The last article concerned itself with the common use - or misuse - of the cowl flaps on the ground for warm-up and other bad things. The following is slanted more towards operations, applying itself to in-flight usage and a little history of the cowl flaps. This can be a flight safety thing, believe it or not! Airplanes have been lost from cowl flap misuse.
The DC-3's cowl flap controls have a detent position placarded TRAIL. This simply means that hydraulic pressure is removed from both sides of the actuating piston and the cowl flaps are free to move to whatever position the airflow drives them, usually this results in being approximately halfway open.
On the EAA's B-17 we utilized the halfway position for takeoff for the first few years, sometimes verbally referring to it as the "TRAIL" position. However - in actuality - this wasn't really the same thing. The B-17 cowl flap system has no equivalent to the trailed position, instead it allows the cowl flaps to be positioned to any position visually, then to be locked in that position by selecting the LOCK position. This position traps hydraulic pressure on both sides of the actuating piston.
In later years we've adopted the procedure of taking off with the cowl flaps open on the B-17. This relieves the pilots of actuating the hydraulic selectors at a time when it is far more important that their attention be directed outside the airplane when taking the runway for takeoff. In actuality, all the military pilot's manuals prescribed this procedure for B-17's. However, it's also been largely true that all B-17's in civil service seemed to use a halfway setting for takeoff. This was probably influenced by the preponderance of DC-3 operators after the war with the inevitable spillover. We've been unable to detect a noticeable degradation of
takeoff performance with the cowl flaps fully open. However it does require the pilot to subsequently partially close them for the climb (after checking temperatures). I also should mention here that many of the other operators of B-17's that I'm aware of still use the mid/half/trail/x position for takeoffs.
On the B-25 we always took off with the cowl flaps fully open. After takeoff we'd move them to about halfway, after checking the cylinder head temperatures. All the a
On the other hand (and there always seems to be at least one other hand), there's the matter of the F6F Grumman Hellcat. With the cowl flaps open, this airplane's rudder capability (in my experience, anyway) for counteracting "P" factor and torque is marginal at best. It possesses one of the more marginal rudders during a high power "wave off" of any airplane that I've ever flown. I'm sure that this characteristic was more than adequately impressed upon fledgling naval aviators back in the airplane's heyday. It's rudder characteristics were sort of left for me to puzzle about, until I happened to describe my impressions to either Steve Hinton or Johnny Maloney, one or the
other, one day during a photo shoot. One of them was able to immediately diagnose the problem when he asked if I had been taking off with the cowl flaps open. When I answered in the affirmative, he mentioned that all Hellcats displayed this rudder airflow interference from wide open cowl flaps. When I made the next takeoff I used the halfway position and noticed a radical difference in the effectiveness of the rudder. Now, the problem with this. Unlike many other airplanes, the Corsair for instance, the Hellcat's cowl flaps are completely independent in their movement. They’ve, of course, been wide open on the ground. Prior to the takeoff roll, as you move the control towards close, one side may move rapidly all the way to closed before the
other moves at all. So you then finish closing them and then try it again from closed to open, striving to obtain some semblance of a halfway position. It usually takes several attempts; each side seems to have a mind of its own! Ultimately, you’re satisfied that they’re good enough for government work so you can then make the takeoff.
Some airplanes are placarded against flight with the cowl flaps opened beyond a certain number of degrees or inches. Witness the Boeing B-29, which has a prohibition against flight with cowl flaps opened beyond x degrees x inches. With this airplane and its inherent ground heating problem, the flight engineer characteristically partially closes the cowl flaps on the early part of the takeoff roll, not before. The Boeing 377 Stratocruiser (in military colors the C-97) has at least one accident on its record due to cowl flaps.
This involved a dual qualified Lockheed Connie/Boeing Stratocruiser flight engineer, combined with switches on the two airplanes being the reverse of each other for the Open-Close operation. The resulting elevator buffet caused an apparent increase in the stall speed and was serious enough that I used to demonstrate it to all pilots newly checking out in the airplane. In the C-97 a prohibition existed that the cowl flaps not be opened more than 3 inches in flight.
We should probably also mention the 1,000# additional allowable gross weight that the Convair 440 was certificated with, beyond that of the nearly identical 340. This was the partially the result of a rather (to us) controversial method of obtaining a small increase in demonstrated single engine climb performance after takeoff. The manufacturer redefined the MID position of the cowl flaps from that used on the 340. On the 340 the four cowl flap doors were all partially, but uniformly, closed in the MID setting for takeoff. The
engine’s cylinder head temperatures all remained within limits using the openings of this position. On the 440 however, for the MID position, the manufacture faired the upper cowl flap doors, while retaining the true mid position on the cowl flaps on the lower portion of the nacelle. This allowed some small reduction in drag, but at the expense of a noticeable increase in the CHT after operating the engines at full power during the takeoff. Accordingly, anyone who flew this machine much as a co-pilot quickly learned to, immediately after retracting the gear on takeoff, automatically reach up and give them about a “one potato - two potato” count
towards open. As in their method of ice protection, General Dynamics (Convair) used some rather interesting tradeoffs, I know I wasn’t alone in thinking these made some excessive demands upon the R-2800 engines. But - so be it, life went on in the big city!
things to write yet:
dc-3 trail, b-17 open/locked, b-25 half?
is "one thousand one-one thousand two" clearer?
f y i from some manuals I just happen to have here, sampling, some mention excess runway required or else buffeting, must be test pilots reports or finding, I don’t really need to find out at this late date
Connie-50% with curtis electrics or 30% with hamiltons.
consolidated pb4 privateer-
a20-closed top and open bottom
R. Sohn © 1999
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