Talk:Flight

Lift-to-drag ratio

Today, Flight#Lift-to-drag ratio was edited to include the following new sentence:

• Lift-to-drag ratios are usually found using a wind tunnel. See the diff.

The cited source for this sentence is grc.nasa.gov. This website mentions wind tunnels only once. It states Lift and drag coefficients are normally determined experimentally using a wind tunnel.

We now have the situation where Flight states that lift-to-drag ratios are usually found using a wind tunnel, and this is purported to be sourced from a site that talks about wind tunnels only in the context of lift and drag coefficients. Readers of this Talk page will know that lift coefficients and drag coefficients are not the same as lift-to-drag ratios. Wikipedia should not talk about one when it means the other.

I challenge the accuracy of the statement and I particularly challenge the legitimacy of using this citation to support the statement. I have raised it at the originator’s Talk page but the originator appears to disagree. The conversation is available at User talk:GliderMaven#Lift-to-drag ratio. What do others think? Dolphin (t) 02:49, 2 May 2012 (UTC)

After a week nothing has been written here in defence of the above new sentence, so I have deleted it. Dolphin (t) 07:09, 9 May 2012 (UTC)

Hey, here's an idea, how about you make constructive edits to the article?GliderMaven (talk) 13:36, 9 May 2012 (UTC)

I see that GliderMaven has reverted my deletion, without comment or edit summary. See his diff. I have now listed the matter for a third opinion. Dolphin (t) 02:26, 10 May 2012 (UTC)

---

Hey, guys, I'm here form the 3O board. It seems to me that the source does support the statement; it says that "Lift and drag coefficients are normally determined experimentally using a wind tunnel." Since the lift-to-drag ratio is just the lift coefficient divided by the drag coefficient, and there doesn't seem to be any other way that it's measured, I don't think it's much of a stretch to say that the lift-to-ratio is calculated this way. However, in the interest of following the source closely, could we change it to something like: "The lift-to-drag ratio is equivalent to the lift coefficient divided by the drag coefficient, which is usually calculated with wind tunnel data.(source)"? It gives a little more data, and removes even the appearance of OR. Thanks. Writ Keeper ⚇♔ 14:43, 10 May 2012 (UTC)

Hello Writ Keeper. Thanks for volunteering to provide a third opinion. To the layman, it must seem reasonable to say that if lift and drag coefficients are normally determined in a wind tunnel, then the ratio of the two must also normally be determined in a wind tunnel. In science and engineering there are many examples where two numbers can only be determined with difficulty but the ratio of the two can be determined easily. For example, a mariner may be able to see a coastline with a high mountain but unless he goes ashore with suitable equipment and climbs the mountain he has no idea of its height. Similarly he has no idea of the distance to the mountain unless he goes ashore and determines the latitude and longitude of its peak. However, with nothing more than a hand-held sextant and a page of trigonometrical data he can determine the ratio of the height of the mountain to its distance away. It is simply the tangent of the angle he measured using his sextant.

A similar principle applies in the aerodynamics of the aircraft. The relationship between the lift coefficient and the drag coefficient of a particular airfoil section or a design of aircraft is not easily determined, particularly if no aircraft of that design actually exists at the time. Engineers must sometimes resort to a wind tunnel to obtain the desired data. But when a prototype of the aircraft is available it is a simple matter to measure its climb angle, or descent angle, across a range of speed and use that data to determine the lift-to-drag ratio at each speed in the range.

Building an aerodynamically accurate model of an airfoil or an aircraft and testing it in a wind tunnel is an expensive business. It is an activity used extensively with proposed designs of new military aircraft and the largest civilian passenger aircraft. The majority of aircraft designs are smaller, civilian designs for carrying up to 9 passengers, flying training, recreation, gliding etc. These small aircraft are not supported by the budget necessary to build aerodynamically accurate models and test them in wind tunnels. For these small aircraft, the lift-to-drag ratios over a range of speeds are determined by test flying - measuring the airspeed and rate of descent with the engine at idle power - no wind tunnel necessary!

