In a helicopter, drooping is a term that refers to a slowing of rotor rpm below what is considered normal. In the Navy, we used percents, but in some helicopters, the actual rpm is used. But either way, the concept of drooping is the same. For the sake of discussion, and as a Naval Aviator, percentages will be used in this post.
So, if the rpm under “normal” operations is is 102% and the rpm decays for whatever reason, to 96%, then the pilots have “drooped” 6%. Normal operating rpm is determined by the manufacturer and is usually the ideal rotor rpm where the most lift is generated. Go much above the normal range requires more power, increases wear and tear on the rotor system and generates less lift. Go much below, and lift drops off, abeit slowly.
Why is this important? When back in the bad old days when helicopters didn’t have the surplus power they have now, to get off the ground, we had to pull up the collective, watch the rpm drop and hopefully, at some point, the helicopter would lift off before the rpm got to some bottom limit.
In the H-3, the magic number was 94% because that was 2% above the speed when the generators would drop off the line. At 88%, the hydraulic pumps would stop pumping and at that point, nothing mattered anymore because the helicopter was now uncontrollable and soon, you would be dead.
I flew four models of the H-2, the single engine A/B, both the UH-2C, the HH-2C and the HH-2D. All had the same main rotor system. The HH-2C and HH-2D had four bladed tail rotors while the A/B and UH-2C had tail rotors with only three blades. The A/B and the UH-2C also had the same transmission while the one in the HH-2C/D was an upgraded unit.
In another post on the site, I describe the difference in flying the different models. Simply put, with only one engine in the A/B, one barely had enough power to turn the rotorblades. In the C, you had twice as much power feeding the same transmission and could easily over torque the transmission. Too much torque and the main shaft in the transmission could fail and the main rotor blades would depart the helicopter and ruin your day. The other problem was that at high power settings, we could run out tail rotor authority and the helicopter would rotate around its vertical axis. The HH models fixed those problems.
In the twin engine H-2s, drooping never happened. In the earlier single engine A and Bs, drooping was a way of life.
I started our flying the A and B models and the first thing I learned was that we would take off, depending on the density altitude with much less than fuel. According to my NATOPS manual (Navy’s version of the pilot’s operating manual) you could load 1,883 pounds (277 gallons) of jet fuel into the internal fuel cells. And, the helicopter could carry an addtional 780 pounds (115 gallons) in external tanks. Never did I ever take off in an A/B with external tanks. Most of the time, we launched with the forward, a.k.a. the sump tank full which mean we had about 700 pounds (103 gallons) on board. A short roll forward got us past 15 knots, into translational lift and the helicopter was airborne.
Hovering with 700 pounds of fuel on a hot, humid day at sea was another story. To hover, we routinely dumped fuel down to about 400 pounds. Even then, the rotor rpm would drop from 100% to 98% or even as low as 96%. No big deal. Once whomever or whatever you were picking up was on board, simply dump the nose, get back into translational lift and away we went.
Since my introduction to the H-2A/B was in the summer and by the ocean, the weather was always hot and humid. On the pilot’s or HAC’s collective, there was an rpm switch which enabled the pilot to beep up the engine speed to force the engine to develop more power. This had its limits and drooping was routine. As a brand new co-pilot, I watched several HAC’s calmly let the rotor rpm droop to 92%!
I’ve talked to many Army UH-1 Huey pilots who flew in Vietnam and asked them about drooping. They laughed and said they drooped, banged their skids along the runway trying to get into translational lift.
Today, in the H-60 series helicopters flown by the Navy, drooping is considered a bad thing. My view is that teaching pilot how to use drooping to expand the performance envelop is important. The technique gives pilots confidence that the helicopter flies quite well at less than optimum rpm. This is important to know if one engine has failed, and the helicopter is heavy and is flying at either high altitude or hot and humid conditions or some combination.
To an old guy helicopter pilot, to droop or not to droop is a matter of perspective.