30 DECEMBER 2008

“Switch off?” I called out.“Switch off!” was his reply.“Brakes on?” “Brakes on!” “Okay, give it a couple of shots

of prime, and I’ll pull it through four blades.”

I was standing at the front end of a 1943 Aeronca L-3 that a client of mine had recently purchased. His insurance mandated that he get 10 hours of training in the airplane prior to soloing it, and the Federal Aviation Administration mandated that he receive a tailwheel endorse-ment prior to acting as the pilot in command of the airplane. This was to be our first flight.

As I pulled through the prop of the 75-hp Continental engine, I made a mental note of the fact that the propeller seemed to give less re-sistance than I had expected. I ra-tionalized that the engine was cold and of low horsepower, and that I wasn’t pulling through a big Pratt & Whitney radial. Nonetheless…

“Okay, switch on?” I asked.“Switch on!” he confirmed.“Brakes set?” “Brakes set!” (Notice that I, the person provid-

ing the motive power to the pro-peller, was calling the commands, and not the other way around, as I have sometimes seen.) With that, I pulled the propeller, and the engine quickly fired to life, murmuring in a smooth idle.

I walked around to the side of the cockpit and groaned a bit as I man-aged to pull my aging bones up into

the back seat of this World War II observation plane. Once I secured my seat belt, and put on the headset that was connected to a portable in-tercom (at least I wouldn’t have to be shouting at the top of my lungs, and I have long ago given up the con-cept of smacking my client upside the head with a rolled-up sectional), I told my client we could start to taxi to the end of the runway.

The airport we were operating out of is a small, privately owned/public use airport, with a 2,300-foot runway. The runway is composed mostly of grass, but the underlying surface is shale and gravel. Although fairly wide at the north and south ends, it narrows to about 45 feet in the north third of the runway as it dips down into a hollow that is about 25 feet lower than the rest of the runway.

The winds that day indicated we would need to take off to the south, so we taxied to the north end of the field. As we came up out of the hol-low it took quite a bit of power to keep the airplane moving up the gradient to the north threshold. I was starting to wonder a little about the power of that engine hanging up in the front of the airplane. Was it really making all the horsepower advertised on the dataplate?

The engine run-up seemed nor-mal, with rpm drops of 75-100 rpm on each separate magneto, and there was no evidence of any carb ice when we applied carb heat. My client then went through the short and simple before-takeoff checks,

moving in a nice flow from the right side of the cockpit to the left. Every-thing was set.

Prior to walking out to the air-plane my client and I had discussed the lesson plan for the day. In the previous lesson we had spent a fair amount of time taxiing the airplane, including several high-speed taxis to gain the sight picture and feel of when to pick the tail up, and then as we cut the power, how to com-pensate for the loss of flight control effectiveness as we decelerated, still keeping everything headed straight down the runway. In this lesson we would be taking to the air, and thus all the important flight speeds had been briefed.

Rotation would be at 45 mph in-dicated airspeed (IAS). With some trees blocking our path another thousand feet beyond the end of the runway, we would then look for 60 mph IAS as our best climb speed. We briefed what we would do in the case of an engine failure, dependent upon when and where it might fail. We briefed the best glide speed and final approach speed. It was great for someone like myself, who suf-fers from CRM (can’t remember much), that those speeds were all the same…60 IAS.

So now, after checking for any other traffic, we taxied out to the center of the runway, and he ap-plied full takeoff power. It would be an outright lie to say that I was pushed into the back of the seat, but aided by the descent into the hollow, our acceleration was accept-

BY DOUG STEWART

Sometimes you’ve got to push . . .

THE Vintage Instructor

VINTAGE AIRPLANE 31

able. The flight controls came alive, and my client fed in forward stick and picked up the tail. Our airspeed indicator came alive; we had static power indicated on the tachometer and oil pressure. Now all we needed was rotation speed.

But we were now at the bottom of the hollow, and climbing up the other side diminished our accel-eration. However, by the time we reached the top of the hollow we had also reached rotation speed. With a gentle pull on the stick, my client lifted us off and we acceler-ated in ground effect up to 50 mph IAS. Yet now with the trees loom-ing out there beyond the end of the runway, my client started to ap-ply some more back-pressure to the stick. Doing so lifted us up out of ground effect, but then our climb stopped. The airspeed indicator was still indicating 50 mph.

