Technical question for a novice

I used to teach a bit and one clear way to understand why there is down force on the horizontal tail is to look at the planform of a boat's hull. Remove one half of the hull from the center line to outboard. What is left is essentially like an airfoil, flat on one side and curved on the other. Now, run that shape through the medium in which it operates, in this case, water. What will that shape do? It will not follow a straight course, it will turn towards the flat side UNLESS you have a huge rudder on the stern (in the rear) that can be deflected enough to overcome the instability of the shape that is being driven through medium. Hence, a STABILIZER. Change the boat hull to a wing that is trying to pitch downward and the boat's rudder to a horizontal tail that is keeping the wing moving horizontally and you can see why there is DOWN FORCE on the horizontal tail. This applies to wings that do not have a symmetrical section. Symmetrical sections produce lift in any direction when they have an increase in the angle of attack in any direction.
Switches

 

[QUOTE/]

Let's say you are going from an airport at sea level to a second airport at 2,500 feet above sea level, then to a third airport at 5,000 feet above sea level.

Although this would never happen, but if it were a standard pressure day at each airport, then if the altimeter is set at 29.92 then when the plane is on the ground, the altimeter will read the true altitude of the airport or the airport's field elevation, whether it be 0, 2,500, or 5,000.

So far, so good.

If it is a standard pressure day at the sea level airport and a low pressure day at the 2,500 feet above sea level (second) airport, then the control tower, ATIS or ASOS might broadcast to you a number of 28.00 in/Hg, which would be their location's barometric pressure in inches of mercury, corrected to sea level.

If you leave the altimeter setting at 29.92, then when on the ground, the altimeter reading will still show you in the air, but if you set it to the new pressure per the control tower, then the altimeter will read 2,500 feet as expected.

This one you have backwards. If you were in a "low" pressure area, (remembering that low and high pressures are relative terms) your altimeter on the ground would read LOWER than field elevation if the altimeter were still set to 29.92. This is because in order to "reset" the dial hands on the altimeter to the field elevation you would be setting the 28.00 inches in the altimeter to bring UP those hands to indicate 2,500 feet.

The Mnemonic you will learn to remember this is "High to Low - Look out below."

When you leave the second airport to go to the third at the 5,000 feet elevation, then you will need to call the control tower again to get the altimeter setting and if it is a non-standard day due to a high pressure area having moved into the airport vicinity, then the control tower may broadcast a number of 30.1 in/Hg. If you set the altimeter to this, then when on the ground, the altimeter will read 5,000 feet above sea level or the airport's actual field elevation or true altitute. If you neglect to re-set the altimeter, then when you land, the altimeter reading will be too low.

Almost! You have the first part correct. That is when you set the altimeter to the local "corrected" pressure of 30.1 inches you will indeed show 5,000 feet on your altimeter. But if you didn't reset the altimeter and it stayed at the lower pressure setting from whence you took off, the hands on the altimeter would read higher than the field elevation.

A little counterintuative until you think about it for a second and then it will be crystal clear forever.

Remember, the altimeter is nothing more than a sensitive barometer measuring local air pressure and with a way to move the hands manualy to normalize to field elevation and as such keep everyone in the proper altitudes as you fly from areas od differing pressures.

[/QUOTE]

Planeflyr

 

OK..... a little ambiguous term was used... "excessive". A matter of degrees.
( No, the temp hasn't got a place here...) ;-)

This ambiguity, only to accentuate the difference, from conditions of level flight.
The slightest addition of down force will change the attutude from level, to climb, which is "excessive" if you wish to remain level.

It all depends on, from where you are looking at it.


See! You're correct!
I had been unnecessarily ............."confused" about it.

heh heh!

Charlie

 

Now how'd I miss that?

My error!

Thanks for keeping tabs on me.....

Charlie

 

What the heck re you guys talking about , anyway

 

Bob,
It's just an item in Steve's missive, that I missed, and failed to correct,
the first time around...


Charlie

 

Charlie, I know what you guys are kicking around, I was trying to be a little bit stupid sounding with all that stuff with big initials....Hg, IFR, IMC,IFIS, UPS,UFO.FBO....you know, all that techinal stuff. It remended me of when I worked at Boeing when all the aero guys and the stress guys got together for a chat. You couldn't understand a thing they said when they got going. Therefore, "What the heck are you talking about ?" Switches

 

Hey Steve,
Understood. Sometimes the books aren't edited by pilots...
It can get confusing when there's no "air time" to make it all come together.

There now, See what I mean..... ????????

Someone with no experience might ask:
" Is air time, different than ground time?"

( Aww... leave me alone guy's!!..... Just having a little fun with words)

and OH! You're very Welcome Sir!

Charlie

 

Hey! It worked!!

DUH!!! Ya fooled me!


(Nah! I just thought you had missed a point, was all) hee hee!

Charlie

 

Ya know sumthin' After all that I GOT IT WRONG! The "High to Low, Look out below" memory aid is in fact correct and jibes with Kershners explanation.

The statement implies that when you traverse into a lower pressure area from one in which you originally set your altimeter, the altimeter will read HIGHER than you actually are. If there are obstructions along your route like radio towers or cumulus granite, as there is here in the west, you will definately have a false sense of security.

All things being equal, descending to an airport, the altimeter will still think it is HIGHER than you actually are and read higher on the ground than field elevation.

Once again, apologies for the confusion. Just goes to show ya that even after 30 years of flying one can get all wrapped around in the theory.

Planeflyr

 

Heyyyyyyyyy!!!!!

Waddya mean I fell down , died, and they're beating me with whips...???

Just cause I'm an old goat, doesn't mean there's snow in the boiler, or fire on the roof..

Huh??? what did I just say???

Aww ! nemmermind!

I think mebbe you mean hindsight....
( That's tryin to remember what we saw, awhile back)

Anyway no thanks needed, for screwups, like me.... ;- )

Charlie

Yes Nursie , you sweet thing, I WILL take my meds now!!!


.

 

Charlie,

There is a recent short video from '60s folk singer Tom Rush of a song called "Remember" which he performed in concert somewhere in the Northeast [I think]. It is a HOOT! .... and will explain e-v-e-r-y-t-h-i-n-g. Make a look for it on the internet and if you cant find it send me a PM with your e-mail and I'll send you a copy.

Planeflyr

 

Will do ..


Right now, I'm getting ready to leave the house....
We ( C.A.P., USAF Aux. ) are doing the "Wreath's Across America" ceremony .
Placing wreaths on graves of veterans, that died defending our way of life.
ceremony will be across the USA at the same hour (w/time zone offsets).



Charlie

 

Corey,
Name sounds familiar.....
But who is Gwendolyn, and what makes her think I love her?????

Found the phone under the covers,
but the battery is all gone
So I stepped out to get a new one,
and now I'm standing alone

The door's locked, key's are on the counter
Now I have to call the locksmith
and I for some reason,
I can't work this damned phone......

:edit: ( my verses)

Charlie

Great song!!