{"id":2013,"date":"2019-07-22T06:57:13","date_gmt":"2019-07-22T10:57:13","guid":{"rendered":"https:\/\/alecmyersflighttraining.com\/info\/?p=2013"},"modified":"2021-03-11T22:33:08","modified_gmt":"2021-03-12T03:33:08","slug":"front-side-of-the-curve","status":"publish","type":"post","link":"https:\/\/alecmyersflighttraining.com\/info\/front-side-of-the-curve\/","title":{"rendered":"Front side of the curve"},"content":{"rendered":"

The “power required” curve for a sample airplane, from <\/em>Theory of Flight (Richard Von Mises, McGraw Hill, 1945). 150 feet per second on the horizontal scale corresponds to about 90 knots, and 100,000 ft.lb per sec. on the vertical scale is about 180 bhp, so this aircraft is faster and more powerful than a modern training aircraft but the shape of the curve still applies.<\/em><\/p>\n

Raising the nose<\/h4>\n

The scenario: our single-engined training aircraft airplane is set up in a stable descent on approach to land. The configuration is appropriate, perhaps with partial or full flaps extended. Airspeed is somewhere between 60 and 80 knots. What happens to the flight path of the airplane if the pilot pulls back on the yoke and raises the nose? Stop and think about the answer for a minute, then read on.<\/p>\n

Short term response<\/h4>\n

Before we start, the aircraft is in a steady state descent: the forces are in balance and the power from the engine and the gravitational potential energy released by the descent are exactly enough to pay for the work the airplane is doing moving against drag.<\/p>\n

\"short<\/div>\n

The short-term response to a pull-back on the yoke is for the aircraft to “balloon” upwards and to slow down. The balloon occurs due to the increase in angle of attack creating an increase in lift which causes an immediate upward vertical acceleration. <\/span>1<\/sup><\/a><\/span><\/p>\n

The slow-down occurs because with the airplane not coming down as fast, not enough gravitational energy is being released to pay for the drag at the old airspeed. This is like a car starting up an incline: without added engine power it will slow down. John Denker calls it the law of the roller coaster<\/a> in his excellent (but very technical) online book See How It Flies.<\/a> <\/p>\n

As an aside, the balloon doesn’t have to cause an increase in altitude: it could just be a sudden relaxation in the rate of descent.<\/p>\n

The short term response is over in about three or four seconds, but what happens next is more interesting.<\/p>\n

Long term response<\/h4>\n

Once the the angle of attack has increased and the airspeed decreased, and the vertical acceleration has ceased (because the forces are back in balance) how does flight path angle – the angle of descent – differ from before? The answer depends on whether the aircraft is on the front side<\/em> or the back side <\/em>of the power curve. <\/p>\n

\"aircraft<\/div>\n

The power curve relates the power required to maintain stable flight (along the vertical axis) against airspeed (horizontal axis). To fly either very fast or very slow both require a lot of power; To fly at intermediate speeds requires less power. The very bottom of the curve is at the airspeed at which the aircraft can fly using a minimum amount of power – this is pretty close to the airspeed for “best endurance” which you will recall from your pilot training. <\/p>\n

We say that an aircraft is flying on the front side or back side of the power curve when it flies faster or slower than the minimum power airspeed, respectively.<\/p>\n

If the airplane stays on the front side of the power curve – the airspeed remains faster than the minimum power airspeed – pulling on the yoke causes<\/p>\n