Frontal area vs. drag vs. wetted area vs

[Nicknick]

Fascinating...

Having been blown off my feet at our camp-site near Holyhead by a Vulcan doing a low pass prior to 'Bump & Go' at RAF Valley, I remember staring at underside of its vast delta for what seemed a very long time...

Must have been quite a side! The Vulcain and the Victor were both great looking planes with a touch of typical British "Weirdness".

The Vulcain is more a delta wing in my view, but there is no strikt border between flying wings, delta wings and blended wing bodies.

 

[HoHun]

Hi Nicknick,

Interesting document, in my view it fits very well to my argumentation. The YB35 could have been (without the mechanical issues) a very capable airplane which is confirmed by this report. The reason behind this is, that it offers low induced drag and high lift by its “fat” wing with an high wingspan. The parasitic drag of this design tends to be high, when related to the typical small wetted area of flying wings, but surly low when related to the high lift capacity (unfourtunally I haven’t found the L/D of this plane).

The NACA report actually scaled the aircraft to an identical wing loading, so the flying wing was not at a disadvantage there, and still surpassed the performance of the conventional aircraft. So, comparing comparable designs, the wing can't be the one to blame.

The YB-49 compares not so well against the B-47 because it's an aircraft operating off its design point. Still, the combat radius of the YB-49 is actually slightly greater, at the same bomb load, than that of the B-47A. It's just not that fast (and definitely not that fuel efficient)!

(Check out https://alternatewars.com/SAC/SAC.htm ... the Standard Aircraft Characteristics sheets have a lot of interesting information, and there might be data on other types relevant for the comparison, too.)

Regards,

Henning (HoHun)

 

[Nicknick]

According to what I’ve found, the combat radius of the B47 was larger than that of the YB49, but I don t know the exact boundary conditions.

To fly most efficient, the aircraft should operate close to its best glide ratio, which wasn’t the case for the YB49 when flying at high speeds and medium height. If other jets would have been available to enable a higher ceiling height, the YB49 could have beaten the B47 but jet engines these days seam to be limited to about 12.000 m.

intresting link!

 

[red admiral]

The NACA report actually scaled the aircraft to an identical wing loading, so the flying wing was not at a disadvantage there, and still surpassed the performance of the conventional aircraft. So, comparing comparable designs, the wing can't be the one to blame.

But interestingly one of the issues with flying wings is that you can't have big trailing edge flaps for high lift because you have nothing to trim these out. So you end up with equal wing loading but poorer field performance, or more normally a much lower wing loading - hence mass, zero lift drag.

But its all trade offs dependent on mission and technology

 

[Nicknick]

As the NACA report states, it depends on the mission. If there would be no limit in engine performance in great height, the low wing loading wouldn’t be a disadvantage, because it could be used to reduce the drag simply by flying higher.

I don’t really understand, why no slats can be used on the leading edge to balance out the flaps on the trailing edge on a flying wing. I’m sure there will be more than one NACA paper on this topic and there must be good reasons for it. Here is an example for going into extreme with slats:

View: https://www.youtube.com/watch?v=JcckHHiYkL0&ab_channel=MikePatey


Might not be the best approach for a laminar wing, but seemingly it works and looks supercool...

 

[HoHun]

Hi,

But interestingly one of the issues with flying wings is that you can't have big trailing edge flaps for high lift because you have nothing to trim these out. So you end up with equal wing loading but poorer field performance, or more normally a much lower wing loading - hence mass, zero lift drag.

But its all trade offs dependent on mission and technology

That's what I was thinking too, but the NACA report points out that for the same mission, the flying wing has a lower structural weight and thus a better power loading (HP were held constant for the configurations), so thanks to its better acceleration, the flying wing has the shorter take-off run anyway.

The maximum take-off coefficient of lift was assumed as 2.0 for the flying wing and as 2.4 for the conventional aircraft.

Of course, stability and control of flying wings at that time hadn't been fully resolved yet, so potentially, with weaker control only a lower clmax would have been attainable.

