Advantage of rounded tips?
 

A number of WWI German biplanes had a low wing with a rounded tip. What aerodynamic advantage does rounding only the tip of the lower wing offer?

RE: Advantage of rounded tips?
 

Was more likely done for simple structure and very light weight. Like the trailing edge on many was just a cable strung between ribs, giving the classic scalloped trailing edge when the fabric pulled in between the ribs.

RE: Advantage of rounded tips?
 

probably less likely to dig into the turf on a ground loop.

RE: Advantage of rounded tips?
 

Neither of these suggestions seems very likely. Rounded tips would appear to represent a more advanced construction technique and preventing ground loops would not explain rounding off the TE wing tip! No, I'm pretty sure there is an aerodynamic reason for the rounding.

RE: Advantage of rounded tips?
 

I'm pretty sure Dick hit the nail on the head with the idea that it's to prevent the tips digging in. For the same reason they added 1/2 circle hoops to some other WW1 planes. If they had done it for an aerodynamic reason then why not do it to both wings?

Later WW1 planes like the Neuport 28 used rounded tips for more aerodynamic reasons as they had both wings set up with the same tip shape.

All else being equal the rounded tips offer a bit less drag. Whether or not it would be noticed with all the drag of a typical WW1 design is the question.

RE: Advantage of rounded tips?
 

Then you'd pretty likely also be wrong. Sometimes the correct answer is just as simple as it is blatantly obvious. Don't try and invent one more complex where it's apparently not. Wood for the trees and all that.

OK, I don't claim to definitively know the answer. But as an professional aviator of more than 30 years having an intelligent educated guess, I'd agree 100% with Dick. The moment I read your question and before I read Dick's answer, it was immediately apparent to me that the reason would almost certainly be ground handling, as aerodynamics weren't a major consideration during the era. Staying alive was, and ground handling attributable take-off and landing accidents arguably killed as many pilots as the enemy.

The consideration I should think might otherwise have influenced shape would be individual design aesthetic, considered much more important than aerodynamic advantage until the very late war period. Even so, by then, production (output) invariably over-ruled technological (aerodynamic) advantage as ultimately war is about attrition.

Keep in perspective that for more than half the years of The Great War, Tommy went into battle wearing a peaked cap to 'protect' him against shrapnel, Fritz wore a leather Picklehaube and Francoise the 'cloth reinforced' Kepi. Hardly confidence inspiring protection against flying iron and steel propelled by high explosive. That's why the venerable steel helmet was eventually introduced, although the 'coal scuttle' Stahlhelm not until mid-1916 after both the fashionable French Adrian (summer 1915) and 'Brodie bowler' (early 1916). And even the Stahlhelm shape, such excellent design that it was that it has been copied by the world's armies today in modified kevlar form, was certainly influenced by design aesthetic considered as important as both production ease and protective form...well to the Germans at least. Gott Mit Uns! :eek:

RE: Advantage of rounded tips?
 

From what I've read the science of aerodynamics during (and even before) WWI was far more advanced than is popularly believed. It wasn't all just seat of the pants design. The Wright brothers, for example, built one of the first wind-tunnels and I've read one German treatise on the subject of aircraft design from before the war. A number of old photos show streamers attached to wings to allow airflow analysis tests. While trial and error, as a "sceintific process" was almost certainly a greater part of the design process than is true in today's computer-driven aerospace industry, functionalism not fancy was still at the heart of the vast majority of design decisions.

Let's take just a couple of examples. First the classic Fokker comma-shaped rudder. This wasn't just a flourist of Fokker's pen. Many rudders of the era were rounded in the back (another feature I don't understand the reason for but accept that there probably IS a reason for). Initially these rudders were "unbalance" with the entire surface behind the pivot line, which made control more difficult. A clear understanding of the basic aerodynamic forces led to the development of balanced control surfaces (such as those elegant tips on the the DRI's upper wing). The Fokker eindecker series was the first of the German planes to use a balanced rudder. And remember that at that time, most aircraft did not use a vertical tail fin. Fokker's elegant solution was to balance the rudder by adding an additional curved surface ahead of the pivot line -- resulting in the comma shape. It wasn't until the DVII that a viertical fin was added and the shape of the balanced rudder modified to fit around the fin.

