A question for you physics gurus

[Guest HercengTN]

Here is the question : Two aircraft, one heavier than the other, descending at the same airspeed. Which aircraft

will have the greater VVI and why?

The question came up today in a conversation with some friends and most agreed the lighter acft would have the greater VVI but couldn't agree on why.

Excuse my ignorance on the subject. Thanks in advance for your input

Edited February 17, 2008 by HercengTN

[Guest rotorhead]

Here we go...you're going to get a score of opinions on this, just like the treadmill takeoff debacle.

Not enough information for an argument or discussion. Too vague to answer. Methinks a U2 at 90 knots will descend a tad slower than a T-38 at 90 knots. It's not merely a weight discussion.

It all comes down to the lift equation.

If you change the discussion to two identical airplanes, wings, configuration, CL, rho, S, and V2, and they produce identical lift, then the heavier one will descend faster. Both planes have the same lift, one has more weight, and descends faster. Refine the question, or you're going to get too many opinions.

[Guest HercengTN]

Thanks for the response.

Ok lets say 2 C-130's, one weighing 150,000lbs and the other 105,000lbs. The reason I was

leaning towards the lighter acft is that it could maintain a lower angle of attack therefore descending

faster than the heavier acft that would have to maintain a higher to maintain airspeed.

Again, I don't know the answer just wanted to let you know what I was thinking.

[tac airlifter]

In your hypothetical situation, the lighter aircraft will have the higher VVI. Look at penetration decent tab data, you'll find that a 105K C-130 takes less distance to lose the same amount of altitude as a 150K C-130 if they both maintain the same airspeed (250KIAS). The only way to lose equal amounts of altitude at the same speed in different distances is if one aircraft is losing more altitude per unit of time.

[WheelzUp]

Gonna have to chime in here. Are you talking an equal power setting or not? Because i can take a heavy aircraft, and a light one, and descend at the exact same speed and VVI in both, but the power will be different. If power and lift are equal the heavier aircraft should descend faster, but there are a significant number of variables to be considered here. At any rate, great question, and definately a skull scratcher.

Cheers

Wheelz

Edited February 17, 2008 by WheelzUp

[Guest HercengTN]

Engines at flight idle

[Riddller]

Wow, that is a good question...

At first glance, I'd have to agree w/ rotorhead. Assuming you go to flight idle and nose down your herk to maintain 250 KIAS, it would make sense that ater stabilizing in that configuration, that the heavier aircraft would descend faster just from the weight/lift/thrust/drag equation.

But it make also makes sense that becasue the heavier a/c is heavier and DOES want to descend faster, the indicated would want to creep up, so you pull back on the yoke, resulting in a net loss on your VSI. I think tac airlifter sealed it though when he looked up the penetration descent info. If the book says shorter distance for lighter aircraft, and everything else is equal, then the only variable in that equation is vertical velocity. Lighter a/c wins.

[HossHarris]

Here's the answer ... weight is irrelevant. period.

The only thing that will matter is the L/D for the airframe and that's based on wing geometry mostly. In a static, unaccelerated glide, weight is exactly countered by lift. A 100k pound "glider" made out of lead with the appropriate wing span and area will descend slower than a paper airplane. It's all L/D these days.

edit: I re-read the question. If operating at L/Dmax airspeed weight is irrelevant. If operating at some random airspeed, see my post below.

Edited February 18, 2008 by HossHarris

[D-ron]

Here's the answer ... weight is irrelevant. period.

The only thing that will matter is the L/D for the airframe and that's based on wing geometry mostly. In a static, unaccelerated glide, weight is exactly countered by lift. A 100k pound "glider" made out of lead with the appropriate wing span and area will descend slower than a paper airplane. It's all L/D these days.

No, if they are identical a/c, except for weight, and they both have the same airspeed, then they will both be producing the same amount of lift. If they are producing the same amount of lift, but have different weights, then the heavier a/c will have to be descending faster.

[Buddy Spike]

Here is the question : Two aircraft, one heavier than the other, descending at the same airspeed. Which aircraft

will have the greater VVI and why?

The question came up today in a conversation with some friends and most agreed the lighter acft would have the greater VVI but couldn't agree on why.

Excuse my ignorance on the subject. Thanks in advance for your input

I think the formula is Indicated Mach x 1000 x Degrees Nose Low = VVI

So a lighter aircraft will require a steeper descent to maintain the same airspeed, thus a higher VVI.

Edited February 18, 2008 by Patch

[RangerMateo]

In your hypothetical situation, the lighter aircraft will have the higher VVI. Look at penetration decent tab data, you'll find that a 105K C-130 takes less distance to lose the same amount of altitude as a 150K C-130 if they both maintain the same airspeed (250KIAS). The only way to lose equal amounts of altitude at the same speed in different distances is if one aircraft is losing more altitude per unit of time.