The sentence in question is not saying Lift-to-drag ratios can be determined in several ways and one of them is testing in a wind tunnel. It is saying Lift-to-drag ratios are usually found using a wind tunnel. That is incorrect - lift-to-drag ratios are not usually found in a wind tunnel. For every military aircraft or passenger jet-aircraft that is given the benefits of testing in a wind tunnel there are many more small civilian aircraft and recreational aircraft that never see the inside of a tunnel.

We must focus on the source of the information. At present we have a website that says lift coefficients and drag coefficients are normally determined experimentally using a wind tunnel. This must not be paraphrased to say lift-to-drag ratios are normally determined experimentally using a wind tunnel.

I am happy to participate in this discussion, but be aware there is no burden on me to provide information to support my point of view. There is a burden on the User who inserts material into article-space to provide a reference to adequately support the material. Dolphin (t) 22:56, 10 May 2012 (UTC)

No, given that the reference supports the assertion, and particularly as Writ Keeper agrees that this is so, I require you to give a reference to support your view, otherwise I will continue to revert your removal of this.GliderMaven (talk) 23:22, 10 May 2012 (UTC)

Far, far too much time has been wasted on this already, please spend your time more profitably improving the article instead of arguing over this kind of thing.GliderMaven (talk) 23:22, 10 May 2012 (UTC)

I have no problem rewriting it as Writ Keeper suggests.GliderMaven (talk) 23:22, 10 May 2012 (UTC)

Dolphin, what do you think about my proposed rewording? I think it addresses your point without requiring the entire removal of the information. (There is a slight typo in my proposal: it would read "which are usually calculated", rather than "which is usually calculated", since it's the two coefficients that are calculated rather than the ratio itself.) Writ Keeper ⚇♔ 00:49, 11 May 2012 (UTC)

Hello Writ Keeper. Unfortunately your proposal is unacceptable for two reasons – firstly it is technically unsound, and secondly it is remote from the cited source.

Dividing the aircraft lift coefficient by the aircraft drag coefficient is one way of determining the lift-to-drag ratio, but it is not the only way. Doing so using lift and drag coefficient data determined with wind tunnel testing of an aerodynamically accurate model is the minority way of obtaining this data. To imply that anything to do with aircraft is usually determined from wind tunnel data is incorrect. The majority of aircraft are small civilian designs that never see the inside of a wind tunnel.

It would be reasonable to say that if a model of an aircraft is tested in a wind tunnel, lift-to-drag ratio would be one of the characteristics measured. It isn't correct to imply that most aircraft have their lift-to-drag ratio measured in a wind tunnel.

Something like the following is technically sound.

The lift-to-drag ratio of an aircraft over a range of speeds can be determined in a number of ways – flight testing, wind tunnel, calculation or comparison against another similar aircraft whose lift-to-drag ratio is known.

In flight testing, the glide ratio of the aircraft is measured with the engines at idle thrust or power. With zero thrust or power the lift-to-drag ratio is equal to the glide ratio.

A wind tunnel can be used to measure many aerodynamic characteristics of an airfoil, wing or complete aircraft. Lift-to-drag ratio is one of the characteristics determined during testing in a wind tunnel.

The lift-to-drag ratio of a proposed design of aircraft can be calculated using the dimensions of the aircraft, its proposed weight, and published aerodynamic characteristics of the airfoil sections used in the wing and horizontal stabiliser.

As you can see, if Wikipedia states that lift-to-drag ratios are usually found using a wind tunnel, Wikipedia is wrong. Dolphin (t) 02:17, 11 May 2012 (UTC)

We would need additional sources to say all that. Do you have any? In the meantime, I don't see how it is remote from the source. The proposed wording doesn't say that dividing the one by the other is the only method, or the most common method, or that most aircraft do it this way, or anything like that. It just says that it is a method, and the data for the method (i.e. the lift coefficient and drag coefficient) is usually obtained through wind tunnel testing, which is exactly what the source says. If you don't like that, how about "One method for determining the lift-to-drag ratio is dividing the lift coefficient by the drag coefficient, which are usually calculated with wind tunnel data.(source)" I'd also have no objection at all to adding another sentence after that to say "the lift-to-drag ratio can also be determined experimentally, through measuring whatever by whatever in-flight; this approach is commonly used with smaller aircraft." or something like that, as long as you have a source for it. Writ Keeper ⚇♔ 02:31, 11 May 2012 (UTC)

Where you propose suggesting that lift coefficients and drag coefficients are usually determined from testing in a wind tunnel, I have a similar objection. For the majority of aircraft, lift coefficients and drag coefficients are not measured in a wind tunnel because the majority of aircraft never see the inside of a wind tunnel.

