Cooling a turbo.

[djschaut]

>>So let's talk about stage cooling a turbocharged engine coming down from up high. Improper stage cooling of a hot (1650 degree TIT) turbocharged engine can easily ruin it, or at least prevent it from going to TBO--or require a top end. What I didn't know was that an engine can be damaged by NOT pushing it to full power on take-off, too. The engine needs full-power on take off to take advantage of the cooling of the rich mixture--though you can easily take off at less than full-power. I used to mistakenly think I was extending my P-Baron's engine life by taking off at less than full power, but the contrary is true. The rule about descents is to decrease manifold pressure about 2mmHg every couple of minutes--or come down gradually (500ft/min) and start the descent earlier. Another way is to 'dirty' up the airplane (gear down, speed brakes), to keep full power as long as possible while enjoying a rapid descent. At high altitudes, I used to turn off the autopilot, pull the nose up to get the indicated airspeed down to gear extending speed (167kts), extend the gear, then push the nose down to the previous airspeed, allowing a nice descent without touching the power settings. You could increase the airspeed after the gear was down and locked. Of course the cardinal sin is to pull the power back and simultaneously push the nose down, open the cowl flaps, and increase the airspeed and supercool the heads. Pulling the power back decreases the generation of heat in the cylinders, while pushing the nose down (esp. with cowl flaps open) increases cooling by air--shock cooling the different metals in the heads and cylinders of the engine, causing unequal contraction and warping. I need an A/P mechanic here. Graphic engine monitors (GEM) are useful to watch for shock cooling. Turbine drivers don't have to worry about this phenomenon--only recip drivers. That's makes it fun, though, to fly a turbocharged recip. It's all about flows, temperatures, speeds, and pressures--all monitored in the cockpit. Shoot, the airplane will fly itself! Navigation these days is cake! The challenge is to preserve the life of the engines and minimize costs at annual. Any other thoughts on supercooling would be welcome. Any suggestions, too.

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DJSchaut

[pawpaw]

Your proceedure is a goodone to use (what is the max gear extended speed in your Baron?)
In my C-401B, to come down from altitude, I set the vertical speed at 500 fpm down, leave the cowl flaps shut, and sometimes even push the mp up and inch or two. I have to start my desent earlier than what your method is, but I come down like a bat out of H--ll. If you plan you desent correctly, when you arrive at the airport, desend a couple of hundred feet under pattern altitude, pull the nose up, decrease power to your normal settings for the pattern, push up the props, and when below 180, catch the first notch of flaps. This puts you at pattern altitude at max gear ext. speed. My CHT never get under 360 degrees so the Victor engines like it and it gets you to the Hamburger line ahead of you buddies.

[djschaut]

I sold the Baron a few years ago, but I think the gear operating speed was at 167kts. indicated. After extending them, though, you could take it up to 200kts. Extending them always dropped the airspeed quickly, as you know. This allows you to push the nose down, keep the power settings the same, and lost altitude without an increase in cooling. They just act like speed brakes.

The thing about your approach would be the approach speeds. I used to fly into Centennial in Denver and PRC in Prescott, Arizona. At Centennial, there is a lot of traffic. In Prescott, they have Embry Riddle University, and much traffic there. So, I always planned to get the speed bled off to 100kts or so for safety reasons. But, I enjoyed announcing at about twenty miles out and over 200kts only to show up at the airport about two minutes later--blowing the tower away. Doing that makes them give you the earliest landing clearance. They'll make way for the speedy twin blazing into the airport by making the 150s, 172s and Duchesses extend or circle. I've never used the procedure of dipping below pattern altitude to then pull the nose up and bleed off some speed. Close to the ground and near the patter, things are happening fast at those speeds--which is why I'd always slow down for the flight school traffic. Landing at Phoenix Sky Harbor once, I was actually asked to reduce my groundspeed to 180kts, like you. Kind of makes you feel....big.

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DJSchaut

[pawpaw]

Your right, forgot to mention that I adjust the proceedure for the traffic. Next time you fly to an airport that is not busy, try it. It gets you there quick and is fun. Not the proceedure for airports with 141 schools or lots of traffic though. I did it going into MLU once and the controller was thining maybe 160 or something and I hit so quick from my 10 mile call that he got flusterd and made an AF C-130 orbit (that was cool)

[djschaut]

Paw,
Reading our posts about losing altitude and modifying airspeeds made me think about an article in the April, 1999 issue of AOPA pilot, by Barry Schiff. It's entitled "No Go-Around" about the approach the space shuttle uses during its glide down to land. Here we are talking about losing 500fpm and maintaining speeds of about 200kts or so in piston twins. You should read the approach data in this article--it'll blow your mind. For example, when the orbiter enters the earth's atmosphere, it's initial airspeed at 280,000 feet is 25kts. and gradually increases. Twenty six minutes prior to touchdown, it's descending at a speed of Mach 19 to Mach 25. At 142,000ft., it has slowed to Mach 7.3. At 120,000ft., Mach 5. At 83,000ft., Mach 2.6. At 3:57 out, Mach .9 at 43,000ft. At 1:31 out, 280kts. at 15,000ft. At 0:33 out, 300kts at 2000ft. At 0:20, 288kts at 300ft. At ten seconds out, 261kts at 30 to 80ft. Finally, it touches down at a descent rate of 200 to 300fpm at 200kts. Then, the speed brake is fully deployed and the chute is deployed.
The thing doesn't even get to the typical pressurized twin ceiling until its been descending for 24 minutes at many times the speed of sound. The descent from the twin ceilings to the ground almost seems like an afterthought. The pilots must think of airliners and general aviation craft as groundhuggers when descending out of earth's orbit in a spacecraft costing 2-4 billion dollars.

I add this to lend a bit of perspective.

[pawpaw]

THINK I'LL STAY AT 200kts.
THATS AS FAST A MY BRAIN WORKS.

[Rowan]

h

[Rowan]

Hi guys this topic interests me, I fly a Cessna turbo 210 and I too have a different method to you. In my descents I leave the cowl flaps closed and I start my descend really early. I do not adjust the mp as I believe pulling back the mp slowly during descend allows the heads to get too cool. I know the twins are a lot faster machines. I also religiously allow the motor to idle for 5 minutes after every flight. Some people say 3 minutes is ok, but in the operators manual it reccomends atleast 5 minutes and I take the manuals word above word of mouth!

[djschaut]

Rowan,
You're basically doing the same thing that I do to prevent stage cooling. It would be poor form to open cowl flaps on a descent, and it's best to try and descend without touching the power settings, as you do. But, instructors would probably call us extreme, and that you can safely decrease manifold pressure about 2" every few minutes without doing any harm. Then again, they're not paying for the overhaul.
I think your habit of cooling the motor on the ground is an excellent one--especially if you've landed in Phoenix in the summer. It's probably just as important when you land in sub-zero temperatures in the winter. This gives the different alloys in the heads and cylinders a chance to cool at a similar rate and avoid wear. I'd like to hear from an experienced A&P on this one.
Thanks.

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DJSchaut

[djschaut]

Previous post: meant to say prevent 'shock' cooling.

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DJSchaut

I have a turbo Arrow IV that can easily be shock cooled. The manual states that the manifold temp should not dip below 240 degrees for a period not to exceed 5 minutes. That means during a descent you should be sure to keep some power in and you'll avoid any problems. Sure makes it difficult to practice emergencies.

You should also allow the turbocharger to cool down before shutting down the engine. That means letting the engine run at idle for a few minutes before pulling the mixture and throttle back. If you have a long taxi, this time counts in the cool down process (as long as you are not taxiing at take off speeds).