The purpose of the report was to provide an indication of whether it made sense to pursue flying wing designs at all. If no advantages were to be expected, there would have been not much of a motivation to try and resolve the configuration's initial problems.

Regards,

Henning (HoHun)

 

[red admiral]

I don’t really understand, why no slats can be used on the leading edge to balance out the flaps on the trailing edge on a flying wing. A

Slats don't really change the pitching moment

 

[Trident]

If two planes with similar wetted areas can differ by a factor of more than two in terms of drag and the “worse” plane is a very clean design, than this shows clearly there is something wrong with the criteria.

That's because a consideration of aircraft drag solely on the basis of area (no matter whether that is frontal, wing or wetted area) remains incomplete and inaccurate without any reference to planform. This brings us back to Raymer - the B-47 and Vulcan perform similarly, even though both their areas (wing and wetted) as well as their wing aspect ratios differ widely. Clearly then neither wing-area based aspect ratio nor wing area (nor frontal area, had the comparison included it) in isolation can give a reliable indication of relative drag levels. Not for designs of such radically different configuration, anyway. Combining wetted area and planform in one measure (wetted aspect ratio) however does, with both bombers scoring similar values corresponding to their similar performance. The YB-49? Has a markedly lower (wing) aspect ratio than the B-47, so with similar wetted area is at a disadvantage.

Frontal area raises at least as many new problems as it fixes, using it isn't wrong per se, but (for aerospace industry purposes) is less intuitive to derive and makes insights harder to discern. Thanks to their wings, aircraft are dominated by skin friction and lift induced drag as compared to form drag in cars. For supersonic aircraft form drag (in the shape of wave drag) gains in importance, but the behaviour and countermeasures are sufficiently different from subsonic form drag that it is best considered a separate issue.

I don’t really understand, why no slats can be used on the leading edge to balance out the flaps on the trailing edge on a flying wing. I’m sure there will be more than one NACA paper on this topic and there must be good reasons for it. Here is an example for going into extreme with slats:

Leading edge devices don't really add lift at a given angle of attack, and hence don't affect pitching moment very much. They merely delay stall/separation so that the wing can safely operate at higher AoA, which is what gives a benefit in lift (but also adds to the trim requirement).

 

[Nicknick]

I would have liked to give a more well-established quotation than a mildly insane (in a positive way) homebuilder on Youtube, but so far, he always seems to have been very honest about things which worked out and which didn’t. Of course, judging your own creation might not always be objective. But maybe his double slat system makes the difference, he stats that this would counteract the moment from the flaps (he tested it).

Let’s hope, he unintentionally found a way which could help making flying wings more effective. Of course, double slats will disturb the airflow somewhat and increase drag.

I totally agree in that, flying wings should potentially be lighter than conventional aircrafts with the same lift capacity due to the more even mass distribution and high thickness of the wing in the middle without the wing root problem. Strangely the B47 had a lower empty weight than the YB 49.

I never stated that using frontal area as reference would be a better choice than wetted area, I just believe it is not totally unimportant just because it is not the standard reference. I think, the truth is, it is a complex three-dimensional problem which cannot be pressed in a simple one-dimensional identification number without compromises.

BTW, it’s a nice coincidence that we have 3.5 airplanes which all share some important numbers with another one from this group, so that they can be compared.

 

[red admiral]

That homebuilt hyper stol design definitely doesn't have a conventional slat - it extends massively forwards compared to normal, so i could understand a bit more impact on pitching moment from that. But such a system would be verynheavy and volume hungry for a much larger aircraft.

Also remember that they still have a seperate tail for a) stability and b) control

 

[martinbayer]

That homebuilt hyper stol design definitely doesn't have a conventional slat - it extends massively forwards compared to normal, so i could understand a bit more impact on pitching moment from that. But such a system would be verynheavy and volume hungry for a much larger aircraft.

Also remember that they still have a seperate tail for a) stability and b) control

Let's see them go through the paces of a regulatory compliant flight test program.

