pierre smith

pierre smith

Well Known Member
Hi guys,
I have a buddy with a beautiful 7 and a 4 cyl supercharged sube, and a three-bladed electric C/S prop. The airplane has all the fairings, is painted and clean.

The problem is that he can't seem to get more than 162 MPH True out of it, and this at 34" MP (34" not a typo!) and 2500 prop RPM at 6500'. On a trip back to Georgia from Memphis, he only burned 7.2 GPH. I told him that he can absolutely NOT be making more than 140 or so horsepower or he'd be burning more gas. The Subes that had the flyoff at Van's earlier in the year almost matched the Lyc's numbers.

I think there's a problem with the computer retarding the timing causing the power loss but don't know how to troubleshoot it.

What are your opinions? I didn't think that the sube RVs are 30 MPH slower than my 6A! when I can only get 23" at 6500" ?? still do over 195 . :confused:
 
pierre smith said:
Hi guys,
On a trip back to Georgia from Memphis, he only burned 7.2 GPH. I told him that he can absolutely NOT be making more than 140 or so horsepower or he'd be burning more gas. :
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It's even worse than that...he can't be making more than 100 HP. 7.2 GPH is 42 lbs/hr. Say the SFC is 0.42 lbs/hp-hr to keep the math simple. That's 100 HP. Even then, with 100 HP he should be going about 185 MPH (based on Van's numbers, which say that at 75%, an RV-7 goes 207 mph).

So one possible problem: he's not spinning the engine fast enough to the power he wants. What's the gear ratio?

Another is that the prop is just not well-matched to the engine: that would explain why, with ~100 HP, the prop is turning some of it into heat and noise rather than thrust.
 
That is nice of you to check

pierre smith said:
What are your opinions? I didn't think that the subie RVs are 30 MPH slower than my 6A! when I can only get 23" at 6500" ?? still do over 195 . :confused:
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Haaaa, install a Lycoming and Hartzell? :D Sorry I can't help myself, but let me try and help.


Pierre, to be serious and helpful the supercharged Subie should be much faster than that, no doubt. They tend to be slower than a Lyc however, but they will start to gain on the Lyc at higher altitudes, say 12,000'.

Your a good guy to help him out. When you say beautiful RV-7 I take that as good fit, finish and rigging. It is not totally impossible that there is some large airframe drag? I know there was a guy who had a speed problem on the forum recently looking for advice. He found his gear and intersection fairings where gaping in flight, gaping a LOT! After taping them down he picked up 10 mph.

Obviously the 34" and 6500 rpm is impressive, but I am not sure if that is up to snuff. The prop should be up turning a little faster I would think? Being constant speed prop I would think he could "pull" a higher RPM at fine pitch. If he can't he is then down on power. That is my pure guess. Why? Don't know.
 
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The engine could be down on power. Folks who know more than me about Subarus will have to help work on that angle.

The aircraft could have some extra drag hiding somewhere. Take a close look for fairings not close to the airframe, etc.

How has he determined the TAS? If he took IAS and converted to TAS, then there may be large ASI instrument errors, and/or static source position errors. If he used GPS data to derive a TAS, he should be sure to use a known good way to do the calculation. Most of the commonly wrong ways result in a too high calculated TAS though, so this is probably not his problem.

Has he done enough flying to have calibrated his fuel flow indication? The fuel flow could easily be off by a GPH or so. Checking actual fuel used against the fuel used calculated by the fuel flow system will allow the fuel flow indications to be corrected.

Have the tach and MP indications been checked for accuracy?

Which prop does he have? How has this prop performed on other Subaru powered RVs?
 
High revs

Martin, George,
He's turning the engine around 4900 RPMs and as I recall, the gearbox ratio is around 1.8:1. My other buddy, an A&P/IA, flew the airplane to Gastonia, N.C. a couple of days ago and tried 2700 prop RPM and 5000 engine RPM AND 34" and the TAS according to the Dynon, was still 162 MPH! This was the A&P that helped build the airplane...his sixth RV and it is straight and well rigged.
He doesn't have gaping fairings or loose canopy skirts....I'm telling you, it's just slow. I've flown with him and it's as though you're pulling a drogue 'chute behind you. :(
It feels draggy but also down on horsepower in my opinion and the fuel burn rate supports that conclusion.
 
pierre smith said:
Martin, George,
He's turning the engine around 4900 RPMs and as I recall, the gearbox ratio is around 1.8:1. My other buddy, an A&P/IA, flew the airplane to Gastonia, N.C. a couple of days ago and tried 2700 prop RPM and 5000 engine RPM AND 34" and the TAS according to the Dynon, was still 162 MPH! This was the A&P that helped build the airplane...his sixth RV and it is straight and well rigged.
He doesn't have gaping fairings or loose canopy skirts....I'm telling you, it's just slow. I've flown with him and it's as though you're pulling a drogue 'chute behind you. :(
It feels draggy but also down on horsepower in my opinion and the fuel burn rate supports that conclusion.
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Pierre,

What is the fuel pressure in flight?