The bad logic here is "if they maintain the same speed". With penetrations you maintain a constant IAS, and therefore a heavier airplane must maintain a lower gradient to keep from overspeeding (since the ratio to gravitational force / drag is higher for the heavy airplane), and therefore a slower descent rate. In this situation, they will be at different speeds because V (L/Dmax) changes with weight. The optimum glide AoA for a lifting surface is always the same. So to keep the same AoA at different weights, it will require that the craft glide at different airspeeds.

AoA is the angle between the relative wind and the chord of the wing. The heavier a/c at the same pitch attitude will generate a greater vertical component of relative wind, and hence a greater angle of attack. To counter this, the nose must be pushed over to seek that optimal AoA, and also generating a higher airspeed and sink rate. The lighter a/c holds a higher pitch attitude (same AoA), lower airspeed and lower sink rate.

Here's the part I had a hard time believing until I put the math to it. For realistic values of weight, ie - empty to max gross, the range is the same. The heavier a/c will get there faster, but the light plane will end up in the same place assuming they are both flown at optimum AoA. In fact, most POH's give best glide at Max TO Weight, and if you glide at that speed regardless of weight, the heavier plane will actually make it further. Didn't believe that one till I saw the graph in my Dash One. That actually ends up being more of the situation that you were referring to with the penetration data...if you fly the airspeed and don't adjust for the angle of attack...the lighter plane actually develops a higher sink rate.

Edited February 17, 2008 by RangerMateo

[disgruntledemployee]

In a Herc, the lighter one decends faster. I've done hundreds of pen. descents...idle power, 250 IAS, and the lighter plane always takes less distance, and thus has a higher VVI. And if anyone argues with me on this, I'll pull my 1-1 out on you!

Out

[Guest that_guy]

The only thing it depends on is what the airspeed is in relation to the min drag airspeed for your weight. That is because since airspeed is set, the only thing VVI depends on is descent gradient, which only depends on L/D (minus idle thrust).

In case A, the specified airspeed is 250 and your min drag airspeed based on your weight is 230. In this case gaining weight will bring your min drag airspeed closer to 250, so you will have less drag at 250 than when lightweight, and therefore a shallower descent gradient. So you will descend slower with more weight.

In case B, the specified airspeed is still 250 and your min drag airspeed is 270. In this case gaining weight will increase your min drag airspeed even further, increasing total drag, which increases the vvi you need to maintain 250. In these case you will descend faster with more weight.

Put another way, it depends if you're on the back side of the drag curve or not.

[tac airlifter]

The bad logic here is "if they maintain the same speed". With penetrations you maintain a constant IAS, and therefore a heavier airplane must maintain a lower gradient to keep from overspeeding (since the ratio to gravitational force / drag is higher for the heavy airplane), and therefore a slower descent rate. In this situation, they will be at different speeds because V (L/Dmax) changes with weight. The optimum glide AoA for a lifting surface is always the same. So to keep the same AoA at different weights, it will require that the craft glide at different airspeeds.

AoA is the angle between the relative wind and the chord of the wing. The heavier a/c at the same pitch attitude will generate a greater vertical component of relative wind, and hence a greater angle of attack. To counter this, the nose must be pushed over to seek that optimal AoA, and also generating a higher airspeed and sink rate. The lighter a/c holds a higher pitch attitude (same AoA), lower airspeed and lower sink rate.

Here's the part I had a hard time believing until I put the math to it. For realistic values of weight, ie - empty to max gross, the range is the same. The heavier a/c will get there faster, but the light plane will end up in the same place assuming they are both flown at optimum AoA. In fact, most POH's give best glide at Max TO Weight, and if you glide at that speed regardless of weight, the heavier plane will actually make it further. Didn't believe that one till I saw the graph in my Dash One. That actually ends up being more of the situation that you were referring to with the penetration data...if you fly the airspeed and don't adjust for the angle of attack...the lighter plane actually develops a higher sink rate.

Where is my bad logic? The original question was "Two aircraft, one heavier than the other, descending at the same airspeed. Which aircraft

will have the greater VVI and why?"

The answer to the OP's question is the lighter aircraft will have the higher VVI. Although I could guess, the truth is I don't know why.

[FourFans]

Here's the answer ... weight is irrelevant. period.

I'm gunna guess you don't fly airplanes that are large enough to have significant weight changes.

disgruntled shacked it. The heavier the airplane will always require more distance to loose the same amount of altitude as a lighter airplane (in theory, the heavier airplane has a lower VVI than the lighter airplane) In reality, it's not the VVI you're concerned with. It's the VVI compared to the ground speed that you want to worry about. In my mind, it's a simple concervation of energy problem.

