|
|
----- Original Message -----
From: "Greg Fuess" <gregory_fuess@yahoo.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Sunday, March 21, 2004 1:19 AM
Subject: [FlyRotary] Fuel Consumption Questions
> Hi Fellow Rotor heads,
>
> I am getting very close to starting construction of an RV-7 (much closer
> than last year when I said this! - should start in 8~10 weeks) and very
> interested in the potential for the Renesis engine in this application
> (can't wait for Tracy to get his installed so we can read the performance
> reports). I do have one question that is difficult for me to get my arms
> around. What kind of fuel burn are you guys experiencing that are flying
> your 13Bs? Turbocharged 13Bs? -at various power settings and altitudes?
Is
> there a data reference site anywhere that could provide this information?
>
> Ed Anderson's Rotary Power vs. 2-3-2 Cooling BTU calculation spreadsheet
> indicated 12.4 to 14.9 gph at 5k to 6k rpm (all for 1,500' alt). This
seems
> not to compare well to the lyconosaurus published consumption rates for a
> 180 hp engine, though I have read elsewhere on the internet that the
rotary
> consumption should be similar. Am I misreading or misusing Ed's
> spreadsheet, or should I get used to the higher consumption rate? Are the
> Lycoming consumption rates bogus?
>
> I am something of a lurker here, but as I get closer to the "date", I am
> getting more and more detail conscientious. Appreciate any and all
> thoughts.
>
> Best Regards,
>
> Greg Fuess
>
Hi Greg,
Some good questions. First regarding the spreadsheet. The spreadsheet
will accurately transfer fuel burn into HP figures regardless of whether the
input parameters are operationally realistic.(Accurate does not ensure
realistic {:>)) If you want to get fuel burn that relate to actually
operation then you must input operational figures. Yes, you can force the
model to give you 180 HP at 10,000 ft altitude - BUT, you would never get
that with an NA 13B. Lets take a more realistic example. Say you have a
hot 13B that produces 180 HP at sea level, then at 6000 rpm that would have
a fuel burn of 16.8 GPH. Now you fly the engine at 7500 MSL at WOT, due to
the lesser density and oxygen content of the air at that altitude, 6000 rpm
at full throttle (if you can get it) would only produce 140 HP at a fuel
burn of 13 GPH if you have it enriched for full power with an Air/Fuel ratio
of 12.65. Now at WOT and 6000 rpm you can vary the fuel mixture and get
quite a bit of variance in fuel burn (and therefore in HP). Since the 13B
can be leaned out more than the Lycoming, I think you would find that fuel
burn at cruise leaned out would be lower with a 13B.
The figures you quote seem about right at 1500 MSL for a 13B producing
between 140 and 160 HP. I don't have the Lycoming figures to compare with
so don't know what state of operation the Lycoming is in. However, I do
know regardless of any charts and figures that you do not produce HP without
a commensurate fuel burn regardless of engine. I've seen figures quoted
that would indicate the Lycoming produces 180 HP on as little as 10-12 GPH -
can't happen, 10-12 GPH will not produce 180 HP in ANY operational internal
combustion engine of current design.
But in actual operation, for example, I cruise at 170 MPH TAS in my RV and
burn around 7 GPH. If I want to fly at close to 200MPH then my fuel burn
will go up to 12.5 GPH and I have a fairly aerodynamically dirty airframe.
Tracy Crook can get 170 MPH TAS burning as little as 6 -6.5 GPH. I think
you will find these figures comparable with the actual Lycoming burn
(perhaps slightly better). Your mixture (air/fuel ratio) control has a
great deal to do with the fuel burn you see in the rotary. For example, at
7500 MSL at 85% throttle if I lean my engine back to where I am turning 5300
rpm I would be producing 71 HP at a fuel burn of 7.1 GPH and still cruise at
170 MPH TAS at 7500 MSL.
Tracy Crook and Bernie Kerr (Lycoming powered RV-6) have actually flow side
by side and I believe I recall the fuel burn of both were in the same ball
park, with some operating regimes where one did slightly better than the
other, but no significant differences as best I recall. Tracy? Bernie?
My personal opinion is that the 13B and Lycoming 320-360 come out similar in
Weight, Power and Fuel Consumption. So I don't think either of those three
factors would necessary tell you which engine to select. However, if you
consider reliability and cost, then the 13B clearly has an advantage.
Back to the model, if you actually operate a 13B at the parameters you plug
into the model then the calculated HP and fuel burn have been shown to be
accurate. I have compared the results with a number of flying 13Bs but in
particular with Tracy Crook and the comparison has been good.
Can't speak for turbo 13B engines as I have no experience with them and very
little data, but I believe that the turbo version of the spreadsheet will
still produce accurate figures for HP and fuel burn.
Hope this helps address your question
Best Regards
Ed Anderson
|
|