What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

H6 Subaru at 11,500'

D

David-aviator

Well Known Member
I took the H6 (normal aspiration) up to 11,500 on a 4.9 hour round robin to Nebraska from St. Louis yesterday. Here are some numbers and observations regarding the flight.

It seems the MT prop works best at 2500 rpm in climb. Also, 90 KIAS appears a best climb rate speed up high although I have become accustomed to a cruise climb configuration of 2300/110K at low altitude. The airplane was climbing steady at 600 fpm through 11,000.

Once level, after trying several different rpm settings, I let it roll at 2200 rpm. Even after some 2 years with this engine/prop combination, I still haven't figured out the best configurations at various altitudes for speed and fuel flow.

Prop RPM 2200 (engine 4036)
Manifold Pressure 19.1 (WOT)
Fuel Flow 6.4 gph
OAT 35F
TAS 140 knots (Dynon computation)
(The Dynon IAS is about 3 knots less than Van's IAS so TAS is
conservative.)
Total burn for the trip was about 33 gallons with 4.9 on the Hobbs.

Note: this is the #1 H6 installed in an RV. The EGG factory has improved the engine/prop combination considerably with turbo charging and a 4 blade Sensinich/Quinti prop.

Cabin heat was very comfy. We use a 3 speed fan to extract heat from a small radiator on the aft side of the fire wall. It works quite well. I did use a blanket for the upper body half due to much canopy air leakage but there was plenty of heat from under the IP running the fan at mid speed. The worst source of cold air is from the aileron push tube holes into the wing. That air comes up through the stick cut out and is on a list of things to improve.

The EGG factory has an informal deal with flying Subbys to inspect on going projects. That was the purpose of the trip. This Nebraska 9A with a 2.5 engine is of excellent quality and should be flying soon.
 
David,

That sounds like a conservative cross country setting at 2200 prop rpm and 6.4gph. 140kts at 6.4/hour is pretty good! Being normally aspirated, the altitude would be high for optimum numbers. Unboosted engines usually turn in the best numbers between 6k and 8k. Same for the H6?

Great report!
 
dan said:
Was that as fast as she'll go? What's your air intake setup like?
Click to expand...

It will go faster with more rpm, but the fuel flow goes up at a greater rate so I don't do it.

From a flight test on 9/29/04 at 12,500'
RPM 2600/4830 (prop/engine)
BURN = 10.1
TAS = 148 KTS (170 MPH)
MPG = 16.83 (Still Air)

I've discussed the efficiency of the MT with Jan and Robert, but they do not believe that to be a problem. Robert's airplane does much better than mine with the same set up so it has to be a drag issue. I am not the greatest builder in the world when it comes to important cosmetic stuff and that's my problem. :( I'd 10 times rather fly than build or have the airplane grounded for any length of time. I really need 2 airplanes and continue to dream about the 8. :)

The air intake is through a filter at the right radiator. I've thought about a ram air by-pass ala some production airplanes. That might make a difference.

I meant to mention fuel type - the trip out was with 87 mogas and the return with an approximate 50-50 mix with 100LL. The engine will run fine with either but prefers mogas, IMO, even though it has a 10.7:1 comp ratio. The knock sensors adjust timing to keep everything OK.

I bought the mogas at Walmart for $2.059, the avgas on the other end was $3.95. 100LL probably will produce more power per gallon and that too is a factor in all out speed considerations.
 
Last edited:
cjensen said:
David,

That sounds like a conservative cross country setting at 2200 prop rpm and 6.4gph. 140kts at 6.4/hour is pretty good! Being normally aspirated, the altitude would be high for optimum numbers. Unboosted engines usually turn in the best numbers between 6k and 8k. Same for the H6?

Great report!
Click to expand...

For general flight planning purposes at 8500' I use 8 gph and 143 KTAS, but have found I can reduce the rpm from 2300 to 2100 and see 7.7 gph with the same speed. Like I said, I'm still learning what is best for my purposes which is not all out speed but getting from point A to B in a reasonable amount of time and cost.

I also believe running any engine at something less than 75% power will extend its life considerably.
 
cjensen said:
Why are the flight tests so high though? :confused:
Click to expand...

Don't know for sure except there are no turkey buzzards up there. Huge beetles there are, but no turkey buzzards - or many Cessnas either. :)

Originally, I went up to see if the engine liked it, actually nosed up to 14,000 one day for a bit but did not record any data. The engine ran fine. Fuel flow really drops off as it does in a propely leaned Lyc. The flow numbers are less now than 2004 because fuel pressure has been reduced. The ECU computes flow based on injector pulse time and fuel pressure. Reduce the pressure, less fuel flow. Robert Paisley did all the grunt work on this and varified the A/F ratio is OK running at about 32 psi. The H6 handbook calls for something in the 40's, either 42 or 47, can't recall. The lower pressure reduces fuel flow by at least 10%. EIS fuel flow calibration is still off a bit since doing this but it is getting close. It's burn was off by about .8 gallon burning 18 gallons yesterday, so the numbers I reported could be 4% high from actual burn.
 
Kahuna said:
David what is your empty weight?
Thanks
Click to expand...

1143 when the 2.5 engine was removed and H6 installed. But I have since switched props with a pencil estimate, it should be weighed again.

I did re-weigh it with uncertified EAA Chapter scales and it came in at 1285. The original 1143 number was with certified scales so I threw out the 1285 number and stuck with the original weight, estimating the weight change with the new prop. I think it is close because I weighed both prop systems.
 
You must log in or register to reply here.
Back
Top