Assumptions:

- higher induced drag created by need for more lift to compensate for the heavier load onboard Heavy is degligable (at least in a herk)

- same config, airspeed, and piloting techniques

Heavy and Light fly throught the same air with roughly the same coeff of drag. But Heavy has more potential energy than Light does. Therefore, if they are burning that energy at roughly the same rate, it will take a longer time for Heavy to discipate its energy. Additionally, Heavy has greater momentum, requiring more energy discipation as well.

Grain of salt: I'm a history major so there are probably errors in my physics logic. However, I'm also a herk driver who has done enough Pen-Ds to know what he's talking about.

BL: Experience and the 1-1 tell you that a heavier airplane will take a longer distance and time to get down.

[HuggyU2]

Fun debate. I'm not reading all the posts thoroughly, but I agree with that_guy's logic.

Going with what I know: a T-38's L/Dmax is 230+fuel. So,..

If I plan to do 250KIAS and the jet has 2000# of fuel, I'm right at L/Dmax.

If I'm out of gas and doing 250, I'm flying 20 KIAS above L/Dmax,... hence, more total drag,... so I'd descend faster, right??

[HossHarris]

No, if they are identical a/c, except for weight, and they both have the same airspeed, then they will both be producing the same amount of lift. If they are producing the same amount of lift, but have different weights, then the heavier a/c will have to be descending faster.

Not to be a dick, but you are wrong. It's a static situation, lift will exactly equal weight or the A/C will be ACCELERATING up or down.

Aero Engineer.

[HossHarris]

Ok, to answer the question a little more completely.

The glide ratio (descent angle) will be set by the L/D max which is entirely SHAPE dependant and has nothing to do with weight. That L/D max will occur at a certain airspeed and the descent angle combined with airspeed will yield the VVI.

So for this question, if you have two identical airframes, one heavier than the other, the L/D max stays the same. The heavier airframe will acheive this L/D max condition at a higher airspeed. L/D max is the same, so the descent angle is the same, so the higher airspeed will have the higher down VVI if both aircraft are operating at an optimal speed. If both aircraft cut power at the same point, they will hit the ground at the same point. The heavier aircraft will get there first.

If they are flying at the same airspeed there is no way to tell which will descend faster, VVI wise. Again, assuming the same airframe, whichever aircraft is closer to it's L/Dmax airspeed will have the better glide ratio. Since the airspeed is the same between the two, the shallower glide of the aircraft closer to L/Dmax will have less VVI. A "slow" airspeed will favor the lighter aircraft. A "fast" airspeed will favor the heavier aircraft. You'd have to know which a/c is closer to L/Dmax at that given airspeed to make the call. In this scenario, the A/C farther from it's L/D max will hit the ground in the shortest range. You can't tell which one will hit the ground first (that's VVI dependant) and that get's into MAX Endurance, which is a whole 'nother ball of wax.

For a less theoretical and more practical perspective, look to competition gliders. They will add balast to hit L/D max at a higher airspeed to cover more ground and dump balast to hit L/D max at a lower airspeed. Across a wide range of weight, the L/D max value and glide ratio is the same.

Even though I don't fly heavies, fighters do see a significant change in weight, especially when you look at the change as a percentage. You can't just use the "in the C-130 this is true" analogy since the T.O.s don't always follow the most efficient path from an aero perspective since there are other considerations outside of pure aerodynamics. None of these aircraft mentioned require any distance to descend. I can certainly point 90 degrees down and overspeed everything (on a heavy). Now, if I'm descending in accordance with the -1 it's different ... but this question has nothing to do with -1 airspeeds does it? So your real-world, I've done this a thousand times examples don't apply do they?

And in the end, Aero is Aero whether its a paper airplane or a C-5.

Edited February 18, 2008 by HossHarris

[HossHarris]

Fun debate. I'm not reading all the posts thoroughly, but I agree with that_guy's logic.

Going with what I know: a T-38's L/Dmax is 230+fuel. So,..

If I plan to do 250KIAS and the jet has 2000# of fuel, I'm right at L/Dmax.

If I'm out of gas and doing 250, I'm flying 20 KIAS above L/Dmax,... hence, more total drag,... so I'd descend faster, right??

If you're out of gas and doing 250 you would not make it as far before you hit the ground, you're glide ratio would be worse and combined with the higher airspeed you'd have a higher VVI and hit the ground sooner as well.

T-38s also have some interesting efficiencies at high airspeeds, around 400-450 for gas mileage. There isn't much difference in gas mileage between the -1 most efficient cruise speed and just parking it in MIL. Check it out sometime when gas isn't critical ... The same range in half the time.