.
The terms are interchangeable, and in the context of the question, speed fits better.
Speed IS a rate.. distance over time.
The Vne being the same, the planes will accelerate to that speed. After they reach that speed, they cease accelerating.. a rate of change of speed, and the question becomes... how much distance did each plane to require to get to Vne.. If either plane hit the ground before it achieved Vne.. it may have been a slower acclerater. But it may have achieved Vne after the other plane, which got to the ground first, because the other plane accelarated faster.
That's why you integrate the accleration to determine the distance achieved under accleration. If one plane goes down further than the other when accelerating to Vne, its speed to the ground is then a constant rate. The other plane will have lost less altitude, and its Vne being equal to the first plane's Vne ... means it's the distance achieved during acceleration is what counts.

Paradigm, you obviously have a scheme in mind here as suggested by your second post. You've left a lot of holes in this issue so it's open to a lot of free form interpretation such that there is no one correct answer. We'll let this run for another day and then it's up to you to FULLY describe the problem and your solution. And then, if there's any holes in it, we can pick your bones clean....

I really don't think we need another 13 page thread where there's no one right answer.

Hi Paradigm

Thanks for your response. My objective of discourse by the way, isn't to 'win'. And if I end up with drawing a conclusion from a fallacious premise to be proven wrong, I gain nevertheless by learning something of which I was unaware or erronous. However, at this juncture, having considered your post entire, I still don't believe it so although I will re-examine.

I discounted BLE's post because it's inaccurate as is Dick's, which, if you'll forgive me pointing it out other than for the sake of clarity, I still think to an extent is yours re L/D. See 2nd paragraph below.

As I recall, he (or it may have been another, so forgive me if I accuse the wrong person without checking first) made the same error in another thread, but I simply couldn't be bothered correcting it at the time as the person started chest puffing about how he flew a venerable Libelle 30 years ago and was or had ben a gliding instructor ...statements which brought only knowing tears to my eyes considering the demonstrated lack of understanding.

For any given type (specifically stated they were aerodynamically identical ergo having the same serodynamic L/D), the best L/D (a gradient) doesn't alter with weight (addition or subtraction of ballast). What changes is the speed at which that best L/D is achieved with the heavier type achieving it at a higher target IAS. L/D is a gradient or angle of descent, a function of distance travelled versus distance (measured as a height) descended if you will, neither being a function of time per se.

As you already know given the L/D example, although the (heavier) glider flown at the higher IAS for the same L/D achieves the same distance, it gets to the destination in a shorter time because of the increased IAS and therefore (nil wind) GS which is the point of the exercise. If you measure this descent as a rate, ie: descent over time, the heavier glider also descends at a higher rate due to this increased weight (balance of forces). Just as it climbs at a decreased rate due to the increased effect of gravity. Ergo, the heavier aircraft will descend at a higher rate for the same target IAS'.

And this doesn't matter which speed you take, be it best L/D, Vmin sink in the case of a glider or Vmin RoD for a powered aircraft, without any increase in the lift vector (available only through a speed or AoA variation as the lift forumula clearly illustrates), which can't be obtained at identical airspeeds or without increasing AoA, the increased weight (the earths gravitational attraction of the greater mass opposed only by the equal opposing lift for both must result in higher RoD.

This is salient for any case to be considered below achievement of terminal velocity where it becomes the limitation.

I don't think so. ...?????

I initially left it out accidentally, but immediately upon proof reading what I had writtten, edited it to include it although I had difficulty abbreviating it. That clarification of the ommission must have taken less than 2 minutes. In any case, as I understand it, it's irrelevant or spurious to the resolution of either the hypothetical presented example or that of flying a glider we have been using. For any two aerodynamically identical aircraft and therefore assumed identical planforms, the lift co-efficient itself is identical although agreed the resultant of alogrithm isn't if AoA varies. In this case, if we reduce power (thrust) to control or maintain target speed due to the increase vectorial component of the increased weight rather than increasing or decreasing AoA, the Lc remains constant and descent rate increases due to increase weight without proviion of increased lift to balance it out.

It's a bit like the old training paradigm where we use the cliche "attitude controls airspeed, power controls rate of descent" as a useful tool where it is knowingly false with the two being utterly interrelated. However resolved, it holds true that if we alter one, the other is affected and we accept either a change in airspeed or ascent/descent.

However, and thanks for this as I admittedly overllooked it as a consideration of an significance, and concur that if we were to accept as a given an increased AoA, the resolved Lc alogrithms won't be with the heavier aircraft also having resultant increased lift and induced drag all else including speeds remaining a constant. We don't however want an increased AoA and increased lift, as our primary objective is to descend at the highest rate possible at constant (and identical) airspeed - limitation

Again, I would point out that we are not discussing an object in gravitational freefall, but affected by restraint (structural speed limitation in this case) as important as the exageratively illustrative feather's drag in time to descend.

