Performance Comparison

The following message was posted on the Sonex and Aerovee discussion groups on 12/2/2010:

Hello again Sonex eGroups-

There has been a great deal of data comparisons lately and we would like to
comment on the important points that impact individual aircraft performance,
the collection, interpretation and reporting process of the data used for
those comparisons, and the very specific numbers digital instruments

The obvious assumption we must make is that the airplanes being compared are built as accurately to the plans as possible. It does no good to compare an
airframe that is 720 pounds empty, out of rig, and has no fairings to an
airplane that is 650 pounds empty, well rigged, and fully faired. Draggy
airplanes will not only be slower, the engine will not turn up to the proper
RPM and it will run warmer than it should. Builders who call us for help
with their high CHTs are often surprised when the first question we ask is
whether or not all of their fairings are installed. They are equally
surprised when installing the fairings brings their CHTs down. We must also
assume identical props are installed (which is why we recommend and use
Sensenich propellers), proper engine maintenance has been performed, similar
compression ratios (for the AeroVee) or horsepower output (for the Jabiru
2200) between the comparison aircraft, and the pilot has accurately
collected and recorded the data. As you can see, the odds are stacked
against any two pilots of any two aircraft achieving an apples-to-apples

Airspeed Comparison
A book could be written about the different airspeeds (Ground, Indicated,
Calibrated, and True), but the only air speed that matters when comparing
one aircraft to another is True Airspeed (TAS). True airspeed can only be
determined if you have calibrated your airspeed indicator to account for
errors in the instrument and the pitot/static system. This generally
requires hours of flying very precise 2 or 3-way courses with a GPS,
measuring your ground speed and comparing the results to your indicated
Airspeed. And that is VERY simplified description of the process. Again,
this is a topic that a book could be written about and it is way beyond the
scope of this posting. Suffice it to say most pilots do not carefully
calibrate their airspeed indicator, or make any attempt at calibration at
all. Indicated airspeed from one airplane to another is no basis for
comparison, and ground speed should NEVER be used as a basis for comparison
unless both airplanes are flying side-by-side, drag race style.

Many of today’s digital instruments display True Airspeed. This leads pilots
to believe they are being provided accurate information. It is digital, it
MUST be accurate, right? Wrong! This information is only correct if the
instrument and pitot/static systems have no error, if the barometric
pressure has been properly set, and if the instrument’s outside air
temperature probe is also accurate. A cumulative error of only 5% results in
your TAS being displayed as 161.5 mph instead of 170 mph.

Empty Weights

Bathroom scales are never a good idea. Period. Digital scales are only
accurate if they have been calibrated. Remember, digital does not guarantee
accurate. And scales from the local EAA chapter or FBO should not be assumed
accurate just because they came from EAA Chapter and “everyone” has used
them. Maybe every airplane that has been weighed with them is much heavier,
or lighter, than the owner knows. Establishing an accurate empty weight is
critical to determining weight and balance; not only for the FAA when they
license your airplane, but also for that hot day you decide to take your
buddy to Leadville, CO for lunch. The ensuing take-off, after enjoying a
large meal, topping off the fuel, and tossing your cameras and jackets in
the luggage area, is the wrong time to find your weight and balance is
inaccurate by 10%.

Fuel Burn and EGTs
Many things impact fuel burn. A draggy airplane will burn more fuel for a
given speed than a clean airplane. An airplane burning 100LL will use LESS
fuel than one burning car gas with ethanol. Less energy in the ethanol-laced
car gas equals less power, which means more gas is needed to go a given
speed or distance. We recommend all flight testing be done with 100LL as
this eliminates questionable fuel as the source of poor performance. We
recommend autogas with ethanol be avoided entirely. The fuel level gauge and
flow sensor must be calibrated to provide accurate information. And how is
each pilot reporting fuel burn? Average for an entire flight? Indicated at a
particular moment? At what altitude? On a cold winter day, with all that
dense air, fuel burn may be way up, but so is TAS. Again you can see how
difficult it is for any two pilots to compare any two aircraft. We recommend
all flight testing be done with 100LL as this eliminates questionable auto
gas as the source of poor performance.

Leaning is critical to optimizing fuel burn for a given throttle setting. An
AeroCarb or AeroInjector provides the pilot the ability to lean for best
power at minimal fuel burn. The engine still requires ³x² amount of fuel to
produce ³x² amount of power, but by leaning properly you can avoid burning
1.2 times ³x² fuel to go the same speed. When you level out at altitude,
optimizing fuel burn happens in simple, repeated steps:
1. Set the desired RPM.
2. Lean until the RPM peaks. The fuel burn will drop, often dramatically.
3. Reduce the throttle again to the desired RPM (remember, the RPM just rose
while you leaned the mixture). Fuel burn goes down further.
4. Now, at the again reduce throttle setting, you may have the opportunity
to lean yet again.

