Descent Question

[fish314]

Originally Posted by bumblebee

Help me understand what additional factors would be involved.
My understanding is that if your tail stalls, then the downforce is no longer there causing the aircraft to dive.

Well, in any scenario where the tail was producing a tail UP force, and then the tail stalled, you'd have a nose up reaction.

The only one that I can think of off the top of my head might be if the tail were to stall as you were pushing forward on the yoke or stick. As you are pushing forward on the yoke or stick you are either lessening the tail down force from the tail, or creating a tail UP force, depending on how much you push.

If the tail were to stall right then, you'd either get some kind of a nose up reaction. Either it would be less "nose down" than normal for that amount of forward push on the stick, or it would be a real 'nose up'.

Of course, that's the only scenario that I can think of off the top of my head where the reaction would be "nose up" and not "nose down". There may be others, but I think that it's probably safe to say that the reaction is far more often going to be nose down than nose up.

[eagledriver101]

Originally Posted by tonyw

I've got a better one.

What is the airspeed velocity of an unladen swallow?

In all seriousness, here's a rule of thumb from Plane & Pilot.

http://www.planeandpilotmag.com/cont...top_rules.html

The descent rule of thumb is used to determine when you need to descend in terms of the number of miles prior to the point at which you desire to arrive at your new altitude. This is accomplished by dividing the altitude needed to be lost by 300 (clearly a much more pleasant number to work with). So let’s say you’re cruising at 7,000 feet and you want to get down to a pattern altitude of 1,000 feet. The altitude you want to lose is 6,000 feet, which when divided by 300 results in 20. Therefore, you need to start your descent 20 nm out

I think I get this, so if you're at FL30 and you need to hit 10,000, you need to start your decent at 66NM out? 20,000/300=66.67(NM) ?? How do you figure what rate of decent this is and or change to rate of decent? What if you want to descend at 1800fpm? You would have calculate airspeed also.. Right? If you're at FL30 @ 250kts, does the divide by 300 still apply if you were at 180kts vs. 250 kts? Or what if you need to slow from 250kts, to 180kts, get to 10k and you're at FL30. Is there an easy way to figure this out??

EDIT: Okay, I read the thing, I get it, just times your speed by 5. Then you can figure out your ROD.

[eagledriver101]

Okay I got it,(I think) If your ground speed is 320kts, and you’re at FL30, and you need to get to 10k, you need to start your descent at about 67NM out, and descend at 1600FPM.

The formula I used; if you need to get to 10k feet, when you’re at 30,000. at 320kts ground speed.

320KTS(GS) x 5 = 1600ROD

@ 30,000 – 20,000=10,000

20,000/300 = 66.77NM

Therefore, you would want to start your decent at about 70 NM out, at a ROD of 1600 to be at 10,000 at your location... Am I right?

[CamYZ125]

Yes, you're correct. And that would be at a 3 degree descent.

[Bumblebee]

Originally Posted by fish314

Well, in any scenario where the tail was producing a tail UP force, and then the tail stalled, you'd have a nose up reaction.

The only one that I can think of off the top of my head might be if the tail were to stall as you were pushing forward on the yoke or stick. As you are pushing forward on the yoke or stick you are either lessening the tail down force from the tail, or creating a tail UP force, depending on how much you push.

If the tail were to stall right then, you'd either get some kind of a nose up reaction. Either it would be less "nose down" than normal for that amount of forward push on the stick, or it would be a real 'nose up'.

Of course, that's the only scenario that I can think of off the top of my head where the reaction would be "nose up" and not "nose down". There may be others, but I think that it's probably safe to say that the reaction is far more often going to be nose down than nose up.

Does the elevator ever produce tail up force? Or is what we're talking about here is less downforce?
Not necessarily equals in this case. If there is less downforce being produced, then the elevator stalling would create less of a nosedown attitude, but in a full tail stall there would be no downward force being applied. I believe that the only time an elevator stall would produce a skyward attitude is while inverted.

[eagledriver101]

Originally Posted by CamYZ125

Yes, you're correct. And that would be at a 3 degree descent.

Cool, I feel smart now

But wouldn't this only be for VFR? Doesn't ATC call your desents if your IFR/airlines..?

