Discussion in 'The light stuff area' started by topspeed100, Feb 11, 2011.
This is fairly lite !
I always liked the concept. This is one of the old designs the I think Rienk should have been considering for his cheap kits: with corrections and updates.
Corrections and updates indeed. The Minibat was a neat little thing, but the airfoil was just bad and the wing construction was just horrific. Didn't last on the market very long.
Charles Fauvel and his flying wings
Nurflugel is a good flight for flying wing enthusiast.
Forward sweep makes more sense to me, but yaw stability can become an issue. Definitely one light glider.
I've been thinking about a "retractable canard" that could be used in forward swept flying wings for takeoff and landings.
The reality is trusting my life to a bird without a tail isn't something my conscious is ready for, or for that matter my level of engineering ability.
.2c max wing height seems abnormal. The airfoil should be optimized for stability primarily.
This looks good, lets investigate.....
Famous last word,
Sir Wonderous Mountain
The airfoil does not create stability in a flying wing or any other type of airplane. Stability comes from the relative placement of the neutral point (mostly due to planform) and the CG. The whole "stable airfoil" thing you see on all the R/C sites is a complete myth, reflecting a poor understanding of what's really going on. It's the difference between stability and trim.
Designing a flying wing is absolutely no different than designing any other kind of airplane. Just harder, because you have to get the wing to also do all the functions that would've been performed by the tail that's not there. Airplanes are a bunch of compromises flying in formation. Flying wings just require more - and harder - compromises.
True, you can use any airfoil on a plank 'wing and it will be technically stable as long as the static margin is positive. But if you do use say a Clark Y it will only fly stably inverted. Kind of a worthless definition of stability I'd say.
Funny thing just this afternoon I was reading articles by two engineers who referred to the use of "stable airfoils" on their full size airplanes. This dead horse beating about the difference between stability and trim reminds me of physics students who never miss an opportunity to berate us lesser educated about the nonexistence of centrifugal force. It may be true but there is a point in casual conversation where it becomes an irrelevant semantic distinction
I would have said "more interesting" instead of "harder"
We don't eliminate the tail we just hide it
Just a different set of compromises. The shorter coupled an airplane is the narrower its range of operations will be. If the limitations of a design are too narrow then the designer should look for ways to expand the envelope. The easiest way to expand a flying wing's envelop of operations is to put a tail on it but that would be cheating.
Indeed, you do at more clueless than you really are, but I make up for it by acting more intelligent than I really am. Also, if you think designing a tailless plane is more interesting; try making a perfect bell/elliptical lift distribution with fuselage and tail included. Finding the perfect balance between empennage drag/structure, and fuselage drag/structure. Also, remember this relationship will change landing gear requirement which affects Cg.......
But yes, failure in flying wings is often more pronounced. From what I read (between the lines) pitching moment changed during flight, or at least the drag did. IF they were abrupt enuf, or unexpected, this could adversy affect flight conditions.
So yes airfoil does affect stability. This design was actually less short couple than most flying wings. As for the Cg again not qualified to say, but the ability to land with an inoperable aileron hints at a reasonable position.
I don't like to beat dead horses-does that help with processing of a pelt?
Incidentally looking for Flying wing info if anyones has something.
Meet the shadow,
My previous post was rambling and not really of any use. This is what I wanted to say about the Haig Minibat. I think that whatever aerodynamic shortcomings it may or may not have had it was too light as evidenced by the repeted mechanical failures. It doesn't mater if you have the greatest shape that's ever been devised if your parts are too light to handel the loades it's a crappy design
Too light? Surely your previous post was better than pure blasphemy!
I'm in so much objection to this statement I'm not signing it.
I've also been called a heretic but I call myself a Philistine. People are interested in flying wings for different reasons. Many have heard the propaganda put out by Jack Northrop and other early, pre-computer control, proponents and think 'wings are astonishingly better than tailed airplanes. Some simply have space limitations and are looking at a shape that might fit in their garage. I just like the design proses ie the different (more interesting) set of compromises. There is a potential efficiency advantage too but without computer control it's only about 8% and that's if you understand the compromises and are willing to make them in spite of fact that some results would be positively bad on a tailed airplane. I think Haig was one of those who couldn't fully commit to the tailless set of compromises.
Jack was all for reducing fuselage area, wet and otherwise. He also had specific airplanes to sell.
Haig's and any other flying wings have at least some fuselage. This is an unfortunate consequence of having bones. Birds have these too. All flying wings with less than a ~12 ft chord and a "normal" seating arrangement are taking on fuselage drag. Once you are approaching the drag of motor-gliders, if you're not lighter-longer ranged or some other important performance parameter ahead; then you're just doing it for burps and giggles.
The 8% figure seems off, but exceeding the best designs around has never been particularly easy. Let's compare to a highly wasteful, unstable aircraft so we can look better......
On the issue of weight, my original thought was load should be about twice empty weight. However, that left empty weight at 75 lb (34Kg). After several feasibility studies, everything pointed to one fact. Anyone making a 75lb plane should throw performance parameters out the window. Even though my design empty weight has been doubled, it still.....
Has too small wing area, sub goal L/D, excessive landing speed, Cg limiting luggage area, and engine placement issues.
Accepting compromises has been a growing experience for me.
Still glad I didn't have to fall out of the sky to come to that point.
Doesn't matter if it's "worthless" to your mind or not, Norman. The definition is what it is. It's the way the aerospace community defines it. Personal value judgements don't enter into the equation.
It's a valuable distinction in that it promotes more constructive thinking about how to design tailless aircraft. You know and I both know that "stable airfoil" has no meaning in stability and control work, and we both know that reflex applies to trim, not stability. I know you know these things, having seen you say as much yourself. Having a solid understanding of the difference between stability and trim is fundamental to designing a successful aircraft of any type. Why would we perpetuate completely incorrect concepts just for "convenience" in a conversation?
