>

- # Properly Setting Cowl Flap Positions Flight-Sim Pilots will generally use one of three procedures to set Cowl Flap positions. Here they are, from the most simple to the real-life. Read all three procedures and decide which suits you best. Pilot #1 will completely ignore the Cowl-Flap position issue, preferring to fly 'just for the fun of it.' And that's OK. Although it's desirable to grow technically over time, there's room for all in this great hobby. These pilots probably aren't even here reading this section. But if they were, they (and all) should be aware of one fact: When fs9 opens, the Cowl Flaps default to the OPEN position. That will cost this pilot from eight to fifteen knots in airspeed, ignoring the deleterious effects on the engines, which might cost him much more than eight to fifteen knots. So if you see Pilot #1 by the water cooler, tell him to take a few seconds and at least set the cowl flaps to the TRAIL position before take-off. That's not the best he can do, but at least he won't suffer a big bite in his airspeed. Pilot #2 mostly wants to set cowl flap positions properly, as long as the study to do so isn't comparable to earning a PhD. The table below should satisfy Pilot #2, with Cowl Flap Settings from the Pan American Flight Instructor's Guide of 01 August 1952.
  Flight Phase | Cowl Flap Position | Flight Phase | Cowl Flap Position
  Start | Open | Cruise | Closed
  Warm-Up | Open | Descend | Trail
  Taxi | Open | Approach | Trail
  Take-Off | Trail | After Landing | Open
  Climb | Trail | Park at Terminal | Open
- Pilot #3 understands that some thought is needed to properly set the cowl flaps, that it is not always a black and white situation as depicted in the table above. The important thing about cowl flaps is that they are supposed to: keep the CHT from changing rapidly, keep it "in the green" except during take off. All Ground Operations, where the air flow around the cylinders is low, should be done with cowl flaps OPEN. All Cruise operations should be with the cowl flaps Closed, since airspeed is high, and low drag is important for fuel consumption/speed. Climbing is a bit more tricky. You need high power, at low speed, so to keep the engines in the green on a warm day usually means cowl flaps in Trail or occasionally half open, and on a cold day usually means cowl flaps in Trail or Closed. Take off and initial climb is in Trail because drag is critical to getting airborne, but the engines overheat, and have to be cooled by reducing power and increasing airspeed to over 100 knots ASAP. Descent is a different problem. Powered descent at high speed, or lower power descent both provide a potential for "shock cooling", which is as hard on the Cylinder Heads as is high temperature, because the heads may crack. This can really ruin your day, so, cowl flaps are adjusted to keep the CHT slowly decreasing and in the green. Depending on OAT, power setting, and airspeed, cowl flaps may be Closed or at Trail. Having said all of that, here are the target CHT's for Climb, Cruise, and Descent. As you can see, the pilot's eye can't stray from the CHT for very long periods of time.
  Flight Phase | CHT - Celsius
  Climb | 200°
  Cruise | 200°
  Descend | 200°
You will do a better job of controlling the cowl flaps if you understand how they work. Here is Allan Greene's nuts-and-bolts explanation of the DC-3 Cowl Flap system: As to the cowl flap operation in the DC-3, the control knobs you see in the cockpit are actually hydraulic valves (yes there are hydraulic lines running directly into and out of them in the cockpit!) In order to open the cowl flaps, the copilot would move the valve to 'OPEN'. This ports hydraulic pressure through the valve to the 'Open' side of the dual-sided cowl flap actuators in the engine nacelles. At any point, one may stop cowl flap travel by selecting 'OFF' on the valve. This traps hydraulic fluid in the line, and as fluid is essentially noncompressible, this serves the purpose of holding the flaps open to the selected degree without supplying constant pressure to the actuators (such pressure which will, over time, cause the seals in the actuator to fail resulting in loss of hydraulic system fluid.) Of course, if you wanted them open all the way you could just turn the valve to 'OPEN' and leave it there, but that would be supplying constant pressure into the line – again, eventually damaging the actuators. Even when the cowl flaps are open completely (except in one circumstance which I'll get into in a moment) you should turn the valve back to 'OFF' to trap the fluid in the line. Closing the cowl flaps works in exactly the same manner, except that the opposite sides of the hydraulic actuators are used. You'll still want to place the valve to 'OFF' when you've reached the desired cowl flap position. Again, this isolates the hydraulic line from system pressure and the trapped fluid in the line from the valve to the actuator keeps the cowl flaps in the desired position. The 'TRAIL' position is a special position where the actuators are connected to the hydraulic return lines. They are not under pressure. There is no hydraulic pressure whatsoever on the actuators. The balance of the airloads upon the cowl flaps will determine their position - a very simple, elegant solution in my opinion. It is my understanding that in the event of a loss of system pressure upstream of the valve, the cowl flaps will remain in whatever position they were last in as the fluid trapped in the line will still not be lost so long as the valve is not moved from the 'OFF' position. All bets are off if the hydraulic leak is downstream of the valve. A special situation I mentioned earlier is after engine shutdown at the destination. In this case, the cowl flaps should be left 'OPEN'. Again, moving the selector to 'OFF' after opening them is the usual procedure. HOWEVER, if the airplane will be left overnight or for an extended period, there will likely be atmospheric changes resulting in either heating or cooling of the fluid left in the lines. This results in changes in pressure, which, if not relieved, can cause damage to the system. Therefore, in order to equalize pressure, and in this case only, the valves are left in the 'OPEN' position.