With an ever-increasing sense of urgency, my client added some more back-pressure to the stick. (Mind you, this wasn’t a hard pull on the stick, just an almost unconscious ad-dition of back-pressure.) To his con-sternation, rather than climbing we remained at an indicated speed of 50 and started a slight descent.

My client was not the only one starting to experience a pucker fac-tor, although I am sure mine was nowhere near as intense as his, as I understood what was happening and had the solution to our prob-lem. Luckily we had the portable intercom and headsets, as I hate to shout, and as we had neglected to bring along a sectional, I hate to smack someone upside the head with my bare hand. I didn’t even have to shout into my microphone, but in a normal tone of voice I sug-gested that instead of pulling on the stick, my client should instead re-lax pressure on the stick and accel-erate us up to 60 mph. Once there, I was confident we would soon be climbing happily on our way over the trees. And indeed, as we tran-sitioned from the back side of the power curve to the front side, that anemic little Aeronca L-3 started

climbing in a manner that would definitely clear the trees.

The “back side of the power curve” almost sounds like some Star Wars fantasy, yet every pilot has at some time flown on the back side of the power curve. In fact, there is no way anyone could obtain even the most basic of pilot certificates without demonstrating to an exam-iner the ability to fly in slow flight while maintaining altitude. As one learns the techniques, the realiza-

tion should dawn that if our power is reduced to a minimum to sustain altitude at high angles of attack, any increase in angle of attack will ei-ther yield a stall (if you are truly at minimum controllable airspeed) or if not a stall, then a descent. This is the back side of the power curve, a realm of flight also known as the “region of reverse command.” It’s a place where the houses do indeed get bigger, rather than get smaller, when back-pressure is applied to the stick or yoke.

For many pilots, visiting this area of flight occurs only during the training for their initial pilot certificate, and they rarely, if ever, go there again. But the condition can and does exist in many differ-ent flight profiles. The space I am al-lotted in this monthly column does not allow for a dissertation on all of the physics involved with flight on the back side of the power curve; however, if you are truly interested, a Google search will yield numer-ous articles, written by folks much more knowledgeable than I, on the subject. An example would be Barry Schiff’s short article entitled Power and Pitch, published in his Proficient

Pilot series in AOPA Pilot magazine, March 1998.

Suffice it to say that it is an excess of power that really makes an airplane climb. If the engine of your airplane is of low power to begin with, and then if it is not making the rated horsepower that is advertised on its dataplate, you might very well find yourself in a situ-ation where weight, density altitude, runway alignment, surface, and/or slope might all, or in part, conspire to prevent a safe takeoff.

Or, you might find yourself in a situation as we were in. Unless we pushed the nose down a little and accelerated to best climb speed, we were bound to mush along right into the trees that stood in our path. For those pilots who fly small vin-tage airplanes with engines of lim-ited horsepower, remember that it is quite possible that the engine might not be making all of the horsepower it was rated for. In the situation I just described, I would not be surprised if, for a variety of reasons, the maxi-mum horsepower the engine was capable of sustaining was more than 70 percent of the 75 hp for which it was rated, or in other words, about 52 hp. The causes can include some very old and worn-out spark plugs and low compression, which were found on an inspection I mandated subsequent to the flight.

So don’t forge . . . sometimes you’ve got to push, if you want to climb. Yes, it is counterintuitive, and that’s what can get a pilot in trouble. There might also be times when you are going to need to push, but doing so will get you caught be-tween a rock and a hard spot, a place I found myself several weeks later. It can be a sinking feeling. (Yes, the pun is intended.) I’ll tell you more in the next article. In the meantime, may you be blessed with blue skies and tail winds!

Doug Stewart is the 2004 National CFI of the Year, a Master Instructor, and a designated pilot examiner. He operates DSFI Inc. (www.DSflight.com), based at the Columbia County Airport (1B1).

With an ever-increasing sense of urgency, my client added some more

back-pressure to the stick.