 

[Nicknick]

There are clearly 2 slats, one after the other and protuding different from the wing, please take a second look.

The plane is for sure an experimental, only for private use and as the builder himself makes clear, not suited for serial production. Things are different in the US than in the EU, btw. it’s not the first plane he builded and flew.

I don’t want to go to deep into the discussion about this single plane, I like what he did, but I prefer the super lightweight bushplanes.

 

[1635yankee]

Taking a look on YB-35 and the B47 will make it clear. Aerodynamics will always consider the frontal aera as very important, the Cw value (there is a equivalent value for planes, but is has a somewhat different name) is related to the frontal area vs. drag. The parasitic drag of aircrafts depends on aerodynamic quality and frontal area.

It can easily be seen, that very fast aircrafts tend to have low cross sections in combination with small thin wings. Flying wings need thicker wings for storage of fuel, propulsion and payload, this will (almost) always result in a high cross section with high parasitic drag.

The flying wing really shines when it comes to induced drag, or even in the parasitic drag vs. frontal area (Cw value), so it is very efficient for slow flying heavy airplanes.

You should compare the YB-35, YB-49 and B-47, this makes clear, why flying wings were great for long range at low speed, but unsuited for high speeds. Jet engines only make sense for high speed applications, thats why the combination failed.

Umm, as an aerodynamicist, the answer is "no, we don't really think about frontal area." We think about wetted area and separation. If something is fast enough, we think about the distribution of volume (area rule).

There are quite a few issues with flying wings that have prevented their widespread use, most of which come down to "where do you put the people?" but also runway requirements (the nose up moment required for high angle of attack decambers the wing), yaw and pitch damping (the B-49 was worse in this regard than the B-35 as propellers behind the C/G are stabilizing) and very restricted c/g travel.

Of course, the biggest one is "where do you put the people?"

 

[HoHun]

Hi again,

That's what I was thinking too, but the NACA report points out that for the same mission, the flying wing has a lower structural weight and thus a better power loading (HP were held constant for the configurations), so thanks to its better acceleration, the flying wing has the shorter take-off run anyway.

The maximum take-off coefficient of lift was assumed as 2.0 for the flying wing and as 2.4 for the conventional aircraft.

I finally managed to find what I believe is a permalink to NACA-TN-1477:

[Edit: No, I didn't. Sorry for the confusion.]

It's available via https://ntrs.nasa.gov/search ... but it's sort of hard to find the document by its ID. The Cranfield mirror used to have a much better interface, but since it was modernized a couple of years back, searching has become a lot harder over there, too: https://naca.central.cranfield.ac.uk/

Regards,

Henning (HoHun)

 

[overscan (PaulMM)]

Took two seconds to locate with Google. Second copy is a bound volume containing several NACA TN articles.

https://digital.library.unt.edu/ark:/67531/metadc54335/m2/1/high_res_d/19930082130.pdf

Technical Note - National Advisory Committee for Aeronautics

www.google.co.nz

 

[Nicknick]

Taking a look on YB-35 and the B47 will make it clear. Aerodynamics will always consider the frontal aera as very important, the Cw value (there is a equivalent value for planes, but is has a somewhat different name) is related to the frontal area vs. drag. The parasitic drag of aircrafts depends on aerodynamic quality and frontal area.

It can easily be seen, that very fast aircrafts tend to have low cross sections in combination with small thin wings. Flying wings need thicker wings for storage of fuel, propulsion and payload, this will (almost) always result in a high cross section with high parasitic drag.

The flying wing really shines when it comes to induced drag, or even in the parasitic drag vs. frontal area (Cw value), so it is very efficient for slow flying heavy airplanes.

You should compare the YB-35, YB-49 and B-47, this makes clear, why flying wings were great for long range at low speed, but unsuited for high speeds. Jet engines only make sense for high speed applications, thats why the combination failed.

Umm, as an aerodynamicist, the answer is "no, we don't really think about frontal area." We think about wetted area and separation. If something is fast enough, we think about the distribution of volume (area rule).