Nothing like two wild engineers locked in a semantical battle to the last breath!
My birds designed to fly, what are you designing for?
Neither I nor Topaz are engineers. I'm a retired steel worker and he's a commercial artist. Our reference libraries seam to have some of the same books. We have different aesthetics as far as what looks good and what an airplane should be. To me an airplane is just a vehicle and getting to the destination safely and economically is all that matters.
Which actually puts you in the engineers-corner
There is another advantage of flying wings; complexity. Fewer parts to build, cheaper and so on. Nothing less of an engineering challenge, but I can see the merits regarding building.
And I feel the same, so there is definitely some common ground here. I also add the 'fun' factor, but that doesn't bear on this discussion.
I don't, however, believe in 'eyeball engineering'. (Just to be perfectly clear, Norman, I do not mean to imply that you do.) There have been far too many failures in the tailess arena that can be directly attributed to people building what 'feels right' without having even a fundamental grasp of the principles involved in the task. Stability and control have been the needless Achilles Heel of attempt after attempt at flying wings. Perhaps more than any other airplane type, putting stability and control of tailless aircraft into clear and precise terms is not "ivory tower" - it's necessary to success.
This is one of many comments that I have made that would be easier to understand if I used more words. Sorry about that! I type with one finger on my left hand due to an old injury involving my right hand from which I never completely recovered. It's the basses for my early retirement.
As you say, the section characteristics are not part of the stability calculations of a tailed airplane (not at first anyway), but the tail size and position are. If we start with a sailplane we see that there is a rather small horizontal stabilizer positioned a long way aft of the wing. If we move this small stabilizer closer to the wing and calculate the neutral point for both configurations we find that the NP also moves forward thus shortening the static margin if we left the CG in the same place. If we want to keep the NP in the same place as it was with the long tail with this shorter tail arm we can increase the size of the stabilizer. If we keep moving it forward, with a fixed NP, it will get pretty large. Eventually there will be no gap between wing and horizontal stabilizer and then, assuming that this plane didn't have a swept wing, it will be a plank type flying wing. In planks the trailing edge control surfaces aren't just doing the job of the horizontal stabilizer they literally are the stabilizer.
Now consider this: Although the arm and stabilizer size can be adjusted to get the same tail volume coefficient the static stability is proportional to tail size but dynamic stability is proportional to the square of the arm. So as you shorten the arm you can keep the static stability but the damping goes down. What happens to a plane with high static stability but low dynamic stability? ________ This is why planks tend to have very short static margins. In model building 2 or 3% is the norm for leisure flying but the aerobatic and combat bunch try for less with predictable results.
Now add sweep. Suddenly the aft-most end of the wing is the pitch trim surface instead of the whole trailing edge. One consequence of this is that you are no longer tied to positive Cm airfoils (like that inverted Clark Y) but that doesn't have anything to do with stability so I'll save myself some typing and not mention it again. The thing that is relevant to stability is that the wing can be divided into a lifting section and a trimming section. Then if you draw a line between the centroids of the lifting sections of each half span and a parallel line between the centroids of the pitch trim areas the distance between those two lines is equivalent to the tail arm.
Thanks to the new attachment editor it's easy to find drawings that I've used to illustrate this point before so here they are with no further explanation because it's taken me an hour and a half to wright this and I'm tired
Nice. I certainly agree with all of that. For swept wing 'wings, in particular, I think that's a nice conceptual framework. I've been working mostly in flying planks, and I find that a different way of looking at it ties the design process more closely to conventional practice, at least for me.
Instead of dividing the wing into 'lifting' and 'trimming' portions, I simply think of the wing as a unit. A horizontal surface that provides lift and moment about the pitch axis. Move it forward (in relation to the CG) and the neutral point moves forward. Move it aft, and the neutral point moves aft (talking about the pitch axis, of course). I can position the wing in relation to the CG to give any desired static margin. The conventional stability and control equations can be used, with the contributions for the horizontal tail set to zero.
So far, the airfoil choice is irrelevant. Once the wing is positioned fore-and-aft to give the desired static margin, it's a simple matter of determining the pitching moment required (positive, in the case of a 'plank) to trim the aircraft at the design speed. Once that's known, it's time to choose an airfoil with the right pitching moment (and all the other characteristics required) to satisfy that condition. Done and done. This works with swept wings, too, excepting that twist is dominant in setting the design trimmed airspeed, with the airfoil contributing less to the outcome.
Two different ways of attacking the same problem.
I'm not sure he was unwilling to commit, he just didn't seem to grasp some of the nuances of the design and built the thing to TLAR standard. The use of vise grips to repair a primary control mechanism makes me think he was a bit too complacent, or ignorant, when it came to some rather important details.
My interest in flying wings is for a single place minimalist (cheap) motor-glider that meets US part 103 requirements. Single place and limited speed range of a part 103 seems to make some of the compromises one has to make with a wing less of an issue.
With modern materials and methods, which the Minibat was attempting to use, there is no reason a flying wing need to be as labor intensive to build as the Horten or Fauvels. Besides, I just like a challenge and doing something different.
Heretic, Philistine, inspired, delusional or prophetic? Only those that write our history can decide.
I like Norman's method. I've also been using it, but my assumption was elevon controls with a bell lift distribution. Never really considered flat planks. Elevator control might be better with elliptical distribution on forward swept wings.
If we can derive a Tailess tail stability formula, it might be possible to calculate sweep "required" for a given wing. Or aspect ratio needed for a given sweep and span (15M sailplane comes to mind). And just maybe we could TAKE OVER THE PLANE!!!
Sorry, I got a little excited.
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