It is not difficult, time consuming, nor terribly scientific in practice,
but leaning can mean the difference between burning 7 GPH or 5.2 GPH.

When you learned to fly in a Cessna 150 you may have been taught to lean
until the engine ran rough, and then richen a “turn and a half”. You had no
EGT gauge, no fuel flow meter. If you were lucky the instructor took the
time to explain why you were doing it. If not, you just did it because you
told to. Either way, the engine didn¹t melt. Most of us have an EGT gauge
these days, so we use that as a reference, but that’s all it is. It is not
an indicator that parts are about to melt in your engine. Most days ³Metal
Illness² can be leaned over 1400 degrees. Some days I can’t get her above
1300 degrees. And if you try to lean at a low RPM, the engine will quit from
fuel starvation at a very low EGT.

For more on EGTs, we refer you to Mike Busch’s outstanding article. “EGT
Myths Debunked” in the October 2010 issue of Sport Aviation magazine.

Engine Temperatures
This one causes everyone the most angst. First, some basics:
1. New engines will be hot. Even in January. Even in Wisconsin. Limit ground
running, get in the air, climb shallow and fast until the engine breaks in.
2. Aircooled engines will never cool on the ground. We get an amazing number
of people trying to fix high CHTs on an airplane that has never left the
3. The green arc is the green arc, whether it is the high end or the low end
of the temperature gauge. Green is good.

We already touched on EGTs, above. “Metal Illness”, with her Jabiru 3300,
routinely flies above 1400 degrees at altitude and 2850 RPM. It has done so
happily for nearly 500 hours. Generally, if the EGTs are high but the engine
runs well and CHTs and oil temperature are good, I’m happy. But pilots must
make this decision for themselves; exceed the engine manufacturer’s
published limits, or not? Keep in mind a published maximum EGT of 1375F does
not mean the engine will melt down at 1380F. And the indicated temperature
is just that, indicated. Probe location, probe condition, reliable wiring
and instrument accuracy all play a part in delivering an accurate EGT
indication. Use this temperature as a reference, not an absolute number.

Proper CHTs are very important to the life of your engine. As mentioned
above, expect high temperatures on the ground and during your first few
flights as the engine breaks in. If the CHT spikes above the green on your
first flight, level out. If the temperatures do not decrease, throttle back,
land, and investigate. If you have persistently high CHTs, during or after
the break-in period, then something is wrong. The fix may be as simple as
installing your gear leg fairings and wheel pants. All that drag makes the
engine work very hard and reduces your speed – both increase your engine’s
heat. Maybe your cowl openings are not correct. Give us a call, or better
yet, send us an email with photos of your airplane and your cowling

Oil Temperature and Pressure
Oil is the lifeblood of your engine; it lubricates and cools. Unfortunately
oil pressure senders routinely provide false highs or lows. Make sure yours
is properly wired – often adding a dedicated ground wire will correct a
faulty reading. You can also check your gauge’s accuracy easily by
temporarily installing a quality mechanical gauge. Low oil pressure must not
be ignored!

Low oil pressure together with high oil temperature is an indication the oil
is mostly sitting in the sump and not being pumped through the engine. This
is cause for immediate concern. Land and investigate. The fix may be as
simple as adding more oil. Or, ironically, in the case of the Jabiru
engines, you may have added too much oil. The Jabiru engine does not like to
be up to the full mark and oil temps will rise quickly if you have too much
oil. Go figure.

Perhaps the most import thing is to get to know your engine. A sudden or
gradual change in your engine’s EGT, CHT, Oil temperature or oil pressure
are indications something may be up. Park the plane and investigate. Use
common sense and deliberate troubleshooting techniques.

All of this is to say numbers are only as good as the collection,
interpretation, and reporting process. Don’t get hung up on the fact you can
only true out at 163 mph. Don’t fret because your CHTs are 30 degrees more
than Bob’s on the other side of the country. Be reasonable with your
expectations. CHTs will never all match. EGTs will never all match. An
airplane with only 10 flight hours has not been flown and documented enough
to provide ANY comparative data at all. We are flying hand-built,
recreational aircraft for fun, not competing for a government contract or
pink slips. None of us have the equipment to measure our performance to the
knot or the degree, despite what your $8,000 glass panel is telling you.

As always, please feel free to contact Betty, John, Jeremy, Kerry, Mark,
Heather Z, Jason, Stephanie, or Heather W at the numbers or e-mails below
with any questions or Comments.

Blue Skies,

Kerry Fores

Sonex Aircraft LLC
PO Box 2521
Oshkosh, WI 54903-2521
Tech Line: (920) 230-TECH (8324)
Mon, Wed. and Fri. 10 AM to 12 AM and 2PM to 4PM CT

Orders and General Info:
Fax (920) 426-8333

Please use the following e-mail addresses to Contact Sonex Aircraft:
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