There are quite a few issues with flying wings that have prevented their widespread use, most of which come down to "where do you put the people?" but also runway requirements (the nose up moment required for high angle of attack decambers the wing), yaw and pitch damping (the B-49 was worse in this regard than the B-35 as propellers behind the C/G are stabilizing) and very restricted c/g travel.

Of course, the biggest one is "where do you put the people?"

I’m not a promoter of flying wings and I’m fully aware that they would need to big extremely big to give enough headroom for the people (after the failed A 380, it will take a long time until we might see another very large airplane). There might be other applications (e.g., as a temporary sending mast circling over an area where a sudden demand of high-capacity internet connections are necessary). Currently, there are other types of aircrafts being “developed” (at least by start-ups…) for this role, which would fit very well to a flying wing. For autonomous airplanes circling in great altitude for long time, you need a design which enables to fly relatively slow and close to the best gliding angle in thin air. Flying wings could shine in this role (if it ever comes to fruition).

For the obsolete ultra-long range bombers, this design (YB35) could have been the best solution, but there is no modern need for a slow flying ultra-long range bomber anymore.

How do you explain the large drag difference (about a factor of 2.2) between the YB49 and B47 with both airplanes having the same wetted area (see my link)? There is also something like a form changing factor for subsonic airplanes (I read something about It a couple of days ago and forgot the exact name of this factor and no it wasn’t the area rule), but forget where. It should be clear, that you can’t have a smooth area transition from a large frontal are to trailing edge without some length in between, what shows, that none of the aerodynamic factor are independent from each other. Of course, large frontal areas will it aways make difficult to achieve a smooth transitioning.

 

[Mark Nankivil]

No, I didn’t mix up something with the Avro Vulcan, the YB35 was developed as a long-range bomber in competition with the B36, but when Northrop switched to jet engines it lost its long rage capabilities but gained a higher speed. With that changes it had to compete with the B47 which showed more potential than Northrop’s flying wings (higher speed, longer range).

With similar proportions, physics don’t care it you are using the larger wing area with a smaller drag coefficient or the smaller frontal area with a higher coefficient to describe the total drag. If an increase the wing area, you increase the frontal area as well and so does the absolute drag. For good reasons, aircraft designers are used to the wing area as reference and car designers are used to frontal areas, both approaches lead to the same results. By the way, do you believe, the Superguppies or the Airbus Belugas weren’t affected by the large frontal area?

The wings of theYB35/Yb49 weren’t thin at all, according to this thread ( https://www.secretprojects.co.uk/threads/northrop-xb-yb-35-yb-49-and-yrb-49.9969/page-2 ) they must have been around two meters in the center section. Thick wings tend to have higher cd values which corresponds to theire higher frontal area.

Focusing on the wing area blurs the reasons for drag, folding wing aircrafts reduce the frontal area at high speeds by folding its wings (which also helps to improve the aera rule) and reduce drag.

Flying wings could become great tanker aircrafts, were high lift to drag ratios are important (low induced drag) but they don’t do well suited to minimize parasitic drag at high speeds. Flying wings tend to be much larger planes in terms of wing area or frontal area compared to conventional planes which do the same job (please compare the YB49 with the B47 or the B1 with the B2)

The YB-49 suffered from being a conversion and not being designed from the outset for jet engines and the increased speed (airfoil choice for one). As such, it is a poor example - think YB-60 conversion of the B-36 to compete with the B-52. B-2A works just fine but the primary reason for the flying wing not being a common airliner today is the design and structure of a pressure hull for a load of paying passengers, pure and simple. Toss in being seated like your in a movie theater which isn't much help either....

Enjoy the Day! Mark

 

[Nicknick]

I know there are many other airplanes, but I would like to go on with the YB49 and B47.

