Tailless Aircraft - Reflex and other design issues

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Topaz

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... contrary to the nay sayers, ...
Who has been a "nay sayer"? I've seen several requests - and made a couple myself - for some supporting evidence or substantiation of some of the claims made here, but I don't recall a single post in this thread to the effect that low-AR designs are "bad" or "won't work." Asking for substantiation isn't nay-saying. It's nothing more than asking for additional information.

...they [low-AR designs] offer a flight envelope that is not generally available from other types.
That's a good example of what I'm talking about. What exactly is it about the flight envelope of an arbitrary low-AR design that is "not generally available from [unspecified] other types"? Is that an opinion, or is it verifiable by analysis or historical precedent? Can you give us data to support this claim? This is an earnest request for information, not "nay-saying."

I just can’t understand why anyone could completely dismiss the video evidence and eye witness reports just because it goes against their pre-conceived ideas.
The issue is that the "video evidence" is not quantitative, and not a reasonable response to questions that are looking for quantitative answers. "Flies good" is not a design criteria. I can't design to "flies good" without turning that pair of words into a set of design requirements a given design has to meet. We can't pull climb rates from the videos, we can't pull airspeeds, local conditions information, or any information about the airplane in terms of weights, weight-and-balance information, etc. I don't think anyone has dismissed the videos you and others have provided - they simply don't provide the information necessary to answer the questions we've posed about claims that have been made. No more, no less.

There have been a number of references to the climb rate , or lack of, of LAR aircraft the quote from post #321 is just one.

“I'm of the opinion that low-AR designs are a poor choice for STOL operations. Yes, you can use vortex lift and the high drag associated with it to produce very short landings. But any runway on which you land, you have to be able to take off from again unless you want to become a permanent fixture at the location. So an ability to land in extremely short distances is rather pointless unless you can take off again and climb out safely from the same location. Here, low-AR is not your friend, for the reasons we've already discussed.“
You're quoting me here, so let me reiterate that I was posting a very reasonable concern about operation of low-AR designs that use vortex lift in one particular performance context. Somehow pulling "low-AR designs can't climb" out of what I said above requires some mental gymnastics that I don't support. I didn't say, "low-AR designs can't climb" at any point in this discussion. I did say that low-AR designs may require more installed power to achieve the same climb performance as an otherwise-equal longer-span design, but that's not "can't climb" by any stretch of the imagination.

Anyway the trolls have won, I have nothing more tos say.
"Trolls"? "... have won"? What do you mean? Nobody's said low-AR designs can't or won't work. Nobody's said there was no design mission specification where a low-AR design might be an, or even the, appropriate design choice. There's a very large and significant difference between "trolling" or "nay-saying" something, and simply asking legitimate questions and voicing concerns about it. I don't appreciate the characterization as "troll" or "nay-sayer", if indeed you're pointing them at me.
 
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Aesquire

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I'm enthusiastic, even inspired, by this thread.

My preference has been to build a motor glider, but the cute & cool factor of a flying saucer type craft is highly intriguing. As a design challenge, the examples are few, and none have achieved commercial success. But they do fly, and there are some good, rational reasons to try a low aspect ratio design. Light construction for the thing area, possible small storage, and messing with people's minds are enough to look closer at the LAR possibilities.

The perfect plane? Hah! No such animal. Darn good for the specific mission is the best you can hope for. Since the mission is recreational flight & mind bending, with requirement to fit Aussie class rules, and be useful from a modest field, that's what should be the focus.

How to make all the requirements and hopes work with this platform is the challenge. And if you can't, then change the platform, you've learned something along the way.

One disadvantage, to me personally, of a LAR platform is building parts in a basement shop, and hauling them up the stairs for assembly. I can assemble a Xenos wing panel in the 50 ft long space, & get it out. A 12-16 foot disk? Not in one piece. Not a deal breaker, the ribs and spars can be fabricated downstairs, and even a pt 103 trike can't be assembled down there, anyway.

Don't let enthusiasm become proselytizing. Zeal become dogma. No need to defend against what are not attacks.