I am happy to continue the discussion, but I have a problem with the way you are handling it. A statement has been added to Wikipedia article-space, and an in-line citation has been appended to it. I have challenged that statement, and I have challenged the validity of the citation. The burden is on GliderMaven to justify its retention; not on me to justify its removal. Throughout this saga I have initiated new threads on Talk pages; I have contributed to discussions; I have asked for a third opinion; I have participated actively in the debate. GliderMaven has not participated in any meaningful way. He has not written anything to justify his statement or the source he has cited. You have volunteered to provide a third opinion but so far it looks like you see your role as being merely to defend the status quo. The statement should be removed while we discuss its validity.

There is a statement at Flight that has been soundly challenged. Let's remove that statement and then continue the debate to see if there is ground for restoring it, or if we can find some alternative wording and citations that are acceptable to the three of us. Dolphin (t) 02:48, 11 May 2012 (UTC)

My "role", as you put it, is to figure out what's best for the article, based on the reliable sources we have. No more, no less. I have no other interest. I certainly have no interest to "defend the status quo"; in fact, in a different conversation elsewhere, I've been arguing pretty heavily against it. Anyway:

Read WP:BURDEN a little closer. "You may remove any material lacking an inline citation to a reliable source." (emphasis mine) If the material is backed by a reliable source, then the burden of proof has been satisfied. I agree that the original sentence is not really supported by the source; all the same, there's a grain of truth in there, so I'm proposing, instead of just axing it, we reword it to follow what the source actually does say. This statement has an inline citation to a reliable source (at least, you don't seem to be saying that the NASA website is unreliable; that's an entirely different discussion that would be good to have if you do think it's unreliable). That website says direct quote: "Lift and drag coefficients are normally determined experimentally using a wind tunnel." Unless you don't think "normally" means "usually", I don't see what you mean when you say that the source doesn't support this. If you think the source itself is wrong, that's fine, but you do need a reliable source of your own to refute that reliably-sourced claim. Writ Keeper ⚇♔ 03:32, 11 May 2012 (UTC)

Thanks. You have hit the nail on the head when you write I agree that the original sentence is not really supported by the source. We both agree on that. The source talks about lift and drag coefficients but the sentence in Flight talks about lift-to-drag ratio. We could discuss the relationships between the various lift coefficients and lift-to-drag ratio but I hope we can agree that these words are not the same. Where the essential words in an in-line citation are different to those in article space then it is possibly paraphrasing leading to an unintended change in meaning. There is a burden on whichever User added the words to article space to justify the choice of words, especially if that choice is formally challenged.

Perhaps the way ahead is to start with precisely the wording of the relevant sentence in the source, and see what justification can be found for changing those words.

(On a side issue, I don't have a high regard for everything in the NASA websites on aerodynamics. They are popular sites with elementary information for enthusiasts. Some of their statements are ambiguous. However, at this stage I am not interested in challenging the accuracy of the source so I will leave it as a side issue.) Dolphin (t) 04:57, 11 May 2012 (UTC)

The problem here seems to be that the statement Lift and drag ratios are normally determined experimentally using a wind tunnel does not mention the other method of calculating L/D. If it was modified to something like Lift and drag ratios are determined experimentally using a wind tunnel or by flight testing and a source was added for the second fact would that calm the waters? As a wind tunnel inhabitant and glider pilot I know that both are true, to my knowledge even the latest glider designs continue to be flight tested by the manufacturer by taking high tows in still air and plotting airspeed against sink rate at varying speeds, wing loadings and flap settings. The results produce the airspeed/sink rate polar curve, the max L/D and speed at which this happens (the glider's selling point) are taken directly from the plot. Nimbus (Cumulus nimbus floats by) 05:44, 11 May 2012 (UTC)

