For engineers and Eggenfellner installations

pierre smith · #1 09-30-2006, 02:03 PM

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 .

mgomez · #2 09-30-2006, 03:38 PM

Quote:

Originally Posted by pierre smith

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. :

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.

cawmd82 · #3 09-30-2006, 04:53 PM

Wonder what Jan has to say? Seems like the best place to start.......

gmcjetpilot · #4 10-01-2006, 01:35 AM

Quote:

Originally Posted by pierre smith

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 .

Haaaa, install a Lycoming and Hartzell?

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.

Kevin Horton · #5 10-01-2006, 06:19 AM

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?

pierre smith · #6 10-01-2006, 06:28 AM

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.

David-aviator · #7 10-01-2006, 07:57 AM

Quote:

Originally Posted by pierre smith

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,

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

Nuisance · #8 10-01-2006, 08:14 AM

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?

Kevin Horton · #9 10-01-2006, 08:57 AM

Quote:

Originally Posted by pierre smith

the TAS according to the Dynon, was still 162 MPH!

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.

dan · #10 10-01-2006, 09:47 AM

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.