The Drag vs. wetted area chart are not given for a specific speed and the YB49 couldn’t even achieve very high subsonic Mach numbers. It is reasonable to believe, that this chart can be applied. Yes, it was a conversion, but the integrated jets were probably the most efficient way for a jet powered flying wing. I read some papers about blended wing bodies and it’s hard to find a good position for the jets, so the integrated solution like in the Hortens of the YB49 is very likely the best one in aerodynamic terms.

We should go on with this two planes, I wrote a lot about it and I don’t want to repeat here, so why was the drag of the YB49 (for sure a nice smooth looking airplane) so much higher than that of the B47?

 

[Arjen]

B-49 airfoil inherited from B-35? Read previous replies.

 

[Nicknick]

Gues not, why should it?

 

[Arjen]

Because the B-49 is nearly all airfoil.

 

[Nicknick]

thats correct, the YB35 as well...

 

[Arjen]

Which was slower than the jet B-49. The B-35's airfoil was not optimised for the B-49's higher speed.
Compare B-60 with B-52. Read Mark Nankivil #57.

All B-49s were converted YB-35s.

 

[Nicknick]

So why was the parasitic drag of the YB49 so much higher than that of the B47? I still haven’t read anything more plausible than the larger frontal area.

The YB49 didn’t fly in high Mach umbers (724 km/h if I remember it right), so things like area rule can’t be the explanation (despite that, the YB49 was clearly a good design in regard to the area rule).

 

[Arjen]

Again - airfoil? You have been answered by others, but you do not like their answers. Noted.

 

[Nicknick]

Do you think, Northrop would have used an inefficient airfoil? The speed difference between the YB35 and YB49 wasn’t so dramatic, that an airfoil would work at two different top/cruise speeds of these planes.

There have been other answers as well, which indeed pointed out, that’s it’s not all about wetted area. You shouldn’t criticise me for answering you.

 

[Arjen]

Northrop chose an airfoil for the B-35 that, at that time, most suited the B-35's performance envelope. The YB-49s, after being converted from YB-35s, had their maximum speed raised from 629 km/h to 793 km/h - an increase of over 26%.
Supermarine's Spitfire received an entirely new wing, turning it into the Spiteful, to cope with comparable performance gains.

 

[Trident]

To be fair, there is an indirect link to frontal area in the airfoil (t/c ratio). Nonetheless, here again the established criterion of t/c is more useful than bare frontal area, as evidenced by the Vulcan (high frontal area with buried engines i.e. large t, yet good t/c because c is also enormous). YB-49: 19% inboard, 18% outboard; Vulcan: 12% inboard, 10% outboard (pretty comparable to the B-47 and B-52). So yeah, we don't merely think Northrop used an inefficient airfoil, it *did*

 

[Nicknick]

An increase in 26 % has only minor effects on the Reynoldsnumber, so the airfoil will do well at both speeds. The Spitfire was continuously improved, like all WWII fighters, if it was fitted with a new wing, it doesn’t mean the old wing wouldn’t have worked at a higher speed.

Im pretty sure, the frontal area of the Vulcain was much smaller than that of the YB49 (about 52 m²!)

Again, I never suggested to use the frontal area as standard reference base for the drag, I just believe it has relevance and the YB49/B47 proves it does.

To me, everything is said in this thread, but will continue to answer if someone wants to go on...

 

[Arjen]

<snip> if someone wants to go on...

No.

 

[Trident]

I'll just add one final point...

An increase in 26 % has only minor effects on the Reynoldsnumber, so the airfoil will do well at both speeds. The Spitfire was continuously improved, like all WWII fighters, if it was fitted with a new wing, it doesn’t mean the old wing wouldn’t have worked at a higher speed.

At >600km/h, an airfoil as thick as the B-35's (and without modern supercritical shaping) was probably quite close to its drag divergence Mach number already. Increasing speed by a further 26% was definitely courting difficulties, even if intuitively it may not sound like much (the Westland Welkin comes to mind). We're definitely well into territory here where Reynoldsnumber is no longer the only consideration.

 

[Nicknick]

Well, if the thickness of the airfoils matters, we might agree, that the thickness of the wing in a flying wing and the frontal area are very close related, aren’t they?