Thanks Nimbus. In one edit you have managed to capture everything I have been trying to say! Dolphin (t) 05:58, 11 May 2012 (UTC)

I have reliable sources for glider L/D flight testing. Whatever fix is applied here should also be applied to the Lift-to-drag ratio article as the same statement appears there. Nimbus (Cumulus nimbus floats by) 06:42, 11 May 2012 (UTC)

We also need a reliable published source for the statement that lift-to-drag ratios are determined in a wind tunnel. Dolphin (t) 07:55, 11 May 2012 (UTC)

Would have thought the NASA source would be ok for that, it uses the word 'normally' which I would take to mean that they suggest that there are other methods of finding L/D other than a wind tunnel. A third method of calculating L/D is through computational fluid dynamics, I can cite that but only in relation to motor sport applications (this is a summary article on flight so we should stick to its scope), could be added to the related L/D ratio article though if it's not there already. Nimbus (Cumulus nimbus floats by) 08:20, 11 May 2012 (UTC)

The NASA source says nothing about lift-to-drag ratio. It says lift and drag coefficients are normally determined experimentally using a wind tunnel. No mention of lift-to-drag ratio. Dolphin (t) 13:08, 11 May 2012 (UTC)

So it does, I mis-read it. Nimbus (Cumulus nimbus floats by) 13:31, 11 May 2012 (UTC)

Hey, guys, Dolphin asked me to come back here and express my opinion again. Let me preface by saying this about 3Os: they're not intended to be authoritative; they don't really mean anything except the opinion of the 3O Wikipedian who provides it. The analogy is that, if two people are arguing about something on the street, 3O is them turning to a random person who happens to be passing by and asking him what he thinks about it. Originally, 3Os were supposed to end there, too; we are usually expected to drop our opinion and then leave. Most of us who frequently provide 3Os end up taking more of a "mediator"-type role, but we're not actual mediators; we don't have any power, authority, superior wisdom, etc. etc. So, don't take this as more than it's worth.

Anyway, my opinion is this: The original sentence, as it was worded, was unsupported by the source. Not egregiously unsupported, mind you; had I noticed the discrepancy on my own (unlikely), I probably would've just said "eh", shrugged, and went on my way. However, the sentence has been challenged and it is indeed unsupported by the source, so it has to either be removed or reworked to be in line with what the source says. To that end, I suggested a rewording of the sentence to read something to the effect of: "One method for determining the lift-to-drag ratio is by calculation of the lift coefficient and drag coefficient which, when divided, form the lift-to-drag ratio. These two coefficients are usually found with data from wind tunnels." The exact wording wasn't really important here; the meaning is what I was trying to get across. I (still) think that this rewording is clearly supported by the source, which reads: "the L/D ratio is also equal to the ratio of the lift and drag coefficients. [...] Lift and drag coefficients are normally determined experimentally using a wind tunnel." In my mind, the best-case scenario for this rewording is that Dolphin (or whoever) provides a reliable source saying that the lift-to-drag ratio is also commonly found experimentally in test flights, and adds a sentence to that effect immediately after. That would require a new reliable source, though. Even without Dolphin's hypothetical addition, though, I think the rewording (although not the original wording) is good enough to stand alone; it's clearly supported by the reliable source that we have.

TL;DR version: original wording was unsupported by the source; it must be removed or reworded. I suggest a rewording to emphasize that it's the coefficients that are usually found in wind tunnels, and that it is but one way to find the L/D ratio. If someone can add a reliably-sourced sentence that discusses other ways of calculating it, great, but it's not necessary for the rewording to stand.

