I have the disadvantage of not being an engineer but it is my understanding
that drag and therefore power requirements go up rather steeply (based on
the cube) at the top end of the speed curve.
Let us assume that 922F goes 15 knots faster than another similar Legacy
and that we can explain
this difference based either on aerodynamics or on horsepower. (Most
likely the difference is a combination of the two) The difference in speed
between 250 and 265 is approximately 6%.
If we chose to explain this difference in speed solely on a
difference in horsepower how much horsepower do we need? (Here is where it
would be very helpful if I was an engineer) By my (suspect) calculations we
would need 19% greater horsepower. So if the 250 kt plane is making 325 hp
the 265 kt plane would need to make 387 hp. If the 250 kt plane makes 310
hp, the 265 kt plane would need to make 369 hp.
In my discussion with Mr. Frazier he said that he had run his engine at
about 2600 RPM and approx 26 inches manifold pressure. I doubt that his
engine makes 19% more horsepower at that setting than a stock Continental.
Some of the discussions on horsepower become more like discussions of
religion than of science. And I must admit that I am probably of the
"Oklahoma Branch" of the faith rather than the "Southern California
Branch". I simply do not believe that an approximately 15 kt difference in
speed can be explained based primarily on horsepower.
On a more practical level, my engine is already built (IO-550, 10:1
compression, dual mags, built by Monty Barrett) so I cannot change that.
My only chance to get 265 kts is to attend to the aerodynamics. I may
have a chance to actually test and see how much increase (or decrease) in speed
I can get by paying attention to the leading edges. Most likely my plane
will be flying before I get a chance to "template" the leading edges. I
will then have an opportunity to see if there is a "before vs. after"
difference.
D. Brunner
----- Original Message -----
Sent: Tuesday, July 31, 2007 6:12
PM
Subject: [LML] Re: Template of Leading
Edges
It was
922F.
Given this discussion,
I'd suggest a slightly different approach.
Most of the Legacys run
in the 240-250kt range (those quicker ones anyway).
If indeed this one is
in the mid 260's, I doubt subtle changes to the bodywork to better match wing
airfoils is the magic as that is a 15kts differential from the 250 mark (which
very few stock legacys make)
This particular
airplane makes that mid 260 number on 19.5 gph (the owner had a card in the
airplane with the specifics). It gave no MP/RPM numbers
however.
Also, it has a highly
modified LyCon IO-550 engine with 10:1 compression and a supposedly dyno'd
375HP. I heard a rumor that it actually made 390 on the dyno and had to
be "turned down" (dunno how you do that with a normally aspirated engine) to
make the 375HP.
As everyone knows, the
only semi accurate way to determine accuracy in TAS numbers is to use a
spreadsheet by the National Test Pilot association, here's the link http://www.ntps.edu/Files/GPS%20PEC.XLS
Use the tab that
requires 4 legs. It's pretty simple to do this, just pick an altitude
where you can make 75% power, setup for your runs and level out and let your
speed build. Now turn on your Autopilot and slave it to the GPS.
You want to fly 90 degree legs (90, 180, 270, 360) all 4 of them.
For each leg, write
down the following information
Alt, RPM, MP, OAT,
PALT, DALT, IAS, TAS (if you have onboard indication), GPS TRACK, and GPS
GROUND SPEED
Now when you have
landed, plug in the GPS TRACK and the GPS GROUND SPEED into that 4 leg tab on
the spread sheet and see what the TAS was. If it is higher or lower than
your indicated TAS, then you have an error in your IAS (which is what feeds
the TAS CALC in most GLASS panels), or your OAT.
I did notice that this
particular airplane had GRT Glass panels in it and that it used a slightly
different ram air setup.
As a note, Scott Alair
ran the AVC at 297mph or 253kts in a stock engined legacy, but I believe he
was turning 2900 RPM and I don't know what his MP was. Last year at
reno, a legacy right out of build assist with no paint or body work turned
290mph laps at reno but this one also had a modified motor from performance
engines altho it turned stock RPM (2800).
The reason I make these
last 2 points. HP is king when trying to put up speed numbers. So
if the mid 260's is everything full forward, what is the typical cruise in
this airplane with a more standard fuel burn? That will tell probably
more as it relates to speed than a speed run will.
My .02,
Alan
What is Mr. Frazier's N Number?
Jeff
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