I became interested about aerodynamics through a experimental project I started building with Kate. It was Cozy MKIV. We have not been building that plane for quite a long time, but we got couple of parts done, for example the canard foams were cut with help from Rauno Viljanen and I managed to do quite poor quality chapter 4 bulkheads with zero understanding what I was doing structurally or otherwise.
Back then there was a concept of "speed modifications" very popular on canard forums, and I think it still is happening there, I haven't followed for a while. They are being invented most frequently by people that don't have even pilot's license yet or don't have flown any aircraft to the date, and they don't necessarily have much understanding on the aerodynamics either.
Back then I was really interested in them, and it felt like magic, you bolt on this and that improvement, and it becomes this and that much faster and more efficient. There were all kinds of concepts like cutting lower winglets, shortening wings, or even placing vortex generators to a laminar flow airfoil. I did not see back then what was wrong and why they wouldn't work as expected. Now I know. They were very entertaining reading, and actually inspired me to start thinking these things in more detail. And I am still on that road. They are not essentially bad but they may not work as the builders expect them to work because they don't understand why they are doing them, but are relying on non-scientific reasons to bolt them in.
Couple of years have passed and I have been reading about aerodynamics and trying to find out how it all works. It occurred to me at one point, that it is not a bolt-on feature you can add to existing design, but aerodynamics is all about the flow. And understanding it as a whole.
Someone might say that "by adding vortex generators, you get 5% fuel savings", that can be true only in a case where the flow otherwise preliminary separates. Good aerodynamics, is not fixing this and that with little this and that, but trying to get it all right and if still a problems persist, try to fix them then with some additional fix.
What happens if you consider adding vortex generators to Cozy MKIV front wing, in other words, the canard? The canard has Roncz RMS1145 airfoil which is about 45% laminar. Depending on where you put the vortex generators, you can vary between 0% laminar and 45% laminar. You can't get more than 45% laminar with that shape.
However, what else can happen is that, the turbulent flow attaches to a higher angle of attack on the airfoil which was designed to maintain its lift even if the laminar flow is disturbed by bugs or rain? You may get some more Cl out of the airfoil with the added vortex generators, and may be able to delay the stall angle of attack some.
But think the whole picture: the main wing-canard relationship was tuned so that the canard always stalls before the main wing. If this does not happen, the plane can enter into deep stall which is not recoverable on the particular type in question. If you delay the stall of the canard to a higher angle of attack, you are trying your luck with the main wing's Clmax and maximum angle of attack before it stalls. And it might be that you can achieve higher angle of attack with the canard than the main wing can function without stalling, and the result is pretty severe, everyone on board most likely die as a result, unless you are super-lucky like some that have survived from a deep stall crash. But wait, there was someone who also dropped from a passenger jet without parachute and survived. I would not try my luck based on the few exceptions.
Same thing what happens if you shorten a wing. Jet fighters have shorter wings and they are faasstt. Right? In case of subsonic aircraft you actually increase induced drag if you shorten the wing. You also increase wing loading, which also increases induced drag, although it reduces the wing wetted area which is desirable for lower drag. But in this case, the increase in induced drag can be such high that the plane actually becomes slower. There was one manufacturer that was doing light aircraft, and they were thinking how to convert their aircraft to LSA. The LSA version had longer wings, and instead of limiting the maximum IAS to 120 kts, the supposed to be LSA version became in fact faster than the plane with the shorter wing.
One could think also that a plane which would have smoothly rounded shape in the wing tip instead of a maybe less elegant looking cut shape would be faster. And surprise might be great when the person would notice that instead of making a faster plane, the plane actually got slower because of the modification. Here is also the thing: what you are trying to achieve - looks or relying someone's claims, or are you thinking what you are going to achieve in terms of flow and how it affects the wing tip turbulence and is what you are trying to achieve beneficial or not. The sharp cut hoerner shape in the wing tip might be there for a reason, it resists the flow from the bottom side to the upper side because of the sharp corner there. Rounding this shape makes the wing tip potentially worse. Only potentially, because you have to consider what is going on, and what you are going to achieve. There is no "yes this is right" and "no this is wrong", because everything affects to everything. But you always should know why you are going to do something. Because it is faster that way is a wrong answer. Right answer is the understanding of why. Would be better to first understand why before doing it rather than understanding why you did it and why you shouldn't have done that.
