December 2013

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/234576

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Page 38 of 40

Hangar Debrief Dished Out By Keith Phillips, EAA 5973 Lifetime Figure 1 If you have spent any time at the local airport or associated with a group of pilots, you probably have heard stories like "Ole Hotshot buzzed the field, pulled up into a roll, and on the back side he dished out and hit the trees!" Just what does the term dished out mean? Basically Ole Hotshot didn't understand the basics of the gravitational pull of the earth and the 2g reversal from upright flight to inverted flight. Let's review this basic law of physics and how it affects flight, but before we get airborne we need to understand the static g forces. This can best be explained to the fledgling aviator by using an aircraft g meter. In Figure 1 you see a g meter in the upright position; notice the needles are pointing at plus 1g. The g meter has three needles; one moves as the g load is applied, and the other two record the maximum positive and negative g's. The needles can be reset to match the movable needle with the push button. Now look at Figure 2 where the g meter is inverted and note that the movable needle is now pointing at minus 1g. If you do the math, the total change is 2g. Note: The g meter is a self-contained instrument and a worthwhile addition to your instrument panel if you don't have one. Most general aviation aircraft and many homebuilts do not. A g meter not only records the maximum g's you have pulled during flight, but also it will show 2g when you are turning level in a 60-degree bank turn…just like you were taught during flight training. So what has this got to do with Ole Hotshot dishing out and subsequently crashing? Let's now transition to the dynamics of flight and see what happened. Rather than analyzing the varying g loads in Ole Hotshot's Figure 2 three-dimensional roll, let's review the g loads on a twodimensional loop, and to help simplify things, we will take some academic freedom and assume our souped-up Westland Wapiti or SX-300 will perform this loop at 4g and a constant 200 knots true airspeed. But first let's introduce the term radial g or centripetal force. This is the force or load factor that causes an aircraft to turn. Basically all aircraft turn as a function of radial g and TAS. With this in mind, let's review the academic loop. Looking at the loop in Figure 3, notice at the beginning of the loop that we pull back on the stick/yoke to establish a constant 4g on the g meter. The aircraft now has a 4g total load factor but is accelerating in a 3g radial vertical turn. (Remember, we have to overcome the 1g gravitational pull per Figure 1.) As the aircraft reaches the vertical position, the g factors are equal at 4g. However, as EAA Experimenter 39

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