Loose Change - Part IV

[Darth Rotor]

Yeah, I dont doubt you're right. But I always thought drag opposed thrust and lift opposed weight. Meaning more weight wouldnt necesarily lead to more drag, thus not slowing the plane down.

How deep into aerodynamics for beginner pilots do we need to go, this morning?

Your first statement, for summation of forces, is basically correct for our discussion here.

Your second ignores the problem of needing to produce more lift for a given weight. For a given power setting (thrust) and attitude/angle of attack, one would be flying slower in a heavier plane than in a lighter plane.

DR

 

[apathoid]

rwguinn is in the right.

Vertical speed and velocity vector are not the same for an aircraft that still has lift. The aircraft was not falling vertically, it was flying a roughly 5 degree glide slope.

Given the overall weight of the aircraft, the marginal difference in airspeed vector due to fuel would account for a knot or three (five?) of difference in airspeed in a shallow dive like that. The nose/pitch attitude, and angle of attack on the arifoils (below the horizontal) would have more to do with that, as you'd get additive thrust from gravity (a small resultant vector) to add to the thrust from the engines. Less than a percent, if impact was at 400+ knots.

The Kinetic Energy at impact would be calculated at the point of impact to include the vector sum of the forces along the line of travel: gravity's contribution to this is minimal, since the plane had lift, and plenty of it, all the way to impact. Lift is a force that counteracts gravity, or generally works in opposition to gravity.

DR

OK, I didn't really follow that. Are you saying that an extra 40,000 lbs would have a negligible effect on airspeed at a given rate of descent and initial speed?

What I am saying is I know for a fact that heavier airplanes need more room to slow down on descent and often need drag(spoilers), this is a matter of experience. I have talked to Boeing pilots about this, and thats the only reason I mentioned this.

 

[Darth Rotor]

OK, I didn't really follow that. Are you saying that an extra 40,000 lbs would have a negligible effect on airspeed at a given rate of descent and initial speed?

What I am saying is I know for a fact that heavier airplanes need more room to slow down on descent and often need drag(spoilers), this is a matter of experience. I have talked to Boeing pilots about this, and thats the only reason I mentioned this.

No, given the overall weight of the aircraft, the power setting, and the apparent attempt at max speed, the speed difference, while finite, isn't as pronounced at high speeds as at approach speeds, where you are dealing with stall margins and high angle of attack, high induced drag, etc.

At the higher speed, form drag is a more significant source of drag than induced drag. (drag from creating lift) and I don't think you'd have flaps or spoilers out (which add lift) when making a high speed run. As I mentioned above, I'd assume a clean configuration for the attack run.

To be honest with you, I don't know if the difference would be 3 knots, 7 knots, or what, I'd have to look at a 757 performance chart, from the manual, know the temp and barometric altimeter, the max gross weight of the plane in question, at both fuel loads, to give you a precise answer. I am pretty certain that my order of magnitude is correct: single digits.

DR

EDIT: the remark "(which add lift)" was supposed to be "(which add drag)" and thanks go to apathoid for pointing that out to me. Of course, flaps also add lift . . . I'm having a beer. Been too long since I taught this to newbie pilot trainees.

 

[Darth Rotor]

Bah, double post.

Looks like I buy the beer.

DR

 

[apathoid]

How deep into aerodynamics for beginner pilots do we need to go, this morning?

Your first statement, for summation of forces, is basically correct for our discussion here.

Your second ignores the problem of needing to produce more lift for a given weight. For a given power setting (thrust) and attitude/angle of attack, one would be flying slower in a heavier plane than in a lighter plane.

DR

Thats it, I'm getting Billzilla over here pronto

The part I bolded makes zero sense to me, it makes sense talking about level flight as well as a climb - but not descending

Airplane 1: A 757, at 80 tons, descending at a given thrust setting at 2000 fpm starting at 400 mph
Airplane 2: A 757, at 125 tons, descending at a given thrust setting at 2000 fpm starting at 400 mph.

Are you saying airplane 1 will accel faster than than airplane 2?

 

[apathoid]

EDIT: nevermind!

 

[apathoid]

No, given the overall weight of the aircraft, the power setting, and the apparent attempt at max speed, the speed difference, while finite, isn't as pronounced at high speeds as at approach speeds, where you are dealing with stall margins and high angle of attack, high induced drag, etc.

At the higher speed, form drag is a more significant source of drag than induced drag. (drag from creating lift) and I don't think you'd have flaps or spoilers out (which add lift) when making a high speed run. As I mentioned above, I'd assume a clean configuration for the attack run.

To be honest with you, I don't know if the difference would be 3 knots, 7 knots, or what, I'd have to look at a 757 performance chart, from the manual, know the temp and barometric altimeter, the max gross weight of the plane in question, at both fuel loads, to give you a precise answer. I am pretty certain that my order of magnitude is correct: single digits.

DR

OK DR, I gotcha now, except I dont believe that the difference would only be a few knots - but you may be right. I'm also assuming all variables being the same in my example except the aircrafts weight.