It's a conservative bunch of engineering types here. Caution, demanding hard numbers, seems stodgy, but we get dreamers who want to build an airplane from pvc pipe, who never read Stick & Rudder, and they do try to limit the encouragement to suicidal loons.

They also know you are a proven pilot & designer, and the questioning of every idea is a method to optimize, or eliminate the bad ones, to assist in your proccess.

It's like a creative writing class. Harden the skin against the "that's stupid" useless responses, and pay attention to the grammar and "you used "and" seven times in each sentence, try smaller, more concise thoughts" ones. You will find few of the first type of comment and many of the second here.
 

lr27

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There was some discussion of landing gear length. If you want truly short takeoffs, then you need high thrust at low speeds, and a high pitch angle so you can fly at those low speeds. This would be exemplified in the V-173. The high thrust was obtained with huge props rather than with high power.

Even if you can't take off again, short landings at low speeds may be useful in an emergency.
 

BBerson

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Gliders routinely make short landings, sometimes in soft fields that an airplane with tall gear legs would flip over on.
 

Sockmonkey

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The gear length is only an issue depending on the configuration of the plane.
If it's tailless with a high wing, the gear doesn't have to be very long to have it rest at thirty degrees or more.
If the V-173 had mounted the cockpit under the wing, normal length gear would have been enough.
 

BJC

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The shortest takeoff is accomplished by maximum acceleration followed by a rapid pitch to maximum lift, often with simultaneous application of flaps.

For most thrust to weight ratios, maximum acceleration dictates a streamlined profile, i.e., tail up.


BJC
 

Topaz

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The shortest takeoff is accomplished by maximum acceleration followed by a rapid pitch to maximum lift, often with simultaneous application of flaps.

For most thrust to weight ratios, maximum acceleration dictates a streamlined profile, i.e., tail up.
+1 on this. Getting up to flying speed is the thing that creates most of the takeoff distance. Minimizing this portion of the takeoff means low drag, low rolling friction, high power, light weight, and a low takeoff speed. Setting up a high angle of attack at rest creates high drag and lengthens the takeoff distance.

The only exception is an aircraft using powered lift of some kind. The V-173 would count in this category to some extent, but it's not the shape or configuration of the airplane that does it - it's the gigantic slow-turning prop-rotors on the front. Not something you're going to duplicate with a standard prop. It's not like it's impossible to have two giant custom props and associated gearboxes on your airplane, but the cost is going to shove the design well out of the "affordable" category, at which point there are other ways to skin the same cat. The V-173 was a demonstrator for a design intended for a very particular, unusual, specialized use-case. There's nothing wrong with it, but that use-case doesn't really translate to civil sportplane use, IMHO.

To be clear to the proponents of low-AR aircraft on this thread, I'm just saying that V-173-style props and gearboxes are going to be very expensive. Nothing more, and nothing less.
 

Sockmonkey

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Now that makes me think of powering the landing gear, which might be practical depending on how you do it.
The shortest takeoff is accomplished by maximum acceleration followed by a rapid pitch to maximum lift, often with simultaneous application of flaps.

For most thrust to weight ratios, maximum acceleration dictates a streamlined profile, i.e., tail up.
Fair enough. A high-mounted disk wing would still allow a large degree of rotation with normal-sized gear once takeoff speed is reached.
 

Aesquire

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I recently discovered The Great War project/program on YouTube. German show, 100th anniversary of WW1. Interesting and often stuff I never knew. Including viewpoints from the "enemy".

And there's a spin off show, an American collector/scholar of guns, was asked for photos, because the regulatory rules make it hard to get & show WW1 arms, and the "company was afraid of guns" in the political climate. C&Rsenal then started a video primer in parallel to the Great War guys, and I'm binging on the more than 100 videos of in depth history. You may not care about the subject, but I find the mechanical evolution fascinating, and the procurement process is frustrating, amusing, and the same with aircraft as artillery or small arms.

Committees that sometimes have priorities that hindsight shows were wrong, equipment choices based on doctrine from previous experience that no longer applies, etc.