We have been adjusting fuel pressure to get a reasonable A/F ratio since the ECU tables are very liberal in pouring fuel into the engine. General conclusion at this point is that 28 to 32 psi is working OK.

Also, the ECU monitors air flow. Make sure the intake filter is OK.

My Subby, too, is not going as fast as it should, but probably it is an airframe drag issue as this machine has been rebuilt and I would rather fly with a slightly slower machine than spend a lot of time on consmetics, just me. I'd rather fly any day than not.

The cross country flight plan at 8,500 is 8 gph, TAS 143 knots, prop 2100 normal aspiration.

dd
RV-7A
H6 Subaru
 
You should be able to sort the power vs airframe drag with a climb speed test. Climb speed is more dependent on the power to weight ratio...

What kind of take off distances, and climb speeds are you getting?
 
pierre smith said:
the TAS according to the Dynon, was still 162 MPH!
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Are you absolutely sure that his Dynon is set up for mph and not kt?

Has he done any testing to check the accuracy of the airspeed system? He could easily have a 10 kt error in the Dynon due to static system position error + instrument error.

Does the indicated OAT on the Dynon look reasonable? An OAT error would affect the conversion of IAS to TAS.
 
Pierre,

Surely your friend knows Robert Paisley. If he doesn't, he should -- and he can get his contact info from Jan. Robert had a 4-cyl supercharged Subaru on his RV-7 originally, and out of the box it was pretty darn doggy. Robert has made huge strides to improve performance of that setup and others. I don't know the details, but obviously Robert does. Get your friend in contact with him.
 
Yes, their slow

We have a 7a here in Denver that is subiecharged and gets about the same low performance numbers your friend is seeing. He thinks cooling drag may be an issue but me thinks low HP is the main reason.

Fuel flow is calculated from the OBD II computer and I have found that this is very accurate. Actually I should say MPG is accurate as both my wifes and my Vehicross MPH is accurate from the OBD II along with fuel remaining and fuel used. This has been verified at the pump many times.

So if the GPH is correct he should be able to keep up with me with the same GPH. I know I run away from our subie friend at 21" and 2400 rpm. Fuel flow is around 7.2 gph at this setting.
 
Static Thrust Test

Hope you find out the problem...
A static thrust test might help to see what your actually getting compared to other RV's with the same engine / prop. Good Luck. AJ
 
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The problem with a static thrust check is that this is a long, long ways from the design point for the prop. We are really interested in the thrust at the cruise condition. A prop that is optimized for static thrust check will be horrible at the cruise condition. A prop that is optimized for cruise will be not so good at the static condition.

A static thrust check is useful if you plan to use your RV to pull stumps from a field. I fail to see what it tells us about airborne performance.
 
Apples and Apples

I totally agree with you Kevin, static tests on the ground is not like slicing through the air at 200mph. I didn't mean it to sound like if a person had a good prop / engine combo or bad one because your correct that while in cruise it's a whole different ballgame.
 
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Nuisance said:
You should be able to sort the power vs airframe drag with a climb speed test. Climb speed is more dependent on the power to weight ratio...

What kind of take off distances, and climb speeds are you getting?
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Nuisance, I am not sure who the question was directed at, but one hot summer day not long ago I made it to 10,500 in 14 minutes from brake release. The surface temp that day was in the mid 90's. The OAT at 10,500 was about 35 degrees above normal. Needless to say, if the OAT were in the 40's, it would be different.

Take off performance is good, off the ground in less than 500' unless it is a hot day with GW maxed out, and then before 1000'.

Climb performance is very subjective. Van's shows a light weight RV-7 doing 2500 fpm but from where to where? I get a sustained 1500-1800 fpm after take off on a cool day at 110 IAS but not to 10,000'. Pull it up sharply from 130 knots and it is over 4000 fpm, whoppee, but not for long. :)

dd
RV-7A
H-6 Subby
 
Sorry David

The question was directed (through Pierre) at the person who feels he is not getting the speed he is expecting. It was not meant as an indictment of suberu power at all, I was just trying to help his trouble shooting efforts.

Since speed is a combination of power and airframe drag, and climb is more dependent on power alone, I thought considering his climb performance separatly might give him some clues to help find the problem. If his airplane climbs as would be expected with his weight and powerplant (and frankly, I have no idea what to expect myself, but he should, by comparison to many other 160 and 180 horsepower RV7s), then he could suspect that his airplane has more drag than it should. Otherwise, if it climbs more slowly than expected, something may be wrong with the engine/prop.

I am assuming here that he has calibrated his instrumentation, and he is using a stopwatch and altimeter to calculate rates of climb. By comparing the results with the numbers on Van's website, he should get some clues.