Agreed the acceleration afforded by gravity is constant for all objects as Galileo (?) demonstrated. However isn't gravitational attraction a force (magnitude) which is the resultant of mass and (by) acceleration (constant 9.8m/s/s if I recall accurately)? In any case, I'll sit back and re-examine my understanding of the latter. Thanks for the question.

In summary I'd state that Drag, Lift, Thrust and Weight are all important considerations altering the dynamics as applicable in the answer to the original question. As a simple analogy, a dropped/falling parachutist (human cannonball) creates no lift (but drag). Once the drogue/drag chute is deployed, his rate of descent decreases due to increased drag, an opposing force. Once his primary canopy is deployed, it alters the dynamics yet again. A heavier parachutist under the identical shape and size canopy descends at a higher rate due to the balance of these forces in play. One can't ignore these inter-related factors in determining the effects of weight on RoD of the hypothetical case by examining it as if it were a falling cannonball...because it isn't.

look like we have a completely different set of misunderstandings here ------
It is just a simple drag race to me ---

That was kind of my point, Vne is the same for both planes but the lighter the plane is, the more steep the glide slope that achieves Vne will be. Have you ever deadsticked a 12 ounce electric foamy 3-D plane to a landing? You have to fly a very steep glide slope or these super light planes will slow down to a stall.

&*%$&T%?? another endless loop with prolonged dissertations about God knows what. Good luck getting to the bottom of this fellas.

Very nice -- but -- you are forgetting that both aircraft are limited to exactly the same airspeed. The aircraft with the steeper angle of descent will also have the greater rate of descent.

Newton sez acceleration due to gravity is the same for both planes with the same geopotential. If both planes dive steeply without regard to speed, the heavier one will win, since it will accelerate longer and it's "thrust" (weigh-force) is greater and therefore terminal velocity will be higher. But they both have the same maximum speed so the pilot of the heavier one will reduce power and/or dive less steeply to keep alive. So if The two airplanes are assumed to reach the same maximum airspeed at the same time, the lighter one will win, since it's glide angle can be steeper.

paragidm, is my reasoning off here?

BTW, I intentionally did not read anyone else's reasoning before posting this, and I see at least britbrat is thinking the same way.

You are as free as the wind NOT to participate. Of course you are also free to participate in a non-constructive way. Or maybe you believe you are discouraging the rest of us from wasting our time, making the above constructive. [8D]

I though that I just said that

Glad we're on the same plane [sm=lol.gif]

Thisis just plane goofy--
The original question does not include power -but says both have same top speed - so -I assume they are powered .
The planes are the same otherwise -except for weight.
so quickest to the ground ?
go for full power vertical dive - and best power to weight craft hits full speed the quickest - THEN backs off power to max speed.
He wins
why consider dive rates etc? maybe they both have dive brakes -
I wrote my scenario entirely within the original question. and as such the one with the quickest vertical acceleration in a dive -wins.
Looks like everyone has their own idea as to whether or not the planes are gliders or what .
Mine were speed brake equipped high powered racers. and weight has NOTHING to do with which one wins.

Good point, but having a Vne has nothing to do with whether an airplane has an engine or not. Gliders have Vne also.

Using this reasoning the lighter plane still wins, but with a small margin and for a different reason. Two right answers?

the one which hits and holds VNE the fastest wins -weight means nothing -the way it is worded.
IF the qustion was for gliders - different answer absolutely
as it stands - point straight down and hit VNE-deploy speed brakes - hold that speed-- you win.
OR reword the question to use the weight difference as a factor
and specify power - equal or non powered same top speed means nothing

Weight (or really mass) means everything, by your own argument in your own scenario. The lower mass of one airplane allows it to accelerate to Vne faster than the more massive one with the same thrust and drag. mass is important to either the "glide angle" or the "vertical drag race" interpretations of the problem. Using the drag race scenario, the race is really over when the first airplane reaches Vne.

Am I way off here?

paradigm, where are you? britbrat? Calling all engineers and physicists who don't mind wasting time at work!

That's right mesae. There's just far too many loose ends to the way the problem is worded. Either we end the issue at the conclusion of the first nose over drag race up to Vne or we consider the two planes in their respective glide slopes when Vne is stable. Otherwise we'll have another conveyor belt takeoff thread.

.
Yes I have. And when I have the prop brake operational, the glide is shallower. It's drag, not weight on these peculiar airplanes.

With respect britbrat, I'm not forgetting the speed limitation or equivalence at all. It just doesn't work like that. The limiting or critical factor influencing rate of descent is neither that airspeed restriction or descent angle, be it powered or glide.

