SonexAus – Kerry Fores 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 provide.
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 comparison.
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 opportunityto 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 ground.
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 pecision 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 installation.
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