As a second example, let's take that famous scalloped trailing edge. Of course as was pointed out this was not some aesthetic attempt to mimmick nature but rather the natural (and perhaps unwanted) result of using doped fabric over wire to replace a wooden trailing edge. But why was the wire used to start with? One reason was to allow the main spars to be placed further forwrad in the wing, typically the forward main spar was just inches behind the leading edge and the secondary spar was located at about mid-wing. The fact that there was no spar in the rear half of the wing cord made for a very flexible wing which in the very early aircraft allowed effective wing warping and subsequently was at least claimed to promote flight stability.

Back toth the issue of the rounded (i.e. semi-circular) wing tips, I again have to disagree that avoiding ground loops is the answer. While this MIGHT explain why the forward edge would be curved it wouldn't explain the need for a fully curved shape (unless these pilots occasionally landed backwards as well). And given that most aircraft had straight not curved tips I think we can conclude that straight tips were simpler from the production standpoint so that's not an answer. I believe but don't have the technical expertise to demonstrate that the rounded tips somehow improved the in flight aerodynamics of these biplanes, i.e. it had something do to with the dynamic interaction between the top and bottom wing and stability. One major design consideration, particilarly for the 2-seat observers which carried out long (relative to the fighters) missions lasting several hours was flying ease. Having to constantly monitor a "tippy" airplane during several hours of flying could be be exhausting. The early designers were clearly aware of things like "washout."

The more I study these early aircraft the more amazed I am at the rationality behind every little feature. Things didn't always work as planned but in almost all cases the designs were the result of hard-nosed rationalism not creative whim.

RE: Advantage of rounded tips?
 

Check out the major causes of accidents on WW1 aircraft- Even some WW11 craft had baaad ground handling -resulting in accidents - The ME109 had a particularly nasty setup -for unimproved fields - --which WAS also,a big issue in WW1
To design around having the aiframe dig in and flip over, has lots of merit.

RE: Advantage of rounded tips?
 

Hi AB

Full marks for you to thinking about it, and the following commentary is not meant in any way to present as either derogatory or insulting.

But....

Whilst aerodynamics were undoubtedly a consideration, I don't perceive they were given the priority you assign them, nor were they as advanced nor engineers and designers certain about them as you'd prefer to believe. Had they been so, and accepted and subsequently instituted as such, then both sides would have been predominently been flying monoplanes for the duration, not bi and tri-planes ....to offer but a singular example.

If you'll forgive me for saying so, you're demonstrating the classic mistake of thinking like a book researched academic from a comfortable armchair in his air conditioned office, not like an experienced professional aviator (or pragmatic designer of the era) viewing the priorities in design for use by a 19 year old sleep deprived 'Flieger' with all of 10 hours flight training and 'experience' putting his arse on the line.

Such rationalisation derived with the benefit of 20/20 hindsight distant from the stress and furor of violent conflict are deceiving. It's easy to convince oneself to believe what one wants to believe based upon how it should have been rather than how it was. By way of example just ask any experienced cop who has to deal and make instant decisions when dealing daily with the physical danger, emotional reality and immediate consequences of violent street crime, and you'll receive a perspective totally divorced from that of the judge remote from the experience with all the time in the world to reflect upon and think about it whilst picking over legal technicalities with the benefit of 20/20 hindsight knowing what has happened, not how it might have eventuated. Whilst such academic perspectives unquestionably provide comforting hypothesis, they echo an impractical or out of touch remoteness all too frequently accompanied by belief as dogmatically arrogant as it is errant from the practical reality derived of experience from those who know the latter first hand.

As for completely rounded edges being of insignificance to ground handling. Look at a ball. Round is better because as it rotates, nothing snags. It deflects. Same with armour. Why do you think curve and slope was introduced, (eg: T-34) ...and immediately copied? (eg: PzKpfw V) The preference and advantage of a full curve in ground handling with nothing to catch as the tip rotates is abundantly obvious to someone who has flown low wing low fabric covered taildraggers, especially when pertaining to those of an era with horrendous forward visibility, exceptionally unstable tall narrow track undercarriages with NO brakes, abysmal rudder authority, and in many cases on-off throttling. If you can't comprehend why a completely rounded tip is better, it's pointless trying to explain it to you other than to say you are thinking like an academic. You need to look at it from a pilot's persective interpreted as a priority into the design by practical engineers where they and managment could be convinced and production compromise allowed them to do so.

