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Message
Jean-Pierre, Lorn, and LNC2
friends,
In an effort
to level the playing field and make LNC2 racing more interesting I thought it
prudent to provide the following discussion to the List. Stated another
way I'm tired of whipping you all so badly that you refuse to come to the races
so pay attention and improve your planes.
In response to
your speed queries I thought I'd list a few of those items that may contribute
to my airplanes solid performance. It should be noted that cheating is of
course paramount. And therefore I would ask that you gentleman be
judicious in your generosity to share these tips. Also, be careful to not
kill yourself or anyone else while considering any of these tweaks. I
would prefer none of you earn Darwin Awards after seeking another 1/4 kt.
A noteworthy truth is that I've never seen above 213 kts steady state ground
speed on my GPS with upwind and downwind averaged legs. During the Sun 100
I saw 214 kts for a while but mostly never looked down. It is also true
that whatever my speed I've not been beaten ever
(Normally aspirated LNC2) in 6 consecutive races. This may of course be
partly because minimal people come to races :(
In particular
Scott Krueger's analysis of turn radii from the book Aerodynamics for Naval
Aviators applies to the Sun 100. The GPS distance for the Sun 100 is 76
NM. Scott's calculation to add about 6 NM for 360 deg of turn radius
is a reasonable thought. I believe he based this on a 30 deg bank
angle. I imagine it would be legitimate to add another 2 miles
for runway length. That is, the takeoff southbound turn is not permitted
before exiting airport property (One runway length). Additionally, the
arrival turn from south to east adds about 1 full runway length because we
were precluded from overflying a camping area on the south west corner of
the airport. Anyway, my total time was 22:35. Backing this number
out of 217.86 Kts yields a race course length of 82 Nm. These are just
things to think about.
In reference
to cheating I would never turn during a race with less than FAA aerobatic bank
angle limitations. I was dead on top of the turn points and pulled solid G
to get around them. Indeed it would be fair to say that I cut one
gentleman off in the turn. He was however over 1 mile past the point when
I arrived (that'd be about 20 seconds). I thought it safe to assume he was
just out joy riding. Scott's Hi Yo Yo discussion probably holds little to
be gained with the speeds we're considering. (FWIW) In a fighter jet
if one wanted to decelerate to "corner airspeed" then such a climbing manuever
might provide some benefit and compensate for excess bleed rates. IMHO
this airplane has minimal bleed when cornering.
With all that
being said, the other disclaimer I have is that my plane is pretty much as
built, (then crashed and rebuilt). Thus, I have no idea which tweaks
actually cause the slight advantage I'm seeing over the competition. I
also have about zero insight into the engineering value of individual mods as I
was too busy building to do much studying when I built my plane. I heard,
"that'll make it faster" and I did it. Alot of what I heard came from an
EAA meeting with Dave Anders. He had just won the CAFE competition
with his RV 4 and provided numerous tips. I didn't bother with how much
faster each mod would be, I just did them all. I will however list my
uniquenesses in order of my best speed value
guestimation.
1) I turn the Hartzell
68" prop at 2920 RPM. This results in 30.7 in MAP flying at sea
level. Don't do this without some convincing that your prop and crank can
be turned this fast safely. My understanding of a Hartzell certified prop
is that they are designed to handle 150% of rated max rpm (2700 x 1.5 =
4000). Mach analysis and temperature dependance above this rpm is
mandatory. Don't just stay under Mach 1.0. You must be under that
rpm where dramatic drag rise from mach affects ensues. Don't break
anything up here or death will be shortly thereafter.
2) Dual LSE electronic
ignition. Buy'm from Klaus Savier and tell him I sent ya. I get no
royalties just enjoy heckling and beating him with his own equipment.
Didn't ever use mags so can't prove how great they are but sure enjoy the fuel
savings to and from the races.
3) 9:1 pistons in the Injected 180 HP
engine (I'm told they make it an empirical 192HP). Engine was polished,
ported, mass balanced, dynamically balanced, then crash tested with a gear up
takeoff. You may not find any extra benefit from the latter. I
have an updraft sump. The injector servo faces forward with about
4" between the servo and the air intake ring. The required 90
deg turn is a bolted on 90 deg elbow (not pretty). The forward facing sump
argument has me beat by excess cost but also may not provide the total intended
claimed benefits because of internal aerodynamic uglies depending on your sump
modifications.
4) I have aY valve crammed in between
the servo and the ram air intake. Atop this valve sits a K&N air
filter. Since the flapper only closes the ram side of the valve it deters
performance when RAM is selected. That is, the higher pressure ram air
back flows out the filter when it should be going into the sump. Therefore, when
racing I remove the filter entirely and plug that side of the Y valve. The
filter/ram lever is of course safety wired to minimize stupid pilot activities
(which I've occasionally been party to).
5) Aerodynamically, as you suggest
(Jean-Pierre) the plane is almost flawless dragwise. My inboard gear doors
may hang down about 1/8" and my two trim motor drive linkages are not
faired. I've recently adjusted my right elevator up about .025" to better
center it between horizontal stab skins. All else is very smooth. My
horizontal tail incidence was intended to be less negative than spec'd (I
believe 1/2 deg nose down was the design parameter). In the end the water
level as compared to the smart level were at both ends of the factory
recommended incidence tolerance. (+/- 1/4 deg maybe?) Elevator trim does
indeed sit at zero while at 212 KTs but this does not alone convince me that my
elevators are perfectly level to the horizontal
tail.