Again, I feel that I've said all I can usefully say at this point; I freely admit that this is not my area of expertise. If you need further clarification, please post on my talk page (I'll be watching this page for a day or so, but I'm going out of town late tomorrow, so I won't be on reliably for a while after that, so a post to my user talk page will probably be necessary at that point). Writ Keeper ⚇♔ 03:51, 15 May 2012 (UTC)

Thanks Writ Keeper. I have altered the sentence so that it now states: "Lift-to-drag ratios can be determined by flight test, by calculation or by testing in a wind tunnel." We must now turn our attention to provide ini-line citations that genuinely support this statement. To ensure we don't overlook it I have added "Fact" tag immediately after the sentence. Dolphin (t) 04:02, 15 May 2012 (UTC)

I know I am alittle late to the conversation but don't you think that everybody has ignored the operator in the NASA sentence experimentally. i.e. when determined experimentally they use a wind tunnel. Semantics.Petebutt (talk) 12:58, 16 September 2012 (UTC)

What was challenged was the statement that 'Lift-to-drag ratios are usually found using a wind tunnel. This was sourced from a reputable document that merely said lift-to-drag ratios are normally determined experimentally using a wind tunnel. There was no objection to the statement in the reputable source because it is true that, experimentally, lift-to-drag ratios are determined using a wind tunnel. The objection was with the statement added to Wikipedia that lift-to-drag ratios are usually found using a wind tunnel. The former is talking about lift-to-drag ratios determined experimentally, but the latter is talking about lift-to-drag ratios in general. The issue appears to have been resolved satisfactorily.Dolphin (t) 13:09, 16 September 2012 (UTC)

Lift

Shouldn't this article answer the question: how does a wing lift? When I was a kid my Dad explained it to me this way. The air over the top of the wing (convex surface) has to flow farther than the air over the bottom of the wing, so the air pressure is lower on the top and higher on the bottom. He said it's Bernoulli's principle. Was he right? If he was right then shouldn't an article on flight feature Bernoulli's principle prominently? — Preceding unsigned comment added by LFlagg (talk • contribs) 19:31, 18 June 2013 (UTC)

The answer is found at the Lift (force) article, which is linked from here.GliderMaven (talk) 21:32, 18 June 2013 (UTC)

There is also coverage of the topic at Bernoulli's principle#Applications. Dolphin (t) 00:21, 19 June 2013 (UTC)

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Gliding flight: plants do it too

Just leaving this here for your consideration and comment: Winged seeds (samaras) and seeds with fibrous tufts, such as those of the common dandelion, use aerodynamic shapes to delay their descent. Neither this article nor the one on gliding flight mention this. Just plain Bill (talk) 00:43, 25 December 2017 (UTC)

Technically, neither are "gliding". An aerial descent with a glide angle of less than 45 degrees (or more drag than lift) is considered "parachuting" (see "gliding" ants), and dandelion seeds are more akin to spider ballooning. HCA (talk) 15:14, 26 December 2017 (UTC)

"Gliding" implies intent and control. Humans, birds and squirrels etc glide with intent, using a controlled process. What plant seeds do is more like "falling untidily". Wdford (talk) 15:57, 26 December 2017 (UTC)

The question of whether dipterous insects such as midges or gnats, or odonatans such as dragonflies, are capable of forming intent is one for the philosophers, presumably right after they nail down the hard problem of consciousness in terms acceptable to all. I suppose the intent of the builder and operator of an autonomous drone transfers to the device itself, or at least it would be difficult to argue otherwise with a straight face. It's complicated.

The dispersal of maple seeds does not seem particularly untidy; it seems more like elaborate, maybe even refined falling. Proliferation of weedy maple saplings is another matter.

Early designs for re-entry vehicles of the "flying bathtub" type had low lift/drag ratios, and may have relied on ballistic effects to get acceptable glide slopes. "Falling with style" is one way to describe those proposals.

I have no wish to press this issue into a WP article without consensus; your comments are thought-provoking. Thank you, Just plain Bill (talk) 23:36, 29 December 2017 (UTC)

Paper aeroplanes have no intent or control, and yet they seem to glide just fine. Is it not gliding if I drop a paper aeroplane and it flies? And yet I would lack intent as well. I don't see what intent has to do with it.GliderMaven (talk) 00:00, 30 December 2017 (UTC)