Subject: Herb's BSFC calcs
From: ACRE NL
Date: 1/14/2002, 7:45 AM



Bill Freeman wrote:


Paul,

Based on Herb and my memories, the drag number for the LE is 2.05.
If we assume Herb was flying at 8K MSL, the air density is 0.001868
slug/cu ft.  If he was flying at  sea level,  the air density is 0.002378
slug/cu ft.
If it was 10K MSL, the density is 0.001811 slug/cu ft.  Since we
don't currently know Herb's altitude, I'll calculate it for 0,8K and 10K MSL
and we'll see which altitude produces believable results.  Then
Herb will tell us the REAL cruise altitude.
Vel =198 mph = 290 ft/sec.

     Drag = (2.05*density*(vel^2))/2

  for sea level,     0.5*2.05*0.002378*84100 = 205 lb drag
  for 8K MSL,    0.5*2.05*0.001868*84100 = 161 lb drag
  for 10K MSL,  0.5*2.05*0.001811*84100 = 156 lb drag

At 290 ft/sec (198 mph) the HP calculates as:

    S.L.       290ft/sec*205 lb = (59450 ft-lb/sec)/550= 108 HP
    8K        290 ft/sec*161 lb = (46690 ft-lb/sec)/550= 85 HP
   10K       290 ft/sec*156 lb = (45240 ft-lb/sec)/550= 82 HP

Assume 80% efficient prop, if it was at SL  then 198 mph takes 135 HP
        if it was at 8K then 198 mph takes  106 HP
        if it was at 10K then 198 mph takes 103 HP

198 mph for 515 miles gives a time of 2.60 hrs
Fuel burn of 119.2 lbs/2.6 hrs = 45.85 lb/hr

  For SL        (45.85 lb/hr)/(135 hp) = 0.34 lb/hp-hr
  For 8K        (45.85 lb/hr)/(106 hp) = 0.433 lb/hp-hr
  for 10K       (45.85 lb/hr)/(103 hp) = 0.445

We don't know what altitude the race was flown at, but I'm sure Herb will
answer that.  If it was "on the deck", the BSFC is FAR beyond what I
believe that Lyc's can do.  If it was either 8K or 10K MSL, then this is right
in there with about what I believe they can do, just about normal
for a well-leaned Lyc.

Actual prop efficiency could be a bit better or a bit worse, say about
+/- 2% (78% to 82% efficient).  The other variable will be OAT, since
the air density values used are for the std temperatures, which can be off on
a particular day.

Since Herb's slightly souped up Lyc should produce about 175 HP, and
at 8K MSL, WOT should make 75% of SL power; 175 * 0.75 = 131 hp.
He probably can't quite make 198 mph on the deck.  At 10K power is
about 60% IIRC, or 105 hp.  This fits the 10K calcs very nicely.  I will
predict that Herb was WOT at around 10K MSL for this run.

Since most real world data indicates that stock Lycs run at about 0.42 BSFC
or higher,  the remembered 2.05 drag number for the Long EZ seems an
excellent fit, confirming the conventional wisdom on Lycs.  Once we get the
actual cruising altitude, we can nail this down.  So far, no surprises.

No laws of physics were broken in the making of these calculations.  ;-)

Bill Freeman
Long EZ builder & pilot
BSME, MSME
EAA Technical Counselor


Don't get the impression from this that the BSFC necessarily changes
with alitude. It usually does not. The BSFC number just comes out differently
when you try to calculate it the way Bill and I are trying to calculate it
starting from drag and fuel burn.

I think the 10:1 compression ratio makes the engine 160 HP. Up from 150 HP and not 175 HP.
I also I think the 2.05 drag number is a bit high. Herb's original guess
was 1.6 which increases the BSFC number to .48 at sea level. Here is the
race results from the 1998  Sun & Fun 100 Xcountry run at about 1000 feet
above sea level.

Surely a 160 HP Long is faster than a 160 HP Lancair 320?
Well... maybe not.

2.05 drag number and 160 HP I figure gives a top speed of 207 MPH at sea level.
135 HP gives 196 MPH  which is close to Bill's of 198 MPH for 135 HP.
See the chart below.

Herb are you there? What is the top speed?  What altitude did you run
the race? This info will narrow down the calculations.


Note that in measured conditions, the Long was more efficient (faster on less fuel
burn) that the Glasairs and Paser Mustang, but not as efficient as the purpose
built racers.


Herb said his airplane was more efficient than the Paser Mustang which I think
is probably similar to the 180 HP Mustang in this chart that went 235 MPH.
I'll have to run my program on that airplane tomorrow and figure its
drag number. Does anybody have Paser's book?

Paul Lamar



I have Paser's book.  What do you need from it?

AI Nut


What does Paser say the equivalent flat plate number for his airplane is?

Thanks.

BTW I am working on a program where you put in the top speed and HP and it
calculates the drag number or flat plate number.

Paul Lamar
 
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