6) My
pressure cowl has Lo Presti style air cooling intakes which feed into a
fiberglass plenum. The rings (off the shelf from Sam James of RV land) are
the perfect size. It is apparently of value to match the ring
internal airfoil to the external airfoil along the cowl. They sit about
1/2" back from the prop. I sit on 180 - 200 deg C on CHT's with oil
temp while racing at 95-100 deg C. (These CHT numbers are the top of the
green arc for you farenheit fans, oil in the yellow range but much cooler than
many of my competitors) I have no additional NACA scoops or air cooling on
the cowl (except engine feed air). My suspicion is many kts are lost with
all the wild NACA mods and fins that we see on so many
LNC2s.
7) The
air entering my pressure plenum is supposed to expand at less than a 5 deg rake
angle. The plenum is now sealed with Red RTV. Cooling air is
borrowed into an up front oil cooler (in front of #2 cyl.). All other air
is for cylinder cooling. I have no little cooling tubes for such things as
alternator, starter, vacuum pump (don't have it), mags (don't have any),
or the gascolator. These items haven't yet burned up (360 hours TT)
perhaps due in part to #8 below.
8) The air off the cooling fins is
guided with a very large piece of thin AL from the cooling fins down and aft on
a gentle curve to the exit. This air has minimal tendency for eddy
currents. It also performs some degree of heat shielding for the
gascolator. The sheet (actually 3 pieces) makes cowling contact down the
sides with 1/8" vacuum tubing to minimize abrasion. It is ugly and was a
total pain to build but perhaps helpful.
9) I have recently replaced my two
down turned exhaust pipe tips with straight tips (about $20 in carbon
steel). The intent of this project was to slowly neck down the tips and
provide either increased back pressure (MAP & Power) or a better exhaust
velocity match to the surrounding air flow. Of the 4 insert rings which
reduced the exhaust pipe exit area all four reduced speed. This was the
only half way accurate test I've done and I'm certain it's a bad idea for my
plane. The rest of my stainless exhaust was ceramic coated and I'm now
using a piece of Zetex near my one hot spot/ blister (seems great!). BTW,
the straight pipes are a belly soot fest! Can't stand'm for daily
flying.
10) The
empty weight of the plane is 1161 lb. I flew Sun 100 with my 85 lb
son. I weigh 213 lbs. (get your speed over your weight then your
going places). I carried 16 gal of fuel and shut down with about 7.
(At least 3 gallons extra fuel including holding on the Lake Parker
arrival). All that said I don't think 100 lbs affects us 1/4 kt. CG
on the other hand may have an affect. Dont' kill yourself finding
out.
11) I run at 17 gph +/-. This
is slightly up from previous races and may be noteworthy compared to other
slower speed races. I've run at 16 gph in several races (also a little
higher msl altitudes). It may be a push. However, the air was very cool (
< 70 F) and super still on this particular race
day.
12) While building I was
friends with a 235 gentleman (Dave Roach-Great guy/ highest LNC2 houred plane
that I know of). After following much list discussion of late I'm now
convinced that I wrongly built my flaps with double available reflex. That
is, people say that flaps faired to the fuselage fillets is -7 deg. on the
newer LNC2's. I don't remember reading it this anywhere.
I have about 1" above faired available for me. This builder mistake may
give a smidgen of speed but I don't see any IAS difference while playing with
it. I do however blow the fuse when I drop too much flap too fast.
Duh!
13) Along those lines,
I've not intentionally done this but my ailerons seem to sit about 1/8"
above the faired wingtip. Don't play games here for a stalled aileron
could also take one's life in short order.
14) I race with my alternator off to
conserve engine drag (about 5 amp reduction). My 18 amp battery does fine
for the short races. I turn on the alternator at 12 Volts to
recharge. I also have a "Sweet Jesus" 3 amp back up battery that I don't
touch except in emergency and throw away every couple years. In longer
races I turn on the alternator while transferring fuel and to confirm
the gear haven't drooped. Occasionally, I get a chirp from the hyd. pump
when I turn on the alternator. In the Copperstate Dash, I saw a 3 kt
acceleration after the chirp. Know however, that this practice provides no
visible indication of a change in IAS. It just feels like it should
help. Another feel good consideration might be an electric primary fuel
pump (throw out the mechanical). My electrical system has a 3
tiered fault system along the Bob Knuckolls philosophy and yours should be
carefully analysed before anything so outrageous is
considered.
15) More feel
good items ( or not actually feel good to be precise). I
close the intake Naca cockpit vents. I also have two exit air holes in my
aft baggage area that permit ventilation air to go down and out my tail. I
plug these holes for racing. There is still gear door leaks
available if you really want to go crazy. To add to the overheating
pleasure I am careful to increase visibility while racing by tucking my Koger
canopy shade up and out of the way. Racing is all about safe
visibility. This part is painful and perhaps
helpful.
That's
it! In the words of Forest Gump, "That's all I know about
that". What do you suggest I tweak next?
Union votes
tomorrow to decide whether I'll have to sell her or not (possible bummer
ahead).
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