Comments on: Fat folk, airplanes and ignorance of basic physics

By: CaptDMo

CaptDMo — Thu, 09 Oct 2008 16:45:44 +0000

Gliders (sailplanes)-
Gee, it must be nice to account for all that math stuff when the passenger just happens to be at the controls,
which brings me to point two-
Beverage trolleys.
As major airlines tend to frown on me touching all the knobs, wheels, sticks, and stuff, I tend to do my best by assisting in redistribution of the concentration of mass of fore, or aft, fully loaded carts of tranquilizers in tiny little bottles a little further back from the nose where the “first to arrive at the scene” hang out.

I can only assume that pilots of the larger commercial flights have estimated the amount of time after those delectable meals are served that there will be a steady
“live” cargo migration about the cabin, with a subsequent gross transfer of mass from the seats to the alleged black ice “holding tanks”, and trim/transfer fuel/reload-redistribute the trolley mass accordingly.

All in the interest of lowering fuel consumption, ecology,
and public transportation economics of course. At that point they REALLY won’t let me past the door of the forward head, but I don’t care anywhere NEAR as much.

By: Dale Amon

Dale Amon — Thu, 09 Oct 2008 04:10:42 +0000

Hi Doug. You’re just trying to make me jealous. I can tell

So hurry up already and get loads of dosh in so you can have me do some more work for you guys!

I’m dying to see your new engine up close and personal!

Well, not *too* close…

By: Alasdair

Alasdair — Thu, 09 Oct 2008 01:56:20 +0000

Possibly silly question from a small-boat sailor …

I understand (and can teach) why, with increased speed, the centre of lift can change with a non-rigid aerofoil like a mainsail …

With a rigid aerofoil like a plane’s wing, what causes the centre of lift to change with change in speed ?

(or are we talking about the shift from subsonic to supersonic flight ?)

(that latter NOT usually being a problem even with a sailboat with a planing hull (grin))

By: Fred the Fourth

Fred the Fourth — Thu, 09 Oct 2008 00:12:50 +0000

Side note: So now we know how to get the pilots to come out of the woodwork.

By: Fred the Fourth

Fred the Fourth — Thu, 09 Oct 2008 00:04:39 +0000

Somewhat counter intuitively, humidity does not make the air “denser” even though it feels that way. Humidity is H2O vapor which is a lot less dense than the basic N2+O2 mix. Also, with minor exceptions, the extra H2O displaces O2 in the fuel-air mix, reducing the effective displacement of the engine (at least on a recip. Never thought about a jet, but it’s probably the same.) The reduced density from heat + altitude + humidity affects not just the engine, but also the wings and prop (if you got one (and if you don’t, how can you call yourself a pilot?).
Departing Denver on a humid summer afternoon is an exercise in precise calculation of accelerate-stop distance. For extra fun, try LaPaz, Bolivia, elevation 11,942 feet.

By: steveH

steveH — Thu, 09 Oct 2008 00:02:00 +0000

Increasing humidity means increasing density altitude, as water vapor is a bit lighter than air.

I was interested to learn during my last biannual review that at least some business-class turbines aren’t legally able to fly if the altimeter’s kollsman window adjustment exceeds some particular limit.

(The kollsman window is used to adjust the altimeter for local barometric changes; if your airport is a 1500′ ASL, it’s nice to have the altimeter actually read that value on the ground.)

Anyway, takeoff tables are run only so far, and once you’re past the published limits, you can’t legally fly the plane. Nothing for it but to wait for the barometric pressure to come back within the limits.

By: Doug Jones

Doug Jones — Wed, 08 Oct 2008 23:54:29 +0000

Dale, that’s why I like flying in rocket-powered aircraft- with thrust:weight of at least .5 (and rapidly increasing), the takeoff roll is gratifyingly short.

[insert smug smirk here]

By: Dale Amon

Dale Amon — Wed, 08 Oct 2008 23:40:10 +0000

The number I have from a reference book that was close at hand was the takeoff roll increases .42 to .65 percent per degree F above 59F. So at 85F the takeoff roll is 11% to 18% longer. Altitude has another large factor, about 7% per 1000 feet above sea level. ‘Hot and High’ are bad news for aircraft and have caught more than one pilot out.

Humidity also affects the density altitude and has some impact.

By: Laird

Laird — Wed, 08 Oct 2008 23:08:57 +0000

I was flying out of Salt Lake City once on a hot summer day in a completely full plane. I guess the air (which is thin enough in Salt Lake at the best of times) gets even thinner on very hot days. (Engineers, please correct any error here.) Anyway, the pilot got on the speaker and announced that we were too heavy to take off in the heat. They then removed all of the luggage from the hold and we were OK. Even got to Atlanta on time to make my connection.

I’m glad I only had carry-ons.

By: Fred the Fourth

Fred the Fourth — Wed, 08 Oct 2008 22:22:41 +0000

Re: fuel burnoff and balance
My acquaintance once arrived in California from Hawaii driving a Convair 990 (roughly like a B-707) with about 10,000 pounds of fuel trapped in one outer wing tank. The transfer valve stuck. Don’t know if they were precisely legal W&B when they landed, but they did not have a lot of options by that time. Not a lot of usable gas left, and (I think) could not dump directly from the stuck tank. Landed without incident. I should ask him about the details, and if he had to buy new pants afterward.

By: Alan Little

Alan Little — Wed, 08 Oct 2008 17:25:17 +0000

This prompts me to reminisce about when my sister used to live in the Shetland Islands and I was flying back after a winter visit.

Getting from the terminal building to the plane (some kind of twin turboprop thingy) on foot in about a Force 8 was an adventure in its own right. The airfield in Shetland is on a strip of sand with the North Atlantic on one side and the North Sea on the other. Spray from the wavetops was visible over the dunes on the Atlantic side.

Having made it to the plane, we were firmly advised/instructed not to sit in the rear six rows of seats “because of the weight of ice that is going to build up on the tail”.

Oh. OK. If the pilot doesn’t sound worried, then I suppose I shouldn’t be either.

Off we go. Some way out over the North Sea, the plane starts to shudder a bit. The pilot comes on the PA: “do not worry about the vibration. It’s just ice buildng up on the propeller blades”.

Oh. Ok. If the pilot doesn’t sound worried … etc.

(He may have been scared out of his wits for all I know. I doubt it though. I assume pilots and controllers in places like Shetland know where the limits are and don’t actually go anywhere near them)

We made it to Aberdeen and the most spectacular landing I’ve ever experienced, with several inches of fresh wet snow on the runway. This makes a big big splash when a plane lands in it. We were the last flight allowed in or out for several hours.

By: Midwesterner

Midwesterner — Wed, 08 Oct 2008 16:51:04 +0000

Ken Mitchell’s story reminds of something I heard a couple of decades(?) ago about the Stanford Marching Band. Apparently they did something similar at altitude but moving from side to side on a charter flight. I couldn’t find anything with a quick net search but as I recall the FAA, NTSB etc types got involved and the SMB was sanctioned or grounded or something.