I always enjoy reading the articles of my fellow contributors at R&WI. Frank Lombardi in his January column, “The Power Struggle” (January 2018, page 39), gives a great description of profile, induced and parasitic drag and how each relates to the total drag on the helicopter. I’d like to take his explanation out of the academic and into the cockpit.
Total drag is represented by a U-shaped curve where the bottom line is speed and the vertical line is drag. The bottom point of the curve is referred to as the bucket speed, or more technically correct, Vimd. This speed represents two important data points for the pilot: best autorotation speed for slowest rate of descent and best vertical climb speed.
This speed does not represent the best glide speed in autorotation for these two things: greatest distance over the ground for a given vertical decent and best angle of climb that is vertical distance gained for horizontal distance traveled. The difference between each is very important.
As an example, in a Bell Helicopter 412, bucket speed, clean, is 70 kt. So best rate of climb and least rate of descent in autorotation is 70 kt. This is opposed to 60 kt with the main cargo doors open. Best angle of climb is 45 kt, and best glide speed as taught to me by Bell is 90 kt.
The significance of the aircraft with the doors open is that it generates more parasitic drag, therefore the curve or bucket moves to the left on our graph (or slower) and also the rate of climb on our 412 is 275 ft per min less at 60 kt with the dirty configuration versus 70 kt clean. Taken one step further, if you were to maintain the 70-kt speed, but your aircraft was in a high drag configuration, that 275 ft per min reduction in climb performance will become even greater.
So much for the theoretical; now for the practical.
The graphs that are in your flight manual represent a clean aircraft. No lights, cameras, hoists, steps, tundra paws, water drop tanks — you get the point. Most of us are not flying a clean aircraft. When I went to the MBB 105 transition course in Westchester, Pennsylvania, in the 1970s, a couple of things happened.
MBB was intellectually honest, and the helicopter business hadn’t become overrun by lawyers. Unlike today, original eqiupment manufacturer instructor pilots and schools actually discussed things like the fact that a MBB 105 is not a particularly stellar performer on one engine. My take on it was the 105 was an aircraft in which you were twice as likely to have a critical engine failure. As part of its curriculum, the school taught you to develop your own Vimd speed.
This is accomplished on a still day. Pick an altitude and weight that most closely represent your typical flying environment. Start at, say, 55 kt in level flight. Without changing power, accelerate to 57 kt, note your rate of climb or descent, continue increasing speed in two-knot increments and record your rate of climb at each speed.
Assuming that the 55 kt was to the left or low-speed side of Vimd, your rate of climb should increase for some number of knots and then began to decrease. If the climb performance decreases with each upward change in speed, then 55 kt was above the Vimd speed and you’d need to start at a lower speed.
The point where rate of climb begins to decrease is the Vimd for that aircraft in its particular configuration. There are other variables, but they are not nearly as big a factor as parasitic drag. We are not blessed with a full flight-test facility so this is a reasonable approximation.
Now for the war story: After attending the MBB school, I was backing off a one-way in one-way-out heliport on an offshore oil platform probably 150 ft off the water. I had half a tank of fuel and two passengers. The wind was blowing 15 to 20 kt out of the west, and I was backing up with the nose pointed to the west. About the time I had backed up far enough to clear the platform, one engine quit. The immediate response would be to pull all of the power that the other engine would give you without bleeding rotor rpm.
One thing MBB taught me in this situation is to not lower the nose more than 2 to 4 deg from your trading maximum change in airspeed for the minimum decrease in altitude. Fly the aircraft to bucket speed. Things to ponder before they happen. RWI