C-74 Flying Instructions Question

[gwhess]

Hi,

I noticed that in the Flying Instructions, Before Take-off section, for Jens C-74, it states that the cowl flaps should be closed, which seems unusual to me. Is this correct?

Thanks,

Gary

[ozbeowulf]

The cowl flap position for take-off might vary somewhat by aircraft or engine type, but closing the cowl flaps immediately before takeoff is the most common setting.

The cowl flaps are typically fully open while taxiing but that's because there is very little airflow through the cowl. Further, drag is not a concern while taxiing.

On takeoff, though, air is being forced through the engine cowl at an ever-increasing speed. (Okay, okay, it's actually the engine cowl forcing its way through the air, but the effect is the same.)

Drag and smooth airflow over the wings are quite critical on takeoff. The drag from open cowl flaps is one thing, but even worse is turbulence from open cowl flaps that wipes out lift over a wide swath of wing behind the engine. A heavily-loaded aircraft could easily run out of runway or even be unable to accelerate to lift-off speed.

Of course, there are no operational absolutes that apply to every aircraft ever built. In some conditions, certain birds may require a small bit of cowl flap open at takeoff, but offhand I cannot cite an example.

Cheers,

Glenn

[Randy_Cain]

Hi,

Thanks, Glenn. I can think of 2 examples of exceptions. I've never seen a B-29 take off with cowl flaps "closed". Engines were prone to overheating and temp was critical.

P-40 Warhawks (fluid cooled) would overheat very quickly with cowl flaps shut.

(At least these are from historical documentation that came with models and I've seen verified from other sources.) I assume there would also be a big difference in whether or not Water-Meth injection (coolant) was being used for takeoff...or not.

Yours,

[Tom/CalClassic]

Most propliners took off with the cowls at least cracked, but a few took off with cowls closed, to be opened ASAP after takeoff.

[ozbeowulf]

Good points, Randy!

I must admit I was ignoring aircraft with liquid-cooled engines like the P-40 & P-51. Strictly speaking, the P-51 used a dump trough to control airflow through the radiator. I'm not absolutely sure about the P-40. They had two or three flaps below and behind the cowl, IIRC, but didn't they have radiator shutters as well? I may be thinking of a different aircraft, though.

I guess I was focussing on the drag and turbulence issues of open cowl flaps on wing-mounted radial engines and relating the practice to personal experience. We used the "trail" position for takeoff & climb in US DC-3s. "Trail" just dumped the hydraulic pressure from the actuators so the cowl flaps self-aligned with the slipstream. No drag, no lost of lift.

On the other hand, Australian DC-3s used fixed cowl flaps; one set for summer and another set for winter. It cut maintenance costs on the notoriously leaky cowl flap hydraulics, but it sure made it difficult to keep the cylinder heads warm during descent. I always figured that's why we were getting 1600 hours before overhaul in California but only 1000 hours here in Oz.

Anyway, I take your point re B-29s and Tom's mention of cracking cowl flaps slightly for t/o. Checking a few of the manuals I have, I see that standard takeoff setting is "trail" for the C-47/DC-3 & C-54/DC-4, "3 degrees" for the C-118/DC-6 and "no more than 3 degrees" for the KC-97/B377. (I never measured, but I betcha "trail" for those P&W 1830s is pretty close to 3 degrees.

I guess I spoke up too soon. I should have checked the paperwork sooner. Ah, well, just as some say "We are what we eat," I must be living proof that "We are what we flew."

Cheers,

Glenn

[volkerboehme]

Hi,

just my two cents: single-engine liquid-cooled engines frequently have their cowl flaps on the centerline of the aircraft. Here's a pic of a P-40 from Wikipedia:



The cowl flaps are located just below the leading edge, fully open here. Opening them should create some drag, but would not have a 'spoiler' effect on wing lift, particularly if the cowl flap is located well aft, like on the Hurricane or P-51. However, some planes had their radiator located in the wings, like the Spitfire of BF-109.

Apart from that, fighter types usually have quite a good power-to-weight ratio, even at maximum take-off weight, so acceleration would be less of a problem. This might not be true for long-range types at the end of WWII, like a P-51D or a late P-38 with a full load, though.

Best regards,
Volker

[gwhess]

Thanks for the interesting information. I wondered if the instructions to close the cowl flaps was due to drag but wasn't sure. It just seemed that the engines would be subject to overheating when operating with the cowl flaps closed at takeoff power.

Gary

[sunny9850]

The engine or more accurately cylinder head temps would certainly be a factor to consider for the proper CF setting.
The ambient temperature being another one...max weight takeoff at KLAX in July is a whole lot different ball game than the same at Fairbanks in December.

Engine power is different at the higher temps to begin with so chances are you would be disadvantage from the word go. Everything would take a bit longer and the engines would reach redline temps sooner.

I would guess that there were specific guidelines beyond just having them closed in the full operating instructions...and the FE would be in charge of keeping his power plants happy as soon as the Captain called for Climb Power.

For the sim I usually use the cracked method unless it's very cold outside and then use the temp gauges in the climb to guide CF settings.

Stefan

[volkerboehme]

Hi Gary,

you would want to use full power for takeoff. Sure, the engine will be heating up first, but even fully open cowls would not prevent this at full power.

Better get some airflow through the cowling, and accelerating quickly is the best way to achieve it. The engine would be run at full rich mixture, wich is giving it some degree of cooling as well.

But full power is straining the engine anyway, and the sooner this can be changed to a more sustainable level of strain (METO), the better.

The aircraft designers might chose between a fully closed setting or a modest opening, but that might be just a gradual difference. But opening the flaps can cause as much drag as a step of flaps - without any lift benefit, and along with severe buffeting as well. United's 'Mainliner Oahu' was lost partially due to severe buffeting and vibration because of wide open cowl flaps.

Best regards,
Volker

[gwhess]

Thanks, Volker.

Gary