The point is, that the optimization of aerodynamics requires thinking as a whole. Improving something somewhere may not help if something else is really bad, and it can get worse by uninformed improvement somewhere. Only by knowing what you are doing, as a whole, you can do aerodynamic design which results better performance unless you are very lucky. In some cases, you might be lucky, but you could ruin your results by doing something additional uninformed where the whole picture what is going on is not taken into account.
If you want to do a optimized aerodynamic design, you have to begin with that basis, you can't bolt it on after. Cleaning up a existing aircraft is possible to some extent, but only to some extent, which is very small. An optimum design is a in balance from the aerodynamic and structural standpoint and everything is taken into account in every detail and they are understood as a whole with the whole thing. It is not a puzzle with small pieces you just put together, but a puzzle where the little pieces change every time you change something little.
If you want to clean up an existing airplane, what you need to do is that you have to understand what you are doing, in other words, what you are trying to achieve what you are changing. You have to consider all sides of the change, what it does. Things are not so simple as they might at first seem. And some things are simpler than believed. Impossible - there is no such word. You just can't bend what is possible with pure luck, it does not work in the long run. Understanding what you are trying to achieve and what are the potential consequences in good and bad for every detail helps doing less not so good decisions.
5 comments:
I agree in many areas, but what ever one does, testing is the best way to find out what it actually is.
Cutting the wing for example. Why not? If you need a faster version of the plane and you don't need the low speed capabilities. It would be grazy to carry more wing than what is needed, since extra wing just adds drag.
Sure the best idea would be to design it in the first place with the right size wing and have no need to cut it afterwards.
But we live in the real world. There are very many designs wich have modified wings and tail sections after test runs. And several cases where even the structural members had to be strengthened afterwards. All planes get improvements and all models get outdated. You can never plan the ever lasting Holy Grail since there is no such thing.
The calculations are just calculations and the final testing will be the judge. Even the theory behind planning is often errornous. So one should not trust too much anything.
Best to test everything, that is the only way to be sure.
Of course, I agree with testing aspect, but I have to comment to this:
>All planes get improvements and all >models get outdated. You can never >plan the ever lasting Holy Grail >since there is no such thing.
Hey, I would not be so sure about that - for example Cessna has made C172 as a standard something like 50 years ago. And every C172 built since is still as bad as they were. This is the state of personal aviation. They used to design planes back then with ideology: if you want to go faster, put a bigger engine on the nose. Cessna was designed that way, and it is still that way. And somehow it seems to be even a successful strategy for selling these things. Somehow Cessnas are so overrated aircraft still these days that I can't believe it and I can't understand it. Despite there is more modern competition which flies a lot faster and lot more efficiently with the same engine size than the Cessna 172 and can be from handling qualities point of view outstandingly superior to the 172 (for example Diamond DA40), somehow these old 172s still sell very good.
A friend of mine called 172 as a Toyota Corolla. I have to disagree, because I feel it to be more like grandfather's a Lada 1200L. The design overall is pretty much like the Lada, it is about as uncomfortable to sit in there than in a Lada, the doors are as bad (or even worse) than in a Lada and the design was state of the art last time in the beginning of the previous century. And there have been very little if any refinements in the aerodynamics. I am not sure if I was surprised or expecting then trying the C172SP, it felt exactly the same as the old 172 model from the 1970's I had flown before, maybe there was just a little less clap in the controls because the plane was new. The seats look better, but somehow they manage to still feel as bad as the original bad looking seats.
And still, many many people go and buy these pretty outdated planes every year. In terms of cars, this would mean that people would every year go and buy Lada 1200Ls or something else from that era, as a brand new, with lots of money, and thinking that it is good. Somehow in terms of cars this would feel like impossible situation, who would care about some old car which don't even have any museum value? Somehow pilots aren't like that, and they specifically want the Cessna.
I was doing my IR flight training at West Valley Flying club in California and waiting for my flight instructor, when I heard someone new coming in and asking "Is this a Cessna training center, can I get training here in a Cessna". Interesting question because the flying club has so many so much more interesting planes available than only the Cessnas. But somehow it is a synonym of a plane and it is not because of technical superiority. Aerodynamics or comfort or money doesn't seem to affect the decision. They are still willing to invest the price of Diamond DA40 or Cirrus SR20 to a 1950s design. There must be some magic in the marketing of these things.