A more contemporary example of such compromise was the astronaut's periscope in the first manned Mercury program capsule Freedom 7 (MR-3, Mercury 7). The design engineers didn't consider a window necessary. After all, the chimps never complained. The pilots knew it was. I'm certain it was much to the relief of Alan Shepard and John Glenn (backup 'driver') that the compromise was implemented. In this case, aerodynamics or physics had nought to do with why a window wasn't designed into the original or implemented belatedly so much as the encumberance it presented upon production and structural integrity without redesign or delay in the space race for international prestige.

RE: Advantage of rounded tips?
 

You might like this:
One of the reasonsI used a Dalotel for IMAC -besides the fact that it really is one of the best setups for this stuff--was that I could suck the wheels up for an emergency , off field landing.
Far less chance for damage on a big model.
---------Been there ---

RE: Advantage of rounded tips?
 

There's no doubt that as many WWI aircraft were lost to bad landings as to enemy action. Judging from the available photos nose-overs and ground-loops seem to have been an almost everyday occurrence on all sides of the lines. Maybe this WAS the reason behind rounded tips -- maybe it wasn't. If it WAS it was an idea that sure didn't catch on very fast judging from other later designs both German and Allied, for example the Fokker DVII, SE5a and the Camel three of the most outstanding fighters of the war.

As for the superiority of the monoplane, well, science (particularly when controlled by the military) rarely proceeds on a linear track. There was a time that biplanes were clearly viewed as superior to early monoplanes and triplanes did offer certain advantages over biplanes. By the end of the war a couple of monoplane designs began to reemerge.

Regardless, it is very clear from available documents (I’ll take academic research over romantic stereotypes any day) that the Germans military authorities (in particular Idflug) tightly controlled the development, testing, and production of aircraft including the performance of static load tests, the establishment of minimum standards, and other measures suggesting that aircraft design was already, at least by 1915, seen as a technical science. The science might have been flawed but the days of "wacky flying machines" were long passed. While there would unavoidably have been a few “design holdovers” from an earlier century, by and large the features of any particular aircraft were as they were for very specific reasons. Unfortunately many of those reasons now seem lost to antiquity.

--Don

PS. I've enjoyed this academic debate even if I don't have airconditioning! :D

RE: Advantage of rounded tips?
 

Hi Don

I can see you're a hard man to convince. [8D] Surely this is far too amiable to be called a debate. Robust discussion?

As I think everyone here agrees, none of us has THE definitive answer to your question for which we can provide proof beyond all doubt. However, Dick and my assessment of the reason being as stated is anything but based upon romantic notion...or stereotype.

Being descended from and much as I admire the Teutonic approach, the academic illusion of semi-infallible highly developed scientific testing is a bit of a romantic stereotype itself which certainly sounds good in a book 90 years later. A lot like military manoeuvre really -works well on paper. You've already had the helmet analogy from the period - and have you ever seen the Great War German 'scientific' approach to armor as in AFV? Have a look at the result - the AV7? Hardly a striking success.

As for the rounded tip implementation "catching on", well political interference at all levels and the frailty of the ego tends to interfere with sound judgement as is unfortunately all to evident today. Case in point we've already touched upon of that era being monoplane development. Now what if they'd had the DVIII earlier and in large numbers. OK, the strategic outcome wouldn't have changed given the entry of America into the war among other machinations, but it sure would have made a deadly impact.

FWIW the Camel made famous by the fictional Biggles killed as many friendly pilots as it did enemy. I'll let you take a guess why that was and how. And how about some of the disasters like the Fokker Triplane? It's only real claim to fame was the profile assigned it by the outstanding tactical skill of Manfred and the colour he chose to paint it. Another highly scientific German aviation innovation! Red 'camouflage'! - well pick any bright colour you like really. :)

RE: Advantage of rounded tips?
 

Sigrun, maybe you are judging WWI technology and the men who designed it too harshly by modern standards. At the time, these ridiculous machines must have been as awe-inspiring in their technical splendor as an F-114 seems to most people today. Even that hideous tank! Think of the Fokker EIII, not only did it have INDEPENDENT suspension but the shock cord bit was in the fuselage vs down on an undercarriage axel. This must have seemed like a fantastic innovation. And a welded steel frame on an flying machine must have seemed like the height of technological innovation.

