but the question isn't eliminated by using a GPS.
 
I?m the one kind of assuming (maybe wrongly) that he used his gps through some triangular or square courses to help somehow extrapolate and do whatever he needed to do to get his numbers.

Concerning the rudder tab, it seems a little big, but at the same time, I remember quite a few really nice RV?s at Oshkosh that also had tabs about that size also.

Also, he did check his timing on the engine, I'm sure he has that set right.

Overall, the plane looks nice considering its age, and he did a great job with the rivets and sheet metal and all. I don?t know how to check for a heavy wing or alignment problems if that is the case. All of you guys have given me plenty to think about.

I?m still thinking about this plane, but it may take someone that has built a plane before that would probably have a better plan than me to getting it close to Van?s numbers. I don't have an active EAA chapter in my area either, so I'd be pretty much on my own on trying to figure it out. Oh, and I would be bugging you guys here on VAF even more. :D
 
John,
It could be a combination of a few things. I think the trim tab( as stated earlier) is a clue. On my 6 the tab is 5" long but only deflects 1/4 ". If the prop is a climb prop you should be able to over rev down low. Manifold pressure will tell alot.

Pierre,
My statement was in general for all wood propellers. Please don't think I'm boasting about mine. I have two friends with -4s one with Catto the other with Performance Propeller. The Catto out climbs the other but the Performance out runs the Catto once their done climbing. It's all in the mission the propeller was designed for. Also, one customer with a -6 O320 Performance Prop WOT(2850) at 3500' 218TAS.
 
One thing that concerns me a bit, and might not be so easy a fix, but I didn’t like the big rudder trim. I guess it to be about six inches long, sticking out another two inches or so, with a fairly good bend in it. Seems like I read a thread where somebody undertook to correct theirs but haven’t been able to find that particular thread again. Others were also giving him numbers on how many degrees he would need to correct it. I’ll keep looking for it.
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Just so you know, the 6 was designed with a straight vertical stab, while later models have a built in angle. That's one reason why you see tabs or not. The whole idea is to keep the ball centered at cruise speeds. Since speeds will vary, the tab won't be perfect. But without it, it takes a lot of foot pressure on the rudder. I'd say that mine is bent about 1/4" like that of another reply. 3/8" at most............but I'd have to measure.

L.Adamson -- RV6A
 
internal timming?

John,
20 to 25 mph is alot! It would take several things to slow down the airplane that much. I have a suggestion based on personal experience that is quick easy and inexpensive to check. I had a freind several years ago with an RV8 that had the same problem. He tried everything and I mean everything including trying a different CS prop. I considered myself fairly knowlegable spent alot of time with he and his airplane and could find nothing. The airplane was straight, clean and there was no reason why it was not running up to speed. I had pretty much run out of ideas and he called me and said he found a tip from lycoming about checking the internal timming of the engine. Let me say right now for the naysayers that the internal timming can be off and the mags will time properly and the engine will run smoothly. I followed the tip and we found that the cam gear was off by one tooth. I never would have believed that an engine could run that smooth if it was off. We moved the gear to the correct possition and the problem was solved.
Now the thing that would make me think this shouldnt be the case is that you said it was 700hrs since new. Its hard to believe the factory would put it together wrong, but hey we all make mistakes. The extra fuel burn could be another clue that this could be the case. Try this check and you will know for sure.
1 Remove valve cover on #2 cylinder
2 Remove a plug from #1 cylinder
3 Bring #1 cylinder to TDC (piston to the very top)
4 Move the prop 20 deg left of center then right of center the intake and exhaust rockers should both move an equal amount. One with LH movment and the other with RH movement. If one of them is not moving within that 20 deg left and right of center your internal timming is off and you have found your problem.
Good luck,
Ryan
 
L.Adamson said:
Just so you know, the 6 was designed with a straight vertical stab, while later models have a built in angle. That's one reason why you see tabs or not.
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Didn?t know that. Sure would be good to know when looking at 6?s why the difference.

131RB said:
The airplane was straight, clean and there was no reason why it was not running up to speed. I had pretty much run out of ideas and he called me and said he found a tip from lycoming about checking the internal timming of the engine. Let me say right now for the naysayers that the internal timming can be off and the mags will time properly and the engine will run smoothly. I followed the tip and we found that the cam gear was off by one tooth. I never would have believed that an engine could run that smooth if it was off. We moved the gear to the correct possition and the problem was solved.
Now the thing that would make me think this shouldnt be the case is that you said it was 700hrs since new. Its hard to believe the factory would put it together wrong, but hey we all make mistakes.
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Ryan, that is interesting what you have to say about internal timing, and I can only imagine the joy and smiles it brought to all of your faces when that discovery was made. He is the one that pulled it from the Grumman Cheetah, and is the original owner of the 4, so I?m sure he hasn?t messed with that. As you?ve said, it?s not likely the factory made this mistake, but it would eliminate another possibility completely by checking. Thanks for sharing.
 
L.Adamson said:
Just so you know, the 6 was designed with a straight vertical stab, while later models have a built in angle. That's one reason why you see tabs or not. The whole idea is to keep the ball centered at cruise speeds. Since speeds will vary, the tab won't be perfect. But without it, it takes a lot of foot pressure on the rudder. I'd say that mine is bent about 1/4" like that of another reply. 3/8" at most............but I'd have to measure.

L.Adamson -- RV6A
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Meaning leading edge is offset from centerline.
 
Neal@F14 said:
A 150hp O-320 can run smooth as silk, show great compression readings, and put out a lot less than 150hp if the cam lobes are worn down. Even make good static RPM .
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Test the compression.... Use a automotive compression tester and follow the instructions. A bad cam will show as low (10% or more difference) between cylinders.

Do a differential pressure test first to see if the cyl will hold pressure. Then a compression test to see the volume it can hold.
 
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