 

[Trident]

Indirectly, yes - as I said earlier. But like (wing) aspect ratio and wetted area, frontal area - taken in isolation - can be misleading when comparing aircraft of significantly different configuration. You're better off thinking in terms of t/c ratio to also capture the other relevant measure of chord, which is why that is how such comparisons are generally done. And even then, there's the compounding factor of airfoil technology - a supercritical section can be thicker for the same drag divergence Mach number.

 

[Nicknick]

The relevance of the t/c ratio shows, that it’s not all about wetted are alone, so basically, we agree.

If the frontal area/wing thikness of a flying wing is large, you have to choose between a large wetted area or a unfavourable t/c area.

 

[HoHun]

Hi Nicknick,

There have been other answers as well, which indeed pointed out, that’s it’s not all about wetted area. You shouldn’t criticise me for answering you.

You started this discussion with some arm-waving arguments about frontal area. I don't think anyone here made any attempt to use equal and opposite arm-waving to declare wetted area the be-all, end-all in aerodynamics. Wetted area is just what the professionals have good reasons to be more interested in, like the NACA, or Dr. Hoerner.

In fact, your argumentation style in this discussion is to always dumb down the complex answers you're getting into something oversimplified, which you then can disagree with next to no mental effort.

If you could find out everything you need to know about flying wing performance by looking at one single figure, NACA wouldn't have prepared an 85 page report to look at the matter. This report, if you'd read it, teaches you how the professional use frontal area and wetted area. It will come as a suprise to none of the readers of this thread that they don't do it Nicknick-style.

I pointed out this report to you more than a week ago, and it's freely available on the internet (as is Dr. Hoerner's book). I specifically pointed out the page count to make it clear that this is a complex issue. I don't know what kept you from finding and reading that NACA report, but it obviously wasn't lack of time.

The way you apply frontal area is about as competent as a carpenter who uses a crescent wench to drive nails. He might maintain that it's just as good as a hammer because in the end, the nails are driven home just fine - but to every outside observer witnessing the spectacle, it's obvious that this guy has no clue how the tools of the trade actually work.

View: https://www.youtube.com/watch?v=2wc-a4fvHSY


Regards,

Henning (HoHun)

 

[Nicknick]

What’s your problem? You can call it frontal area or t/c if you are more familiar with it, but in the end all parameters are not independent from each other. The integral of the thickness over the wingspan is (Tadaaa!!) the frontal area of a flying wing.

You started an attack on me, and now that’s this thread is coming to end it seams you want to restart it just because your not satisfied with the final conclusion…

 

[Arjen]

Thickness to chord ratio does not equal frontal area. The first one is a dimensionless percentage. The second is measured in square meters.
For another thing, it is likely to vary along a wing's span.
Once again, you oversimplify.

 

[HoHun]

Hi Nicknick,

You started an attack on me, and now that’s this thread is coming to end it seams you want to restart it just because your not satisfied with the final conclusion…

What kept you from reading the NACA report before answering? That's what I mean by "disagreeing with next to no mental effort". It takes a mental effort to read and understand that report, and it is quite obvious that you are totally committed to not making that effort. That's not an attack, that's a statement of fact.

Regards,

Henning (HoHun)

 

[Nicknick]

you cant have one without the other (on a flying wing). If a flying wing has a low wetted area but a high wingthinkness it implies that it has a large frontal and high t/c ratio, the mathematics are quite simple. The frontal area geometry of the YB49 can be well describt as two triangles, with a maximum thikness of about two meters in the middle. So the frontal area is AFrontal = 1/2 * t max * b

with (might be not the best letters)

t = maximum thikness
b= wingspan
A = frontal area

This is no oversimplification but simple mathematics. You can try to blure the facts by overcomplivication, but this is not what engineering is about.

 

[Arjen]

Then, as Trident wrote in #73, a supercritical section can be thicker for the same drag divergence Mach number. Which, when drag is concerned, messes with simple reliance on t/c, never mind simple reliance on frontal area.