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DANGER

BACKSIDE OF THE POWER CURVE! By Dr. Igor B. Bensen, President Bensen Aircraft Corp.

P.O. Box 31047 Raleigh, NC 27622 The following article by Dr. Igor Bensen is from his presentation during the 1981 Oshkosh Design College Forum. Dr. Bensen is, of course, the inventor and developer of the Gyrocopter.

IT MAY BE a quirk of nature. Or, it may be God's

will. But know this: "IT TAKES MORE POWER TO HOVER AND TO FLY SLOWLY THAN TO FLY FAST." If you don't know this and you fly, sooner or later you may find yourself in serious trouble.

standing still takes no energy at all, and it takes less power to move at 20 mph than 40 mph. Right? WRONG, when you fly! True, when you sit on the ground, "terra firma" provides your support against gravity. In that case, it does take less power to move slowly than faster. But when this support is provided by the air, you get into a whole new ballgame. You had better learn its rules now, or else prepare yourself to be "self-destructed". Power Required Curve — PRC

This little-known law of nature applies to all heavierthan-air things that fly: from eagles to hummingbirds, from butterflies to gnats. Did you notice that they never hover, nor fly slowly unless they absolutely have to? Do they know something you don't? Well, they all know this law instinctively, as well as from their daily experience. Only to man does it

In the 1940s the civilized world witnessed the arrival of a new successful man-made flying machine — the helicopter. It was almost bewildering to old-time aviation buffs to see these new wingless birds hang motionless in the air, going nowhere, but with their engines roaring at wide-open throttle. Why? The key to understanding the "whys" of power need-

have to be an acquired knowledge. It seems that it defies

ed for flight can be found in the "Power Required

man's "common sense", which in this case is faulty. Our human "common sense" reflexes tell us that

Curve", or PRC. Engineers measure and express important physical values in numbers, which they then SPORT AVIATION 31

plot graphically in form of curves. For man-made flying machines, they obtained characteristic Power Required

understood and abided by a student pilot. After all, this is what he always does when he wants to walk or

much horsepower is needed by a helicopter and an airplane to fly at any speed at any given gross weight.

the boat faster. More power gets more speed — that's natural. This is the "positive" slope.

Curves shown in Figure 1. Stated simply they tell how

ride a bicycle faster, or to drive faster, or to row

HP*

100

60

FIGURE 1. Power Required Curve.

MRP STALL

AIRSPEED,V

Several important conclusions fairly jump at you from looking at this diagram. First, it is at once obvious that a helicopter can hover at zero airspeed, while an airplane can't. No amount of power can make an airplane hover, because it stalls and simply quits flying below a certain airspeed. Curiously, it stalls in just about the region where a helicopter uses least power. Secondly, the shape of the Power Required Curve is quite specific and should be memorized by all who fly. We see, for instance, that for a helicopter it takes MORE horsepower to hover than to fly forward. If horsepower to hover is pegged at maximum available 100 percent, then to fly at "Minimum Power Required" (MPR) speed may require only 60 percent of that power. That special MPR speed is usually 45-55 mph for most small helicopters. It may be somewhat higher or lower for other types of high-speed or low-speed designs, but the shape of the PRC curve is always the same. The airplane shows the same trend, except that it won't hover and its region below the MPR speed is much narrower. PRC Slopes

Of special interest to all pilots are the slopes of the PR curves. This, incidentally, applies to ALL heavierthan-air flying machines, not only to helicopters and airplanes. Every helicopter and every aircraft (birds and

bees included) have a certain airspeed which requires the least amount of horsepower to maintain level flight. With the foot-pedaled Gossamer Albatross that flew across the English Channel, it was 10 mph. For most private airplanes, it is 60-80 mph. For the Gyrocopter,

it is 45 mph; for a passenger airliner., it is 140 mph, for

the Space Shuttle Columbia, it is 240 mph. The important thing to remember is that at this MPR point the slope of the PR curve reverses. It changes from positive to negative as the aircraft slows down. The power required to fly sort of bottoms out at this MPR point, meaning that more power must be added to fly either faster or slower than at that speed. The faster portion of this requirement is easily 32 MAY 1982

He also knows that he can go faster by going downhill, or driving, by extracting extra horsepower from the available potential energy, at the expense of losing altitude. Now, what happens below that special MPR airspeed is not at all natural to human instincts. Let us add here in passing that for most aircraft this MPR point is also the airspeed of "best L/D (lift-over-drag) ratio" and corresponds to the "minimum rate of descent" gliding airspeed power-off, as well as the airspeed for "longest endurance". So, what happens when you fly below this critical MPR airspeed? You must add power, right? That's what the PR Curve says you should do. So far, so good. So, you add more power and are now flying at, say, 30 mph. Can you forget it now? NO WAY! Now is the time to summon all your attention and skill, because you are now flying on the dreaded negative "Backside of the Power Curve". Positive and Negative Slopes

To a pilot, there is a vast difference whether he flies on the positive or negative slopes of the PR Curve.