Sure the science of aerodynamics and aircrart design was still in its infancy -- perhaps something like the alchemy that led to chemisty. My point was simply that new designs back then -- just as today -- attempted to redress some prior perceived shortcoming. The solutions were not always successful. In which case they were quickly abandoned and something else was tried. This to me is the essense of science.

As for the DrI and "red camouflage" well each needs to be understood within its context. The British had introduced the Sopwith Triplane and this was having considerable success in combat so Idflug pushed German designers to come up with triplane designs. Most companies competed for the prize (military contracts) including Pfalz and Albatros in addition to Fokker. The Fokker was the only one that got Idflug's approval. It was not, however, as good as the Sopwith triplane but it must have had its plusses or von Richtofen would not have chosen to abandon his Albatros D fighter for one. While the fanciful colors of the "flying circus" may have appealed to some sense of teutonic heraldry it also served a very practical function just as important -- if not more so -- than camouflage: self-preservation. These personalized aircraft identification almost certainly reduced the likelihood of death by "friendly fire" in the frenzy of a dogfight. And of course it served organizational goals as well. For a flight leader to paint his aircraft is a bold easily distinguishable color is of immediate and obvious benefit. Losing a plane AND PILOT in the air is a greater loss than just a plane on the ground. British olive grab "camouflage" of WWI aircraft reveals less about strategic thinking that is does about the intranigence of the British high command -- and perhaps the reserve of British society in general.

We've moved WAY AWAY from the original topic and this thread probably no longer belongs here but it's been fun all the same. And it does generally address the issuse of functionalism in design.

RE: Advantage of rounded tips?
 

Not to take away from anything which has been said, I'm going to suggest that making a curved (semicircular) wing tip on a fabric covered wing is actually easier (and lighter) in many cases than a squared-off wing tip. If you take a strip of wood, soak it in water and then bend it around a circular form, you get a nice arc. Now, this curved piece of wood will be able to resist buckling (from the shrinking of the fabric) much better than a straight piece, and thus it can be made lighter. If you use a straight tip piece, it must be wide (heavy) or braced (complicated) to maintain the correct shape.

There is also the factor of style points. Some designers preferred certain shapes which they found to be aesthetically pleasing. This is still true today, of course.

-David

RE: Advantage of rounded tips?
 

Good point David. And you may just be onto a new track with the idea of the tip working WITH the covering rather than against it. It has much of the pactical aspect that drive war time production and design.

From a purely aerodynamic standpoint the raked back tips typical of the Albatross and the earlier Taube are actually more efficient from a drag and lift standpoint. Of course this raked back tip shape shows up on many a design on both sides but it would be hard to pin down at this point just who came up with it originally and why. Certainly there was much copying of design elements. The Eindecker being a copy of the early Moraine Saulnier of Roland Garros and the Fokker Triplane being influenced heavily by the Sopwith. This copycat action oddly produced aircraft that shared the success AND foibles of each. The Sopwith Triplane was excellent at some points and inadequite at others. I don't remember the strengths and weaknesses at this point but I do remember reading that the Fokker shared these issues and this is why both aircraft designs had a short lifespan.

RE: Advantage of rounded tips?
 

Not at all. I hold a particular interest in the Great War and Second World War. Even though the sight of such a behemoth on the battlefield would have scared me witless, the Geman AV7 was an acknowledged unmitigated disaster even by the standard of the day.
Which is why I presented it as a case in point to demonstrate that efficient and logical though the Prussians may be, they are still subject to the same flawed development path and wrong choices available to all. That AV7 btw, is the sole example remaining in the world today. When I was a kid, it was open unguarded display outside at the old museum site, and you could get inside it. Huge mortar hole in the roof - must have spelt an ugly end for the crew. A real and rare treat for anyone interested in armor - and the period.

Similarly to the WW I biplane. Any glamorous notions from childhood have long since been displaced by the clinical harsness of the aviation industry and tens of thousands of hours. Have you ever flown such a machine? I have (full size airshow replica). Far from the romantic notions offered up in Biggle's 'adventures' with 266, they are simply horrible to fly (in terms of performance (lack of), heaviness of controls et al). Full credit to the poor devils of either side went up in them with essentially zero time.

