101 Aviation Lies
I am 28, have 10,000 hours, most as a Boeing 777 Captain
Hey, I with the FAA and I am here to help
There I was at 12,000 feet and I ....................
I made a few changes and my engine makes at least 50% more power
The new "Sky Scooter 3000" does 250 MPH on 150 HP at 6 gal/hr.
AND
My RV cruises 220 mph on 55% power
Airspeed is one of the biggest areas of confusions, misunderstanding and exaggeration. We assume it is "True" but who knows what it is and what errors are involved sometimes.
In the 1980's kit manufactures made some seriously optimistic claims for kit planes that had never flown yet. This is what separated Van's from the others early on, RV's do what the spec sheet says or even better. While other wounder planes proved to have poor performance or ended up never flying, despite selling kits.
Reported airspeed should be true airspeed (TAS), which makes sense. You want to know how fast you are going thru the air (or over ground), like a car. TAS with now wind is equal to how fast you will get there.
To compare apples and apples you need to know flight (air density), power (RPM and MAP) and aircraft weight.
As an
unofficial standard for piston powered planes there are TWO conditions typically reported for doing comparisons. They are:
CRUISE: 75% power at 8,000' DA, typically wide open throttle (WOT) and RPM set to cruise, nominally 2,500 RPM
TOP SPEED: 100% power at sea level, wide open throttle (WOT) and max RPM (This is the typical top speed for normally breathing engines; turbo airplanes may report top speed at the "critical altitude", which is the altitude a turboed engine can maintain 100% sea level power.)
If you-all test at these two conditions than TAS or Ground speeds can be compared directly. The method of determining the TAS or ground speed with wind component accounted for is a matter of good flight test procedures. TAS and ground speed are the same with zero wind for the purpose of our discussion.
One of the best methods of getting ground speed is fly three legs with the GPS on constant track courses, that is course not heading. Note the course and GPS ground speed for the three legs. Using an excel spreadsheet you can resolve the ground speed into aircraft speed with wind effect deducted. RV-8 builder has the spreadsheet for down load:
Kevin's flight test page Even with all the care in the world you still can be off +/- 1% to 3%. If the air mass is not calm, the vertical components can reduce performance. However the above method should be close. Best to test early in the morning. Of course you have to be a good stick and fly smoothly and exact.
You can't account for the accuracy of data with out knowing who gave, how they measured it and in what conditions. How fast does it go is still one of the top 101 lies. By the way I am dating Morgan Fairchild, yea that's da ticket.
George
PS: One of the best top airspeed BS eliminator is racing. Look at the sun-N-fun 100, Copper State Dash or the Airventure challenge races. Even those numbers are slightly elevated. For example the SNF 100 is not quite 100 miles and the speed is distance/time. They use 100 miles not the actual distance that is a few miles less.