How does this apply to home built planes?

In WW1, The German high command had a series of fighter plane competitions to choose the next new fighter. It was a time of very rapid development, and fads embraced, sometimes without understanding.

The famous Fokker triplane was in response to a British Navy plane, that used high aspect ratio, short chord wings, in triplane layout, to improve pilot visibility. It was a success, albeit in small numbers, so Germany must have airplanes to keep up!!! The Fokker product was a failure at the British goal, visibility was awful, but the Germans didn't know that was the goal! It did use cantilever wings, a major advance ( that the British Sopwith didn't have ) that led to the famed Fokker D.VII. And the highly maneuverable Fokker triplane was a success for it's own reasons. For a short time.

A later competition would lead to the adoption of the parasol Fokker D.VIII, which was too late to influence the war, but did lead to a few parasol fighters between wars. If WW1 had lasted months longer, the parasol fighters might have been the standard model for years, instead of the low & mid wing machines we now think of as normal.

Even more interesting, another offering from Fokker at the same competition, was a low wing, cantilever fighter, that would have become the shape of things to come, more than a decade ahead of the 1930s designs that were independently evolved without the influence of a successful model in 1918. That didn't happen because the High Command didn't like that the low wing monoplane had poor visibility, straight down.

This leads me to ask, how important is looking straight down?

Obviously it became less of a factor in fighter design, over the years.

It's still important for helicopters and gyros, with hovering & very steep approaches. Presumably somewhat important for STOL ops.

And any planes that lose forward visibility on landing are problematic to varying degrees.

Is there a standard from a century of flight on visibility, or is it a thing of fads, designer preference, and shifting priorities?
 

BBerson

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On landing, virtually all pilots (even helicopter) are looking ahead and slightly down to the intended touch down zone.
The helicopter pilot will scan straight down perhaps only in the hover. But usually can't see the other skid anyway, so carefully feels the way on the last two feet down to the surface.
 

Sockmonkey

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The point is that a many designs got passed over for reasons unrelated to their actual merit, so success isn't the defining criteria of a good design.
 

Himat

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...
This entire discussion has become quite "silly," in the sense that there's no design requirement context in which all these notions are being discussed, lots of "black magic" design theories are being thrown around, and deep, meaningful discussions are being held about whether things like vortex lift is "better" on circular planforms or delta-like planforms. The latter is particularly frustrating when any rational design process involving "vortex lift" is either well outside the capabilities of those discussing it or, if they do have the capability, they're ignoring it completely in favor of a "design process" that seems to primarily consist of the statement, "I think...".

If one wants to have a "dreamers" discussion about what looks neat and is desirable from an aesthetic point of view, or even a pure construction concept standpoint, then fine. But despite repeated calls for substantiation from several people, including myself, I'm still seeing unsupported statements - unsupported by actual aerodynamic theory, standard design methods that have proven viable for decades, or even historical precedent - that this or that low-AR design is clearly better overall at some vague and unspecified broad class of design missions. And, when myself or others have asked for some kind of substantiation for those claims, we're either pointed at a shaky amateur video of an existing design "flying great" - without any metrics or performance data - or told that we're dismissing low-AR designs as unworkable, or other accusations that we say they "can't climb" and so on. None of which any of us has ever actually said....
The discussion also gets defocused as when comparing the merits of high and low aspect wing designs the metrics are not held constant. Tailed designs of one aspect ratio is compared to tail less designs of the other and large part of the difference in performance may come from tail or not. Not the aspect ratio of the wing.

Then the merits of a moderate to high aspect ratio wing with flaps and brakes are compared to low aspect ratio wing without either. Turning this around, with no flaps and no brakes the airplane with the high aspect ratio wing may have to be designed with excessive drag impairing on the speed range to have decent handling characteristics when landing. The low aspect ratio wing may pitch up for braking.