[Bumblebee]

Originally Posted by eagledriver101

Cool, I feel smart now

But wouldn't this only be for VFR? Doesn't ATC call your desents if your IFR/airlines..?

You are smart!!!

Not always, they will give you a crossing restriction out there somewhere and you need to figure out your own descent profile.

My rule of thumb is Alt(in thousands) to lose x 3 for TOD (Top Of Descent)
20,000 to lose or 20x3=60 (I add about 10 miles to get there before the restriction)

for 3 degree glideslope I take groundspeed x10/2 so in your example 320(10)/2 or 3200/2=1600fpm

I find these are easier for me to do in my head

[KLB]

Originally Posted by eagledriver101

Cool, I feel smart now

But wouldn't this only be for VFR? Doesn't ATC call your desents if your IFR/airlines..?

Not necessarily. This really comes into play when they give you a "descend at pilots discretion" or a "cross XXXX at yadayada altitude. Also you're on your own if you have lost comms.

Futhermore, you can't rely on ATC to plan your descents for you or you will end up in some screwy situations sometimes.

[eagledriver101]

Originally Posted by KLB

Not necessarily. This really comes into play when they give you a "descend at pilots discretion" or a "cross XXXX at yadayada altitude. Also you're on your own if you have lost comms.

Futhermore, you can't rely on ATC to plan your descents for you or you will end up in some screwy situations sometimes.

Okay, thats what I thought...Thanks, this will really help me out a ton!

[fish314]

Originally Posted by bumblebee

Does the elevator ever produce tail up force? Or is what we're talking about here is less downforce?
Not necessarily equals in this case. If there is less downforce being produced, then the elevator stalling would create less of a nosedown attitude, but in a full tail stall there would be no downward force being applied. I believe that the only time an elevator stall would produce a skyward attitude is while inverted.

You're absolutely right about there being a big difference between a tail UP force, and merely a lessening of the tail down force. I am pretty sure that on any aerobatic airplane AT THE VERY LEAST, the tail produces a tail up force, rather than merely lessening the tail down force. Use your example of an aircraft flying along in an inverted attitude as an example.

To fly along inverted (and not lose altitude) the weight still acts towards the earth, but the wing would have to produce "lift" away from the earth (which in this example is in the direction of the bottom of the plane). If you assume that the aircraft has a non-cambered wing, then the location of that lift, and the pitching moment would be exactly the same as if the aircraft were right side up. In other words, the "lift" would act behind the C.G., and would have to be balanced by a force acting towards the earth, from the tail. This would also be towards the top of the aircraft, and hence a tail UP force. Clearly this is possible, because you can go to airshows and see all kinds of airplanes flying inverted and not losing altitude.

Now take that scenario and turn it around. Of course the direction of the weight would be different (as seen from the pilot in the plane, that is), but the AIR doesn't care which way the earth is. The airfoils would still be capable of producing the same forces. So therefore, there must be at least SOME way of producing a tail up force, if only in aerobatic airplanes that are capable of level inverted flight. And if they can produce such a force in inverted flight, they can produce it in NON-inverted flight also. Because the aerodynamics doesn't care which way the force of gravity is acting.

So there must be SOME way of causing a tail stall which produces SOME kind of nose up motion in SOME kinds of aircraft. Now that doesn't mean that it's a common phenomenon, but it has to be at least POSSIBLE. Probably not very likely, though.

[Doug Taylor]

Originally Posted by eagledriver101

Cool, I feel smart now

But wouldn't this only be for VFR? Doesn't ATC call your desents if your IFR/airlines..?

Sometimes you have to pimp ATC for descents, especially when the SID has crossing restrictions. Don't depend on ATC to give you a descent early enough to meet the restrictions.

When I flew the 737, I learned that by the time ATC said, "Delta 2431, cross..." I already had the speedbrakes out.

The 757 is especially bad descending too because the wing is engineered to fly.

The mad dog.. Like a rock.

[eagledriver101]

Originally Posted by Doug Taylor

The 757 is especially bad descending too because the wing is engineered to fly.

The mad dog.. Like a rock.

Are not all wings engineered to fly??? I kid I kid... Yeah, I heard the getting the 757 to stop flying or descend was a pain