Page 19 - Volume 14 Number 8

ASK THE EXPERT
Rudder Boost Ramblings
Why do some King Airs have the Rudder Boost (RB) system installed and others do not? No, the answer is not that Rudder Boost was an extra- cost option, similar to a particular piece of avionics. Instead, some models were certified with RB as standard equipment and others were not. From the first King Air 65-90 model in 1964 up until the model 200 that received its Federal Aviation Administration (FAA) approval in late 1973, RB did not exist. The 200 was the first model that had it installed.
Realize that the 200 was a rather huge step up from its large-cabin predecessor, the A100. Among other improvements, it gained nearly 10 feet of wingspan, 1,000 pounds of maximum gross weight, 30 knots of speed, 1.4 psid of maximum differential pressure and 170 SHP (shaft horsepower) per side! With the engines mounted 25 inches further outboard from the fuselage centerline and with 170 more horsepower, keeping Vmca down to a reasonable figure was a challenge. The T-Tail came as a result of the Vmca-lowering efforts. The engineers had a worry: Would the force required to deflect the rudder to its maximum limit during Vmca testing exceed the FAA-mandated limit of 150 pounds?
Because there was a distinct possibility that the 150-pound limit would be exceeded, the engineering team came up with the design of the RB system and had it installed on the two model 200 prototype airplanes, BB-1 and BB-2. As the flight test program progressed, it was found that the maximum cockpit rudder pedal force required was 147 pounds. Thus, RB was found not to be required. However, it was decided to make it standard equipment on all 200s. Why? I think there were two main reasons. First, since Beechcrafts are known for excellent handling characteristics in all regimes, why make the pilot apply a force so close to the limit?
AUGUST 2020
by Tom Clements
Second, what about when the 200 evolved with more powerful engines, as it surely would someday? Since the 150-pound force limit would be exceeded, why not have the system already in use with a proven track record?
The King Air F90 model came out in 1978 and it shared many design features and systems with the 200, including the RB system. Required? No. The maximum required rudder force was well below the FAA limit, due to having 100 horsepower less per engine than the 200 and with the engine back in its previous location, more inboard. In 1984 the C90A replaced the C90-1. It shared many systems with the F90 and 200, including RB. Necessary? Of course not. Not only do we have the closer-mounted engines but they are putting out 200 HP less than the F90!
The same year that the C90A appeared, the 300 also arrived. For the first time, RB became mandatory to allow certification of this high-performing model. Now we had 1,050 horsepower mounted further out, at the same location as on the 200. The rudder force exceeded 180 pounds in the worst-case scenario. No longer was there an MEL (Minimum Equipment List) allowance to operate without RB. It became a mandatory item, rendering the airplane no-go when it was inoperative.
In a previous article and in my first King Air Book, I presented details about the design and operation of the three different RB systems that Beechcraft used. In short, the C90A (which includes all subsequent C90 variants manufactured after 1984), the F90-series, and the 200-series (including the present-day 250 model) all use a very similar system. The difference in left and right unregulated compressor discharge pressures (P3) is the trigger that tells the RB to operate and the actual force applied to the rudder cable comes from a pneumatic
KING AIR MAGAZINE • 17
(Photo credit: King Air Academy)