John
 
Thanks guys

Mornin' all,
This guy is a friend of mine and lives 13 miles away with his own strip and that's why he has a sube.......to burn mogas from his own strip.

Last November we did our first flight in the 6A with no leg fairings or wheelpants, he came by and his airplane had been flying a few months. He flew over and we paired up and BLEW BY HIM and he had all the fairings/pants on!! It was an omen of things to come.

Guys, we're talking groundspeeds. Slow, slow. Take away the Dynons and the gauges, we blow by him like he's anchored. We take off sooner, climb much faster and his AVERAGE groundspeed in any given direction on just about any day, is around 162 MPH on the GPS and his fuel burn is 7.2 GPH, measured by the gas pump on the ground, not the gauges......that's how much fuel he bought.

Jan did show up at the airport last year to address a fuel pressure problem and turned it up to around 50 psi as I recall. The airplane has never been close to other RVs, even the 150 horse RVs leave him behind! I've often said that there are no slow RVs, but this one's made me eat crow......then again, it doesn't have under the hood what Van says there should be either :D

I'll also try to contact Robt. Paisley....thanks,
 
What is Vans Rate of Climb

David-aviator said:
Climb performance is very subjective. Van's shows a light weight RV-7 doing 2500 fpm but from where to where? I get a sustained 1500-1800 fpm after take off on a cool day at 110 IAS but not to 10,000'. Pull it up sharply from 130 knots and it is over 4000 fpm, whoppee, but not for long. :)

dd
RV-7A
H-6 Subby
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That is a good question. I am pretty sure they test "steady state" or continuous ROC at sea level for example.

They do it at lower altitudes, not at sea level but below 5 thousand, using the watch method. The watch method establishes a steady state climb and you hack the watch at the fixed altitude change, say from 3000 to 4000 feet. The VSI gauge is rarely used as the prime flight test method for ROC, but it is good to note to compare to your watch method. You do enough repeated test you can get an average, throw the odd ones out. ROC is hard to measure and it does vary.

With the fact that ROC changes as you climb and you can establish a continuous climb rate right at sea level, where does sea level climb come from?

I am sure Van's rates of climb are NOT a ZOOM climb as you imply. I know because I can peg 6000 fpm for a bit and get 4000 fpm plus out of my RV with ease, at least for 10 seconds. :D Trading airspeed for ROC is useless and not what Van reports.

So how do you measure sea level ROC? You can't but you can measure higher altitudes and calculate or extrapolate the lower altitudes. Its almost a linear relation.

I know Van and Scott have embraced the "data acquisition" of the new flight instruments. Now you can just go fly and down load the data for later "reduction" and analysis.

TO BE clear vans ROC is a steady state climb not a ZOOM climb trading speed for ROC. Also as you correctly point out, obviously as you climb ROC will go down steadily unless you can keep increasing power, which is not the case with a normally aspirated engine.

I think ROC is a great indicator of how much thrust your power-plant or HP your engine is making, which is what I think he was saying. Climb rate is primarily a function of HP.

I have found my Lyc powered RV's match or exceed the steady state ROC Van posts. That applies to speeds as well. If I take off and establish Vy right away, I see my VSI at or above what van reports. I usually don't do that because its too nose up, I rather a +120 mph climb for better visability for traffic and the climb rate is still spectacular, and that is on 150 HP with a Hartzell C/S prop.
 
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pierre smith said:
I've often said that there are no slow RVs, but this one's made me eat crow......then again, it doesn't have under the hood what Van says there should be either :D

I'll also try to contact Robt. Paisley....thanks,
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Robert is the man on these engines as Dan pointed out and hopefully he has some ideas for your friend. He is quite busy in prepping for Jan's big shipment of engines in a couple months though.

You don't need a Lyco to match Van's numbers, just a healthy engine. There are a variety of Sube, Wankel and LS powered RVs which have equalled or exceeded stock RV airframe/ Lyco performance. Robert had the same package and showed his fin to Dan's very quick Lyco RV a number of months back.
 
What, show me

rv6ejguy said:
Robert is the man on these engines as Dan pointed out and hopefully he has some ideas for your friend. He is quite busy in prepping for Jan's big shipment of engines in a couple months though.

You don't need a Lyco to match Van's numbers, just a healthy engine. There are a variety of Sube, Wankel and LS powered RVs which have equalled or exceeded stock RV airframe/ Lyco performance. Robert had the same package and showed his fin to Dan's very quick Lyco RV a number of months back.
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I hate to disagree but for the other 4000 folks out there on average, the typical Lyc powered RV is faster and climbs better than auto-engine RV's. Yes there are selective cases, you are one, where you can match a Lyc in some respects. However the extrodinay Lycoming powered RV's are in a class their own, meaning the fastest of the fast. The winners of the races are Lyc powered. Its not a put down just facts.

