Some thoughts of flight testing.

Vance

Gyroplane CFI
There is a current thread on Vx and Vy that has expanded to learning what is in the Pilot’s Operating Handbook and applying it.

Some of us don’t fly an aircraft with a POH and we need to find the numbers during the phase one test period and write our own POH.

Even those that do have a good POH, finding the actual data for their particular aircraft may have value.

I find value in just thinking about how to apply the information.

I feel there is value in imagining the emergency procedures and writing them down.

FAA AC 90-89A has a lot of useful information about flight testing and can be found here:
http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC 90-89A.pdf

I admit that it is not all applicable to gyroplanes. I feel there is enough value to spend some time reading it even if your gyroplane has been flying for some time.

We don’t know what we don’t know and a little study may point us in the right direction.

I suspect the FAA is going to get more focused on fundamental flight test data because they realize that many people think of the phase one flight test period for experimental aircraft as fly off time rather than a flight test period. The FAA is trying to address the significantly higher accident rate of experimental aircraft and better flight testing is one way to do it.
 

All_In

Gold Supporter
Hi Bro

Could someone with more time post a link to Chris's testing documents (it's the best I've ever seen) so we could have everything on Vance's most excellent thread.

U-Rock Vance great idea as I and most have never had to do this before!
 

ckurz7000

Senior Member
Thanks, Vance, for opening this thread. As I already mentioned, flight testing your gyro brings a lot of benefits:

  • It really gets you to explore the flight envelope of your machine and become intimately familiar with it.
  • It makes you a better, more precise and aware pilot.
  • It teaches you a thing or two about the underlying theory.
  • It gives you the basis for more accurate flight planning if you fly cross country.
  • It is in line with the intentions of the FAA to evolve phase 1 flights from merely burning fuel in the air to a meaningful flight test regime.
  • It provides a basis to compare different gyros' performances.

The corresponding Advisory Circular published by the FAA already contains a lot of useful information, even though the AC is not written specifically for gyros. It covers necessary ground tests you should perform before taking to the air for the first time. Did you know that 80% of all failures during phase 1 have to do with either the fuel system or the power plant? Most of these issues can be ironed out safely on the ground.

But once you have a flying gyro, the -- to me at least -- fun part of flight testing begins.

First order of the day is to calibrate your ASI. Believe it or not, most ASIs in open frame gyros (and many also in closed ones) indicate 10-15% wrong (almost all too high)! What the ASI indicates is IAS (indicated air speed). And IAS should be as closes as possible to CAS (calibrated air speed) not GS (ground speed). There are some common misunderstandings in calibrating an ASI properly.

There are those people who believe an ASI system can be calibrated on the ground using a predetermined source of pressure connected up to the pitot tube and reading off the indicated airspeed from the gauge. This, however, brings only a relatively minor source of error to the daylight: the instrument error. Instrumen error is the error inherent in the ASI gauge itself. Most gauges come with a calibration chart showing you exactly what that error is over the entire speed range. So there really is no point in measuring it. In a new instrument it is usually of the order of 2%.

What you really need to do is determine the installation error. For this you need to go fly the aircraft and have real static and dynamic pressures on your system.

There are those people who believe comparing your GPS derived ground speed to the IAS will give them a good calibration of the ASI. This does leave out several possible sources of error. Most important is the effect of wind which will be different at altitude.

Going one step further is to fly a course and its reciprocal course and taking the average of the GPS derived ground speeds. Still, this only works if you are flying perfectly into and against the wind. Any amount of cross wind at altitude will introduce errors. Atmospheric conditions deviating from ISA (international standard atmosphere) will also cause errors.

If you really want to do it properly, you need to fly three different ground tracks at a predetermined IAS. Mathematically, this is sufficient information to eliminate the effect of any wind component and separate out the ground speed from the wind effect. (If you fly four instead of three tracks it will give you four data points instead of one for the price of only one additional leg.)

But that still isn't good enough. What you are ignoring is the effect of density altitude on the ASI. On a hot and humid day with low atmospheric pressure prevailing, the ASI will indicate significantly lower than on a cold, crisp winter day when a high pressure system is in the area.

So here is what you need to do to calibrate your ASI properly:

  • Pick a day with a calm atmosphere or, at most, steady light winds. Turbulence or thermal activity will make it impossible to derive any useful data.
  • Go up flying and pick a predetermined height at which you make your suite of test.
  • Record OAT at altitude and atmospheric pressure (QNH) so that you can determine density altitude later on.
  • Pick an IAS and a ground track. Keep IAS nailed to +/-2 kts and +/-3 degrees. Fly for 10 seconds to let the gyro stabilize itself. Then record GS and track. Now change the track by about 90° and do the same IAS for that track. Then repeat it again at a track about 90° off from the second one. And, lastly, turn about 90° with respect to the third track and do it over for the fourth time. In essence you flew a more or less rectangular course at a constant altitude and constant IAS, recording GS and track for each leg.
  • Repeat the last step for every 10 kts of IAS so that you cover the entire speed range between Vx and Vne.
  • Once back on the ground you will need to enter your data into a spreadsheet and do some data analysis. I have such a spreadsheet which I will post once I get some time. (Still need to tidy it up and translate it to English.)

This procedure will give you a fair calibration curve for you ASI. You will be surprised, how far off most ASIs are, by the way.

Once you have the ASI calibrated you know your airspeeds. This forms the basis for all further investigations. You need to defiine a reproducible configuration (e.g., at what weight, with/without doors, etc.) at which you perform all further test flights.

Next, I would suggest to measure the climb performance and derive Vx and Vy for your gyro. Then determine best glide speed (which will be close to Vy) and glide ratio. Then go and measure cruise performance. Next come take-off and landing distance over a 50 ft obstacle. Then you determine the H-V curve. And, of course, the entire suite of static and dynamic stability tests.

Once you are done, you may want to re-do this for different altitudes or loading conditions.

So, you see, there really is a wide terrain for flight testing, limited only by your patience and persistence. During the course of test flying you will become a better pilot, will come to understand your machine better and learn a trick or two on the way.

Greetings, -- Chris.
 
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