A quick correction though, spoilers dont add lift dude - they dump it.

 

[Darth Rotor]

Thats it, I'm getting Billzilla over here pronto

The part I bolded makes zero sense to me, it makes sense talking about level flight as well as a climb - but not descending

Airplane 1: A 757, at 80 tons, descending at a given thrust setting at 2000 fpm starting at 400 mph
Airplane 2: A 757, at 125 tons, descending at a given thrust setting at 2000 fpm starting at 400 mph.

Are you saying airplane 1 will accel faster than than airplane 2?

The variable of staying on attack/glide slope is missing in your set up.

If all you want to do is descend, set a nose attitude for X airspeed, and descend at an identical power setting, my brain says the heavier aircraft will fall (vertical velocity) at a higher fpm, due to lift having less work to do.

If you are descending at a constant rate, a glide slope, an attack angle, you are still in a state of equilibrium of forces, you are unaccelerated.

OK, start with max thrust, so we have a constant power setting. You set an approach path (dive angle) at the face of the Pentagon. (Probably a piss poor assumption, really, given the guy in question is flying by hand.) But, in the hands of a "good stick," and given a pretty stable glide path, which you adjust with the stick/yoke, the ligter plane would of course accelerate (his flight path is more horizontal, less vertical) faster than the heavier plane, for the same targeted glide path. Is the difference significant at max speed? Probably not, in this 400+ knot missile mode.

If, on the other hand, you are trying to dump the nose and fall as fast from teh sky, (that old vertical dive) without using any speed brakes or flaps or spoilers, I'd guess a heavier aircraft would achieve terminal velocity first, if we assume both aircraft remain intact and retain lift.

DR

 

[Darth Rotor]

OK DR, I gotcha now, except I dont believe that the difference would only be a few knots - but you may be right. I'm also assuming all variables being the same in my example except the aircrafts weight.

A quick correction though, spoilers dont add lift dude - they dump it.

EEP, I meant "add drag."

AARRRRRRRRRGGGHHHHHHHHHHHH!

Darn, I buy the beer again!

DR

 

[apathoid]

Darn, I buy the beer again!

DR

Awesome, no need to make the usual Friday beer run then

 

[apathoid]

The variable of staying on attack/glide slope is missing in your set up.

If all you want to do is descend, set a nose attitude for X airspeed, and descend at an identical power setting, my brain says the heavier aircraft will fall (vertical velocity) at a higher fpm, due to lift having less work to do.

If you are descending at a constant rate, a glide slope, an attack angle, you are still in a state of equilibrium of forces, you are unaccelerated.

OK, start with max thrust, so we have a constant power setting. You set an approach path (dive angle) at the face of the Pentagon. (Probably a piss poor assumption, really, given the guy in question is flying by hand.) But, in the hands of a "good stick," and given a pretty stable glide path, which you adjust with the stick/yoke, the ligter plane would of course accelerate (his flight path is more horizontal, less vertical) faster than the heavier plane, for the same targeted glide path. Is the difference significant at max speed? Probably not, in this 400+ knot missile mode.

If, on the other hand, you are trying to dump the nose and fall as fast from teh sky, (that old vertical dive) without using any speed brakes or flaps or spoilers, I'd guess a heavier aircraft would achieve terminal velocity first, if we assume both aircraft remain intact and retain lift.

DR

I agree with absolutely everything here except the bolded part. Glideslope and rate of descent are 2 different things entirely, yes? If the angle was the same(ie 2.5 degrees) I agree 100%. But if we use vertical speed instead, say 2500 fpm(resulting in a different AoA), the heavier airplane will pickup speed faster.

 

[rwguinn]

This has nothing to do with freefall.

I guess I need to clarify. We have:

Airplane 1: A 757, at 80 tons, descending at idle at 2000 fpm starting at 400 mph
Airplane 2: A 757, at 125 tons, descending at idle at 2000 fpm starting at 400 mph.

The extra weight will will cause airplane 2 to speed up because more gravity is pulling it down, the rate of descent is the same in both cases. Again, this has nothing to do with freefall velocity.

WHAT!
I surrender.
Ignorance takes over.

 

[chran]

Debating Korey?

So, what ever happened with Gravy's debate of (with?) Korey Rowe?

His challenge seems to stand, and I remember Gravy sending him an email about it ...

Did Korey back out?

 

[apathoid]

WHAT!
I surrender.
Ignorance takes over.

Look man, I can appreciate your engineering expertise but here, you are just misunderstanding the point. You are looking at this the wrong way.

It is a FACT that an airplane on autopilot, descending at a given rate, and a given N1 % will accel faster if its heavier. Heavier aircraft need more room to descend without busting the 250/10000 FAA speed limit, why is that do you suppose?

ETA - Just PMed Billzilla (747 Captain) and asked him to clarify....

/thread derail

 

[rwguinn]

Look man, I can appreciate your engineering expertise but here, you are just misunderstanding the point. You are looking at this the wrong way.

