Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.
Issue link: http://experimenter.epubxp.com/i/339662
36 Vol.3 No.7 / July 2014 FLIGHT TESTING TECHNIQUES THE FORWARD SLIP. Ah, the solution to a high start on fi nal approach. This handy way of increasing descent rate and angle without changing airspeed is also a method of testing an air- plane's lateral and directional static stability. Big "but" here—before you stomp in full pedal, you should make sure your airspeed indicator is telling you the truth in the sideslip. The airspeed indicator determines airspeed by mechanically subtracting the static pressure, sensed through the static port, from the total pressure, sensed through the pitot tube. A change in pressure from either source will cause a change in the indi- cated airspeed reading. Imagine an airplane with a single static source located on the right side of its fuselage. If that airplane is in a nose-left sideslip, the relative wind is approaching from the right. The air is no longer fl owing purely past the static port, parallel to the fuselage; some of it is forced into the static port. This ram ef ect raises the pressure in the static system. If there's no change in total pressure sensed through the pitot tube, the erroneously high pressure in the static system results in an airspeed indica- tion that is slower than the plane is actually fl ying. The opposite ef ect occurs if this airplane is in a nose-right sideslip. The sensed static pressure would likely be lower, and the indicated airspeed would be erroneously fast. Return the airplane to sideslip-free fl ight, and the airspeed indication returns to its correct value immediately. You don't notice the dif erence in airspeed when you per- form a forward slip maneuver because you adjust the plane's pitch attitude to maintain the original indicated airspeed, whether it's erroneous or not. One reason you see a static port on each side of an airplane's fuselage is to help ensure a truer static pressure in the system. Airplane designers take a lot of care locating the static ports. It's often a trial-and-error process fi nding that sweet spot where the air fl ows parallel to the port during most of the fl ight envelope. The pitot system can experience similar errors. Ideally, the pitot tube would be pointed directly into the relative wind to achieve the most accurate total pressure, but this occurs at only one angle of attack. In a sideslip, the pitot tube is at an angle of attack, just from the side. The errors caused by pitot tube orientation are typically small for reasonable angles of attack and sideslip when compared with the static pressure error potential. A ND A NO T HER T HING Even if there are no pitot or static errors in the sideslip, you may notice a change in airspeed upon releasing the pro-slip fl ight controls. Picture an airplane with its pitot tube near the left wingtip in a nose-left sideslip. So, we have left pedal, right stick, and whatever forward or aft stick is necessary to maintain a particular airspeed. The relative wind is approaching from the right of the airplane's nose. Re-center the fl ight controls, and the plane yaws nose-right toward the relative wind. During this yawing motion, the left wing momentarily moves forward faster than the rest of the airplane. The total pressure in the pitot tube is now the sum of the airplane's forward speed plus the increment of speed at the pitot tube as the left wing swings forward. Add the likely decrease in pressure sensed at the static port on the right side of the fuselage as the ram ef ect is removed, and you'll likely see a jump in indicated airspeed. Perform this mental experiment from a nose-right sideslip, and you'd see the airspeed drop when the pro-slip controls were released. In this case, the total pressure in the pitot tube would be the airplane's forward speed minus the speed of the retreating (relatively speaking) pitot tube at the left wingtip. Airspeed variances due to yaw rate most easily can be observed during slow speed fl ight in an airplane with a long wingspan and a pitot tube located near its wingtip. The longer wingspan means the tip travels faster through the air for a given yaw rate. The slower the airplane is fl ying, the greater the infl u- ence of yaw rate on airspeed indication, because it's a bigger fraction of the airplane's forward speed. Let's get back to that "but" mentioned earlier. Determining airspeed error in a sideslip should be approached cautiously. If the error is such that the indicated airspeed reads erroneously fast during the sideslip, the plane is actually fl ying slower than indicated. Because we're most interested in the sideslip, or for- ward slip, maneuver on fi nal approach, you're probably fl ying low and slow with landing gear down and fl aps defl ected. Now consider that fl ying out of balanced fl ight can increase stall speed. And consider that one wing is more likely to stall before the other in a sideslip. In addition, don't forget the two ingredi- ents for a spin are a stall and a yaw rate. So be careful. HOW T O CHECK FOR A IRSP EED ERROR Apply a little pedal and countering opposite stick while main- taining the original indicated airspeed—just a little. Stabilize there. Then abruptly return the fl ight controls to their pre- slip positions while watching the airspeed indicator. You may notice the airspeed needle bounce a little due to the yaw rate, but if it quickly settles back to its pre-slip indication, there's no airspeed error. Slips Is your airspeed indicating accurately? BY ED KOL ANO E A A E X P _ J u l y 1 4 . i n d d 3 6 EAAEXP_July14.indd 36 7 / 1 / 1 4 9 : 5 8 A M 7/1/14 9:58 AM