And I concur with everyone else, that unless given in a context of conventional operation rendering the absurd or esoteric understandably as incorrect answers, the question presented in this forum would have benefitted from tighter specification and greater scenario illustration...if only to avoid the esoteric.

From the perspective of my understanding taking the presented case as a conventional descent of identical fixed wing type of varing weights constrained only by the stated airspeed limitation influenced by my not inextensive aviation experience and years of examined and applied knowledge your presented argument presents as lacking either a knowledge of or thorough understanding of the fundamentals ordinarily accepted as knowns and which shouldn't require amplification. I sincerely don't mean that disrespectfully. Specific knowledge isn't a function of intelligence, but acquired learning, and it'd come as no surprise that in your specific area of expertise, you know stuff I haven't a clue about. Lest you mistake my directness for arrogance, as Bruce can attest, I'm not averse to admitting when I don't have knowledge about something eg: viz: "electric gumby".

All I can suggest is that you and some of the others do some reading [link=http://www.amazon.com/gp/product/156027140X/ref=pd_sim_b_1/102-0225426-0188145?%5Fencoding=UTF8&v=glance&n=283155]"Aerodynamics for Naval Aviators"[/link], [link=http://www.amazon.com/gp/product/0582237408/102-0225426-0188145?v=glance&n=283155]Kermode's "Mechanics of Flight"[/link] and [link=http://www.amazon.com/gp/product/1854861905/qid=1142641279/sr=1-1/ref=sr_1_1/102-0225426-0188145?s=books&v=glance&n=283155]Simons "Model Aerodynamics"[/link] are all pretty respected and easily assimilated texts which would be a good start.

Now as any good Captain should be only too well aware, we are not infallible. If I am missing something vital particular to this problem, I am open to disproving the premise upon which my finding is based if it is based upon a misunderstanding, provided it is done from a position demonstrating as basis of sound conventional knowledge. The statements presented so far don't and thus I reject them unless they can represent an argument illustrating why they are sound? Paradigm did present a convincing response, but which upon examination, didn't hold up as I have already discussed, although what he said may have been what he had in mind when he constructed the question. Which poses the question, did Paradigm construct the question or plagurise it from somewhere else. If the latter and he did so verbatim, it may open up the avenue to prove or disprove our respective hypotheses.

BTW, I'm not just 'name dropping' that bibliography. Having read all of them and more over many years in the aviation industry, acquired BSc Av, amongs other more practical professional aviation qualifications all used extensively proving the soundness of my knowledge in examination and in practice, I have yet to be offered a convincing argument that the variance in rates of descent for the question posed would be influenced other than as I've already amply explained.

I readly admit I am not treating the issue as a phsyics experiment dropping ball bearings to prove acceleration due to gravity is a constant of 9.8m/s/s, which btw becomes moot when forward flight and a wing is involved. If it were just so, how would be control rates of descent if it were just down to gravitational acceleration? And as we can and do, how do we manipulate them and what are the constraining and salient factors. Like I said previously, the question implied ipso facto by virtue of absence of any specific qualification otherwise a conventional (understood) powered aircraft constrained only by the stated speed factor and differing only in weight.

In essence, the correct answer was already presented in my more concise first which really should have required no amplification whatsoever.

Edit: proof read edit

Insufficient information for even a goofy discussion.

Were they level and at WOT ?
Were they level and at 1/2 WOT ?

If at 1/2 WOT, the lighter plane should accelerate faster to max speed and destruct first.
It won.

Given the carte blanche specifications in the query - The weight & top speeds of the planes and structural limitations are meaningless .
All of the books/referrence writings on flight etc., can add nothing to answer this question -as it stands .
Using logic -you always FIRST reduce a question to it's limits .
There are no constraints in the question to using more power or air brakes or any other devices - in either craft
First one to VNE and holding that speed --wins -
It is that simple

I respectfully disagree.

"Two airplanes identical in geometry are flying at the same altitude above ground."

Ergo by deduction this statement implies by omission of specific to the contrary that (a) the airplanes are in cruise - sustained straight and level flight - (b) at the same position and height above the ground. Note: Altitude used in the context he has is a misnomer because the word altitude per se implies a reference datum of MSL aka altimeter QNH, not "above ground" which is a height aka QNE, or in the case of a specific airfield, QFE.

Accepting that by convention in the absence of specific contradition certain unknowns must be assumed, if flying at cruise they would be flying at (a) cruise power at (b) a conventional mean cruise altitude to type be it. 6000ft, FL180 or FL370. The actual altitude is irrelevant accepting that it falls within the mean norm for type, and we have sufficient data otherwise to presume their config. assuming ops norm in the absence of anything stating otherwise. ie: Stable level, cruise speed, cruise power, etc

The fact that he has specified "identical in geometry", it can reasonably be assumed the two airplanes are to be considered identical types.