It is a FACT that an airplane on autopilot, descending at a given rate, and a given N1 % will accel faster if its heavier. Heavier aircraft need more room to descend without busting the 250/10000 FAA speed limit, why is that do you suppose?

ETA - Just PMed Billzilla (747 Captain) and asked him to clarify....

/thread derail

nope--
I am trying to get some precision, as opposed to pseudoscience in here.
Gravity acts equally on all bodies, regardless of weight. Ballistic coefficients and aerodynamics are additional factors--obviously. The heavier airplane (assuming for a minute that they are both the same airplane type --engines and all) will take longer to accelerate to any given speed for the same power (thrust) setting. That, however, is in addition to the gravitational component of that thrust.
and BTW--in your example, 2 airplanes descending at 2000fpm, both have a 2000fpm down component. They are equal.

/derail-back to CT mythology.

 

[Abbyas]

I've been suspended again at good old LC forums.

Johndoex loves it when you ask a question he can't answer.

Good thing he's stuck on the ground in front of a computer screen 24/7 and can't do any real damage.

He makes me want to up my med dosage.

 

[apathoid]

nope--
I am trying to get some precision, as opposed to pseudoscience in here.
Gravity acts equally on all bodies, regardless of weight. Ballistic coefficients and aerodynamics are additional factors--obviously. The heavier airplane (assuming for a minute that they are both the same airplane type --engines and all) will take longer to accelerate to any given speed for the same power (thrust) setting. That, however, is in addition to the gravitational component of that thrust.
and BTW--in your example, 2 airplanes descending at 2000fpm, both have a 2000fpm down component. They are equal.

You are confusing vertical speed with forward speed(airspeed, groundspeed). Completely independant of one another. This is why I said you simply arent getting what I'm saying(granted I'm not the most articulate fella in the world).

They have the same vertical speed, yes, and if they have the same thrust behind them - then what happens to airspeed when one airplane is significantly heavier than the other(from fuel and pax)?

I am not not trying to rewrite physics books here, I work on airliners and have talked about this very topic with pilots. We've also talked about it in autopilot school where my instructor said the first you need to ask a pilot whose written up an autopilot is whether he was late starting a descent or if he had an unusually high fuel/pax load if the write-up dealt with VNAV.

 

[rwguinn]

You are confusing vertical speed with forward speed(airspeed, groundspeed). Completely independant of one another. This is why I said you simply arent getting what I'm saying(granted I'm not the most articulate fella in the world).

They have the same vertical speed, yes, and if they have the same thrust behind them - then what happens to airspeed when one airplane is significantly heavier than the other(from fuel and pax)?

I am not not trying to rewrite physics books here, I work on airliners and have talked about this very topic with pilots. We've also talked about it in autopilot school where my instructor said the first you need to ask a pilot whose written up an autopilot is whether he was late starting a descent or if he had an unusually high fuel/pax load if the write-up dealt with VNAV.

Heavy airplanes require higher airspeed to generate the lift required to counter the weight. That is due to airflow over the wing. Lift is perpendicular to the wing --which is not necessarily UP
I specifically differentiated between gravity components and thrust components of velocity.
You did not.
That's all you get on this subject on this thread. Any other questions, PM me. We've derailed enough here.

 

[apathoid]

Heavy airplanes require higher airspeed to generate the lift required to counter the weight. That is due to airflow over the wing. Lift is perpendicular to the wing --which is not necessarily UP
I specifically differentiated between gravity components and thrust components of velocity.
You did not.
That's all you get on this subject on this thread. Any other questions, PM me. We've derailed enough here.

Check.

/re-rail

 

[Darth Rotor]

I agree with absolutely everything here except the bolded part. Glideslope and rate of descent are 2 different things entirely, yes? If the angle was the same(ie 2.5 degrees) I agree 100%. But if we use vertical speed instead, say 2500 fpm(resulting in a different AoA), the heavier airplane will pickup speed faster.

For the same thrust, a heavier plane would accelerate more slowly while stying on the same glide slope. Or isn't that what we are talking about? Are you saying the RoD would increase more quickly for a change in nose attitude, given that we are at max power?

What actually would happen, from a stick and rudder perspective, would be that to maintain the picture (fixed on the impact point of the Pentagon) of the glide in attack from 2000 feet to about 40 or zero feet, with fixed power, the pilot has to adjust the angle of attack with his nose to maintain the desired glide path. A heavier aircraft would, all other things being equal, be a few knots slower on that glide path due to needing the marginally increased lift to hold glide slope as constant.

Again, starting at the same point over the ground, about 4 nm away at 2000 feet, to hit a spot on the side of the Pentagon at "0" feet, you have to counter act the increased weight with a slightly increased lift to stay on glide slope, which means slightly higher nose attitude/angle of attack, which means slower airspeed. Trying to stay on identical airspeed with a heavier plane makes for, at identical power settings, a higher rate of descent. But the point is to hit the target.

This is a bit counterintuitive as a pilot flying glide slope based approaches, since most approaches like that are a constant airspeed evolution, with power/attitude adjustments to stay on glide path.

DR