All in all, it is design to requirement. Just observe how different formulas are arrived at in aerodynamic text books. At constant span, induced drag does rise with increasing aspect ratio.;)
 

ypsilon

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At constant span, induced drag does rise with increasing aspect ratio.
You forgot to say: At any given airspeed (for both wings).
At max L/D for each wing, the higher A/R wing will have lower induced drag in many configurations.
 

Himat

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You forgot to say: At any given airspeed (for both wings).
At max L/D for each wing, the higher A/R wing will have lower induced drag in many configurations.
You are correct that I should have stated at equal airspeed, equal airplane weight and atmospheric conditions for both planes. Nevertheless, the high aspect ratio wing will have higher induced drag, drag due to lift, even at best L/D if span is held constant. The reason is the Cl term in the Cdi equation. The lift coefficient that have to increase as the aspect ratio increase and wing area decrease to provide the same lift.
 

ypsilon

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You are correct that I should have stated at equal airspeed, equal airplane weight and atmospheric conditions for both planes. Nevertheless, the high aspect ratio wing will have higher induced drag, drag due to lift, even at best L/D if span is held constant. The reason is the Cl term in the Cdi equation. The lift coefficient that have to increase as the aspect ratio increase and wing area decrease to provide the same lift.
No, the Cl doesn't have to increase. The airspeed for best L/D increases. At best L/D the induced drag is pretty much 50% of the overall drag. The lower A/R wing will have more (shear and form) drag, and therefore also more induced drag. Otherwise one could increase the performance of sailplanes simply by increasing wing cord / lowering A/R and adding more ballast to the ship. That doesn't work though. High performance sailplanes are high A/R for good reason.

As said for any given airspeed the lower A/R wing will produce less induced drag than the high A/R wing of the same span. For any given Cl the high A/R wing does better.
 

Himat

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No, the Cl doesn't have to increase. The airspeed for best L/D increases. At best L/D the induced drag is pretty much 50% of the overall drag. The lower A/R wing will have more (shear and form) drag, and therefore also more induced drag. Otherwise one could increase the performance of sailplanes simply by increasing wing cord / lowering A/R and adding more ballast to the ship. That doesn't work though. High performance sailplanes are high A/R for good reason.

As said for any given airspeed the lower A/R wing will produce less induced drag than the high A/R wing of the same span. For any given Cl the high A/R wing does better.
Note the detail, I said at constant wing span.;)

Lift = ½ *air density * speed^2 * wing area * Cl
Aspect ratio = Wing span squared/ wing area
Turn it around:
Wing area = Wing span squared / Aspect ratio
Substitute and you get:
Lift =1/2 * air density * speed * (Wing span squared/Aspect ratio)* Cl
Hold airplane weight, that is lift, air density, speed and wing span constant the Cl have to increase as aspect ratio increase.

The crux is that at constant weight, lift, and span, the high aspect ratio wing always has to operate at a higher Cl.
 

ypsilon

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Note the detail, I said at constant wing span.;)

Lift = ½ *air density * speed^2 * wing area * Cl
Aspect ratio = Wing span squared/ wing area
Turn it around:
Wing area = Wing span squared / Aspect ratio
Substitute and you get:
Lift =1/2 * air density * speed * (Wing span squared/Aspect ratio)* Cl
Hold airplane weight, that is lift, air density, speed and wing span constant the Cl have to increase as aspect ratio increase.

The crux is that at constant weight, lift, and span, the high aspect ratio wing always has to operate at a higher Cl.
Everything above is correct, however you said constant span, but not constant speed.
What I said (maybe not clearly enough) is that at the same span the high A/R wing will reach it's maximum L/D at a higher speed with (possibly) less induced drag than the low A/R wing. The exact speed of max L/D depends on the parasite drag, but if the fuselage is very clean (as in: sailplanes) you get not only lower overall drag, but also lower induced drag at max L/D for the high A/R wing. As i said before at any given speed the low A/R wing will have less induced drag. That is trivial, of course.

In your initial post you didn't contrain airspeed. If your requirements are indeed span and (low) airspeed, then at some point you'll have to increase area which means lowering A/R. No surprise either.
 
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