On the positive slope, more power is required to fly

faster. When you open the throttle, you reach a new and higher airspeed and stop there. At any constant power

setting, if the craft slows down for any reason, excess power will make it speed up to trim cruise again. If it

speeds up too much, such as in a dive for instance, bringing it back to level flight would slow it down to trim cruise again. This "more power-more speed" is a stable flight condition and is characteristic of the "positive" slope of the PR Curve. The negative side of the PR Curve is a lot more

troublesome to the pilot. "Less speed-more power" is an unstable condition, because at any fixed setting the craft wants to fly either slower or faster than at trim

speed, as well as to lose or to gain altitude. Negative

slope of the backside of the Power Curve thus is unstable and requires the pilot to constantly monitor his air-

speed and altitude, and to "chase" them by continuously

jockeying the throttle to keep them constant. If he fails to do this, especially during maneuvers, he will find himself unexpectedly settling down, or ballooning up, without any obvious reasons. This becomes crucial when

maneuvering in tight quarters close to the ground and has caused many accidents in the past. The annals of fixed wing aviation are overflowing with accident reports of crashes on landing approaches when pilots allowed their planes to slip behind the backslope of the power curve. This, of course, applies also to rotorcraft. Worse During Turns and Glides

More power is required in a turn because centrifugal force is added to the weight of the aircraft. Thus, power must be also added to execute correctly any level turn. It's bad enough to remember this when you fly on the positive slope of the PR Curve, but it may become downright disastrous on the backside of the PR Curve. The effect of the negative slope doubles in a 60 degree turn — a bank any rotorcraft achieves handily — meaning that you either: (1) must double the on-board horsepower output (an impossibility), or (2) be prepared to lose altitude to use up available potential energy, or (3) to lose even more airspeed and get further back on the negative slope of the PR Curve. No matter how you look at it, even if you don't crash, it will be sloppy

piloting. Seasoned pilots, the birds and the bees, don't

fly'sloppily. To the comfort of rotorcrafters, it should be said that their machines never run out of lift, even at zero airspeed, but they most certainly can run out of altitude. And flying on the backside of the Power Curve is the quickest way to run out of altitude. Gyrocopters No Exception

Gyrocopters deserve a special mention here, because they are special hybrids between airplanes and helicopters but must still comply with the same laws. Their PR Curves fall somewhere between helicopters and airplanes. Their flight characteristics are best shown in the Figure 2, which is a diagram of Rate of Climb (and of Descent) vs Airspeed. It is more descriptive and revealing than the standard PR Curve. What we see in Figure 2 is ability of the aircraft to fly and climb at any given weight and horsepower. You will notice at once that an airplane (A) has almost double the rate of climb of helicopters (H) and excels in this ability at higher airspeeds. But look at what happens

on the slow speed end! The airplane quits flying as its airspeed drops below stall, but the helicopter and Gyrocopter (G) reach their maximum in that area. Then as the airspeed is slowed down further, the Gyrocopter loses its ability to climb and at 15 mph

begins to descend. At zero airspeed, a helicopter can

still climb in a vertical ascent, but a Gyrocopter bottoms out in its vertical descent. The important thing to remember is that unlike an airplane, the Gyrocopter does not drop out of control and does not quit flying at zero airspeed, but has a limited and controllable rate of descent. Notice that its rate of descent is decreased even when the Gyrocopter flies backwards, or sideways. For this reason, experienced gyro pilots who wish to lose altitude quickly either slow down their airspeed below MPR, or alternately side-slip right and left to control their rate of descent. Airplane pilots do this, too, but their range of

maneuvering near the ground does not include zero airspeed.

Know Your PRC and Watch Your MPR!

To sum up then, you must chisel in your mind as in concrete the basic knowledge of the Power Required Curve (PRC): (1) Determine as soon as possible what the airspeed of MPR, Minimum-Power-Required, is for your aircraft. (2) Know and react instinctively to nature's law that MORE power must be added to fly either faster or slower than at MPR speed. (3) Flying slower than at MPR speed is flying "on the backside of the power curve". This puts you on the NEGATIVE slope of the Power-Required Curve and is UNSTABLE. It requires constant jockeying of the throttle and monitoring of instruments to maintain level flight. (4) Not only is the airspeed unstable on the backslope of the Power Curve but also the altitude. (5) Turns and banks require more lift and have a higher MPR speed. Prepare to lose airspeed and altitude in almost ANY steeply banked turn. Especially, don't get caught making steep turns at slow speeds near the ground. It's really not all that complicated. If birdbrained birds know it, and abide by it instinctively, so can man. Now that you know it, make your awareness of it instinctive too, and never, never get caught unprepared

on the "Backside of the Power Curve!"

R/C

FIGURE 2. Rate of Climb vs. Airspeed.

R/D SPORT AVIATION 33