Paradigm specifically stated "airplanes" , which again, by absence of the use of the any specifically contradicting term such as glider, sailplane, gyrocopter et al, we can reasonably assume he meant powered aircraft be they turbine, jet or piston, single or multi.

"One of the airplanes weighs twice the other".

Quite specific here. By omission to the contrary, the identical types and thus identical respective performance in all regards is identical save their weight - operational technique notwithstanding but also considered operated by the same robot for the purpose of the exercise.

"The airplanes are going to race to the ground to see who can come down faster."

OK, room for clarity here. Bottom line is interpret the objective of the question. Which, from "race to the ground" and "come down faster" I clearly interpret as "descend faster" as did our dive bombing technique advocate friends. I didn't interpret him to mean however other than a conventional speed limited max. rate descent, something similar to what I consider - profiled descents, driftdowns, emergency ie: max rate limited by Vne - in practice in powered and unpowered flight regularly.

Is this actually what paradigm meant? Or did he actually mean to say "race to the destination"? Clearly he did not, as he clarifies in his sentence following the one presented immediately blelow.

"The hitch is both airplanes have the same maximum speed, meaning they will flutter apart if flown faster than that."

He's clearly saying that for the purpose of the exercise, you can't fly the friggin' things faster than Vne. That's an and the speed limitation.

"So, which airplane reaches the ground first and why?"

Again he reinforces he is seeking an answer to 'that all else being equal, which aircraft can achieve the highest rate of descent given a speed limitation of Vne when one is much heavier than the other', or in other words, what effect if any will weight have upon max. RoD with a the speed limitation of Vne. Implies conventional max. rate descent profile must be maintained. You simply can't push a conventional aircraft nose over and point it straight at the granite and stay within that speed constraint unless it is a specific type designed with deployable speed controlling devices intended specifically for the purpose.

Far from an absurd 'discussion' cyclops2, it appears to me only that you haven't thought the problem through. What has become apparent is that for those without a familiar working knowledge of airplane ops or proficient with the performance issues pertinent to the topic, they need the problem 'spelled out' in greater detail for them to comprehend it adequately.

Apt nick btw

Not to infer we don't trust your assertion, would you like to present your hypothesis detailed sufficiently so the rest of us can be privy to how you arrived at this conclusion? Even were credibility to relent its natural demand, the proverbial 'inquiring mind' insists. [:-]

What's goofy about it? If an airplane experienced cabin pressure failure at 35,000 ft above MSL, I would think that the flight crew would be highly motivated to get down to under 15,000 ft, preferably while they were still conscious.
It seems to me that the maximum safe rate of descent would be Vne x sine of the angle of whatever glide slope results in Vne. If the plane is so light that it takes a verticle dive to reach Vne, then the RoD will equal the Vne.
If the plane reaches Vne with a 45 degree glide slope, then rate of descent will be .707 of Vne.
If the plane reaches Vne with a 30 degree glide slope, then the rate of descent will be .5 Vne.
Deploying spoilers, flaps, putting the landing gear down, and flying in a side slip will increase drag, allowing the plane to go into a steeper dive without exeeding Vne, resulting in a higher rate of descent. Reducing power to idle would likewise allow a steeper dive without exeeding Vne.
This assumes that the plane has the same Vne with flaps and gear down as it does clean, which is not nessesarily true.

It is quite true that increasing the weight of a plane increases induced drag. Induced drag is greatest at stall speed and goes down as the airspeed increases. Parasitic (plain old air resistance) drag is very low at slow speeds and increases with the square of the speed. At a plane's Vne, the airspeed is so high that the drag is almost entirely just plain old air resistance. So, it seems reasonable to me that a heavy plane would have to fly a less steep glide slope to keep the airspeed below Vne, resulting in a lower rate of descent.

There may be exceptions of course. A 3-D electric foamy is so light and draggy that it may need a verticle dive and extra ballast just to reach it's self destruct airspeed.

sure
read your own verbage
and -in your last paragraphs -note that YOU assumed what was conventional.
I do not agree with your assessment of conventional .
-no limitations as such were ever mentioned.
To me conventional includes craft capable of just exactly that - the ability to push over and accelerate at full thrust.
As long as VNE is not exceeded - no harm -no foul.
- the craft may be a 3D foamie or a 747 according to the original question - no parameters were established.
try to keep things simple - less chance for error.
do not read anything into a question which is not there.
If I seem a bit curt - please excuse me - I have spent a lot of time offering depositions on technical matters - so I try to answer only what is asked.
Next question?

This is still going on??!!

Mesae & I (with all modesty) have it.


I see that B.L.E. is also on track

[8D]My Flatana "flaps" its wings at full throttle in level flight. So I don't do that anymore.