Curiosity aroused by our discussion, I pulled out my Janes Aircraft of WWI last night. I noted that most German biplane lower wings actually had a frontal curve with a rear taper. Aerodynamically efficient. In fact that compromise of ground handling friendly with aerodynamic effficiency. As a designer with low time (skilled) pilots in mind influenced by the ground handling idiosyncrasies of aircraft of the day mentioned in my previous post, I'd still favour the fully rounded tip which was liable to save more lives than the cruise or dive speed of the type. If you've ever been in a 1 on 1 air to air, let alone a WW 1 Circus melee, you'll appreciate how disorientating it all is even to an experienced fighter pilot. To a kid with few hours under his belt, aerodynamic advantage he neither appreciates nor can use means diddly squat. He's just another number in a duckshoot with all his chances of survival resting on just being one of the flock. Getting off and on the ground in one piece does.

I understood the concept behind 'red camouflage' very well and its psychological intention. My point was intended to illustrate both an extreme and contradiction both in term and logic. Standing out from the crowd was only of psychological benefit where the proponent had a reputation (and ability) like Richtofen. However the practice of decorating one's craft in accord with ego was widespread within the German side of those who bore sufficient class status to do so. But it also had a downside, as commanders who chose to wear their rank on display everywhere from Stalingrad to Dien Ben Phu discovered to their detriment. The lozenge came very late in the timeline, as it took 'em a while to figure out that that was a better way of minimising the attrition rate.

The plus of the Dr1 triplane was its knife fighting ability. ie: It turned tightly, useful in the hands of someone who'd use that strength. (experienced). The Albatross et al types common then didn't. That Richtofen survived so long in one is testimony to his experience, outstanding tactical doctrine and sheer flying ability interspersed with considerable luck as he himself would possibly be the first to admit were he able to share his thoughts with us today. Generally the doctrine with the Albatross was to single hit and run (dive through), though this wasn't always practised according to circumstances and silly notions of "I never run from a fight" ascendence of ego mistaken for 'chivalry' completely in discord with modern warfare. In tactical doctrine of (fighter) air warfare, it is recognised one never engages in a knife-fight where one can avoid it as it simply sets you up as a target. The spot first, approach unseen, strike and run practice has been successfully adopted from the tactics of all the really successful great aces of both sides from WWI through II and is equally relevant today. Mannock, Hartmann et al.

As you say, we've moved away from discussion of the tips, though all of the above displays that the reasons something is done a particular way (wingtips) may have nothing to do with optimising functionality, let alone aerodynamic functionality, even where such knowledge may be present.

cheers

RE: Advantage of rounded tips?
 

David have a look at Piper's Pawnee. Rounded tips. Steel frame, curve preference is not about weight vs strength. Pawnee is lower tipped than Brave or (Cessna's) Agwagon, and design echoes from a time where (pilot) safety was more important than fuel efficiency (induced drag reducing tips). Pawnee is/was frequently the first 'seat' of low time inexperienced pilots or operated in extremely high risk ag environment in small field work frequently cluttered with obstacles.

My first guess for round (WWI German) is still ground handling, second is individual aesthetic style with aerodynamic efficiency coming in a poor third until very late in the war period when you would think that max. production not requiring skilled woodworkers would be paramount? ie: Curves and laminations (time & skill) where they could be avoided are out.

Have a look at the Dr1's square tips. Why if strength vs weight was the design factor assuming some logical scientific basis? Also, I'm not so sure the majority of German aircraft of the era did have rounded tips. From my glance at Janes, they mostly seem to prefer a taper to me rather like an arrow's shaft's quill.

As an aside, the Nieuport 28 a favourite mount of the AEF and late war type had rounded tips. Co-incidence or aesthetic fancy? As did the late war Sopwith Snipe (semi) and Salamander.

Must away. Gotta' go flying.

cheers

RE: Advantage of rounded tips?
 

All I was saying is that rounded tips are not that difficult or heavy to make, whether from wood or from metal tubes. I really have no idea why they were or were not implemented on various aircraft, and I made no claims about it. Simply, the curved wingtip can be a simple, yet elegant, solution to the question of how to terminate a wing.

-David

RE: Advantage of rounded tips?
 