When ever there have been side by side fly offs of auto verses Lyc, the rhetoric is squelched, but even than there is always an excuse like the ECU was not set right. I am not trying to squash autoengines, just promote realistic expectations. No one take my word on it, just read the RVator and some unbiased fly offs. Its not all bad btw. I am not talking about being way down on power like this plane, but to be fair for you to say:

"a variety of Sube, Wankel and LS powered RVs which have equalled or exceeded stock RV airframe/ Lyco performance."

You need to back that up with the particulars. I think you will find as more and more Autoengines are flown you will see more and more cases like this. To be fair a few Lyc powered RV's end up slow.

I think you might give the novice unrealistic expectations of equal or superior performance as the norm. It is not the norm, with respect. Most Subies have 140-150 hp type performance.

I would recommend anyone considering an auto engine not listen to the dealer (Eggenfellner) but the people who fly them, or call Van's aircraft and ask them. They have test flown both Rotary and Subie RV's against their Prototypes with Lycs. Those results have been published in the last few years. Its very fair and impartial and not all negative either. People love the idea of not mixture contol, but I don't see that as a burden, considering I may move it only 2 times in a flight, and twice on the ground. Noise, weight and fuel burn are all other factors and they tend to favor the Lyc.

If you are happy with 140-150HP performance than you may be happy with an auto engine. If you want the max performance, equal a Lyc you may be disappointed. Also the mazdas and subies that get decent performance are turbo-ed or supercharged, which is more complexity. A simple normally aspirated Lyc is as simple as it gets and gets the job done.

Bottom line is we have enough RV's flying with auto engines now to have a base line, and it's a lower bar than the base line of Lyc RV's, typically or on average. That is my finding.
 
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Nuisance said:
Sorry David

The question was directed (through Pierre) at the person who feels he is not getting the speed he is expecting. It was not meant as an indictment of suberu power at all, I was just trying to help his trouble shooting efforts..........

.......I am assuming here that he has calibrated his instrumentation, and he is using a stopwatch and altimeter to calculate rates of climb. By comparing the results with the numbers on Van's website, he should get some clues.

John
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No problem, John, I knew why you posed the question and it is a good measure of performance if we know how the comprison is to be made.

Too bad there isn't a standard method to measure of rate of climb. Everyone does it differently and reports the most attractive results. Seems like if we all measured it from brake release to 10,500', we'd have a very good base line. The only variable would be OAT, GW, and pilot technique. (and of course, HP :))

dd
 
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gmcjetpilot said:
I would recommend anyone considering an auto engine not listen to the dealer (Eggenfellner) but the people who fly them...
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Just a data point here...

I had an RV-7 H-6 E-Subie visitor at my airport last week, and he LOVES his H-6. I asked him about his real world performance, and he is getting 190mph cruise and can push it right on up to 205mph if he wants to burn some gas. I didn't get his fuel burn numbers, but he will be back later this month (October) for another visit, and I'll get more info.

Sorry, I know this is about the H-4 Subie... :eek:
 
Silly me........

I shoulda done (did?) this earlier........visit Eggenfellner aircraft's web site. These are his numbers, that I copied and pasted, (2005) for a 9A at 7500' WOT: :eek: I don't think a 7 would be much faster then.

RPM 2500/4550 (engine RPM)
BURN 7.8 ($21.68)
TAS 140kt (161mph)
MPG 20.64 (.13)
RANGE 743 (4.61hr)

DUH!! Sorry guys, but these numbers are for a normally aspirated 4 banger. My buddy gets these numbers at 34" !!
 
cjensen said:
DD (David-Aviator), check your PM's...
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And never fly straight and level for more than 30 seconds....:)

Actually, the AM check is very important these days with all the birds migrating south. Saw some really HUGE flocks this past week end, Brant's geese and other unidentified critters. Turkey vultures are the largest around here, but they hang out all year and are not leaving.

David Domeier
Troy, MO
 
David-aviator said:
And never fly straight and level for more than 30 seconds....:)

Actually, the AM check is very important these days with all the birds migrating south. Saw some really HUGE flocks this past week end, Brant's geese and other unidentified critters. Turkey vultures are the largest around here, but they hang out all year and are not leaving.

David Domeier
Troy, MO
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Sorry David...I meant check your private messages... ;) :eek: :)
 
Any experience with the newer E6/200?

I've read through many posts about these Eggenfellner subie engines but haven't seen any recent posts. They are mostly 2006 and older. Perhaps I didn't search correctly.

I'm building a RV-8 and am considering a new Eggenfellner E6/200. Does anyone have any experience with these newer Egg engines with the IVO prop? It seems the older ones, especially the 4 cyl can be a bit slow.

Any real world experience will be greatly appreciated.

Thanks,
 
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