I have to say that C172 is not really bad plane, its flying qualities are acceptable and it does its thing. Because I am not a really a potential customer for a new C172, DA40 or SR20, but I am rather looking used aircraft market (what to own and fly while designing and building), still even the bad Cessna is along with possible options - I can fly a cow if I have no better plane available. Flying bad plane is better than not flying at all. But I still wonder, why people buy new C172s from the factory. The production of C172 was discontinued in 1980s but it was continued 1990s because of high demand. Somehow, the pilots are either very uninformed (they don't know what else is available) or they are very very very conservative and they want a very traditional plane without anything new on it, about the opposite of myself who is a person who loves all kinds of new technology, gadgets etc.
But the thing is, making aircraft for myself is one thing, but making it a product that sells, is another. That another thing may not be very realistic and it seems to be pretty much hobby-like even on the companies that are on the top of the sales figures. They don't do it because it is a very good business (personal aviation at the current state is very bad business, I am hoping that this would change, but it does not happen very easily and it does not happen overnight, and it requires aircraft designed from different perspective - mass production of inexpensive high tech planes, when the price tag is low, they will finally outsell the old C172s), but because they want to do it, because they want to support from their behalf the aviation community. Doing planes for some other category - such as business jets, is more profitable business than private aviation currently. But I really hope that it eventually changes.
One more comment about testing: the refinements done according to testing have to be done with understanding of the effect of the change, and sometimes, to get desired results, for example a new airfoil is needed. Cutting a length of a wing is not so good way to reduce unnecessary wing area. Unnecessary wing area can be reduced by designing a new wing with lower wing area, but keeping the aspect ratio. Lowering the aspect ratio decreases the aerodynamic efficiency of the plane.
Also in my example with the Cozy canard, the fix of deep stall by reducing the length of canard with hacksaw, was a wrong fix to the problem caused by the larger fuselage lift (which moved the center of lift forward), decreasing the aspect ratio by reducing the length of the canard could have actually caused deep stall by stalling at higher angle of attack instead of the lower angle of attack with the steeper lift slope of the higher aspect ratio canard. It was pure luck that this was somewhat successful modification, but a correct fix would have been to redesign both main wing and canard, and at least increase the wing area of the main wing. It would have been beneficial to keep the wing loading of Long-Ez, instead of letting the stall speed get significantly higher than the maximum stall speed of a single engine plane. The plane would have ended up a bit slower, but the stall speed would have been acceptable, and the canard - wing relationship tuning could also have been kept such that it would have allowed more front seat load.
To avoid lots of rebuilding of major parts in a large plane, I think it would not be so bad idea to build large RC planes and try to find out about the flying qualities with those before building the 1:1 size model. It is very expensive to come up with a conclusion that ok, now we need a different wing, this one sucks. A scale model has a lot less materials (and thus cost) on it, and more rapid reconstruction is possible.
Also one thing that worth noting is that building RC plane and modifying it heavily is not regulated by anybody. But to get started with building a full size aircraft, you have to have your drawings etc. approved in Finland by CAA Finland before you start building. If you find out that there is a problem in a particular part, e.g. wing, you can't go and cut it and subsequently test fly, because your drawings that were already approved, indicate otherwise. You have to apply for a modification and it is a lot heavier process than just redoing the wing on the RC scale. This process is more sane and less heavy in USA, and you can see that Burt Rutan does aerodynamic modifications to his full size planes after it has been finished, and he gets them test-flown the next day. In Finland such activity might not be so easily possible and and it could end up with "Tietoomme on tullut että olette kokonaan ilman lupaa menneet muuttamaan koneenne konfiguraatiota joka tarkoittaa että se ei täytä minkäänlaisen lentokelpoisuuden tunnusmerkkejä. Olette vielä jatkaneet rikosta lentämällä konetta ilman minkäänlaista lupaa tai valtuutusta lentää täysin lentokelvottomaksi katsotulla lentolaitteella..."
> you have to have your drawings
> etc. approved in Finland by CAA
> Finland before you start building
Actually you don't have to in the EU. You just start an aircraft company. They can build and test prototypes when ever needed.
Ever heard of approved design organization?
(shivers)
(sad, but true)
Ask Markku Hiedanpää.
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