Were the early pioneers of heavier-than-air craft aware of the science of aerodynamics and how things like wing tip design might effect flight? It think this website makes the answer pretty clear:

http://www.grc.nasa.gov/WWW/Wright/airplane/models.html

This may not add anything to our discussion here as to the primary reason for the rounded lower wing tips on several German WWI aircraft, but I think it says something important about the scientific approach to design probably prevalent among these early designers.

Also, I've looked at a couple websites on wing shape and see lots of mention among the glider folks of elliptical shapes being advantagous in certain ways (even spread of lift load) and I would imagine that the WWI era designers would have been aware of this. The lower wing tips I'm talking about on A/C like the Rumpler and Albatros 2-seaters might be described as semi-eliptical additions to a fixed cord wing. Rather than just a curved tip the wing seems to actually taper (as Sigrun mentions) forward a bit. So maybe there is some aerodynamic advantage after all.

One more argument against the "ground loop" theory: Why not try to address the actual cause of ground loops rather than design in protection for when it does happen? It would have made more sense to make modifications to the undercarriage to minimize ground loops in the first place. Practical "field" modifications are one thing, as with the wing skids added to the DrI, but wing design modifications are another.

PS. I apologize if this whole discussion was begun by my use of the misleading term "rounded" wing tips. I guess what I'm really talking about it eliptically tapering tips (which are of course "rounded"). Naturally that argument that rounding the corners on rectangular wings to avoid ground loops makes perfect sense!

RE: Advantage of rounded tips?
 

Then I suppose the simple answer as to why ONLY the low wings used this shape was that the designers didn't want to mess with the aileron design of the upper wing!

RE: Advantage of rounded tips?
 

I have had the impression from the beginning of this thread that you were prepared to answer your own question, so why ask us? Or were you 'educating' us?

RE: Advantage of rounded tips?
 

I like the "rounded tip for ease of construction" WITH the "non-interference with upper wing aileron" ideas myself.
Other than these two, there's no good reason for the shape.

RE: Advantage of rounded tips?
 

Doug, I've turned to the aerodynamics forum several times in the past for the wealth of knowledge here on just about every conceivable aspect of A/C design. As a glorified English teacher and newbie RC pilot with no real world experience with airplanes I'm not in the position to "educate" anyone on aerodynamics. :D

I was just curious about this common feature of later war German A/C and was honestly surprised when no one was able to offer a "treatise" on the aerodynamics of wing tip design -- so I went searching for something along these lines. Still haven't really found it.

Paul, I can't see how it would have been easier to contruct these tapering round tips which would have required modifying the last few ribs as opposed to a constant chord wing with a simple bamboo or laminated round tip. Also this seems to have been an "innovation" added to later versions with the round tapering low wing replacing an earlier version with squared-off tips.

My thanks to all who have contributed here!

RE: Advantage of rounded tips?
 

Fletch, when I was going to Parks the wizened old aerodynamisist argued that for most propeller driven flight the shape of the wingtip was a case of pure aesthetics. There was very little performance differences between a closed off flat plate end rib, or a nice curved surface. Until Whitcomb played with the winglet, that remained true. Even the winglet, if improperly designed, can't do squat for you as evidenced by we never put them on the KC-135 because on that wing, at best we saw a 1% savings in fuel.

As to your question, we're at the Occam's razor solution. Take a model of the airplane in question and tell me what part of the wingtip hits the ground if you put it in its normal tail low landed position, then push the wingtip down till you make contact. I'd imagine that its going to be a good portion of that elliptical trailing edge. Why? bigger surface area contacting the ground means lower loads at the point of contact, lower loads mean less stress, and less the likely hood of breaking the wing tip.

My point. A bent bow of laminated wood is pretty light and very strong. And if you curve it right it will slide over the ground instead of digging a furrow from a pointed tip.

And in addition to your comment above it's easier, lighter and cheaper to minimize the damage to the wing tip than to completely redesign the landing gear.

Final answer; Ground loop clearance.

Tom

RE: Advantage of rounded tips?
 

OK. I guess I've played the devil's advocate here and in the end heard some pretty good arguments. Given the apparent lack of an aerodynamic explanation, I'm willing to accept the avoidance of ground loop hypothesis! I was just certain it had to be something beyond aesthetics because these birds were seriously funky looking! :D

Thanks for playing!