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Thread: Tailless Aircraft - Reflex and other design issues

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    Re: Tailless Aircraft - Reflex and other design issues

    The Cherokee flap speed limit is 115 mph. But I was only at 60-65 in the pattern. Solo, almost no gas, it was relatively light for towing gliders.
    So I didn't just pull the flaps, that would be tricky. I came in already with full flaps all the way from downwind with a bit of power. Then drop the rope, chop the power, and it just falls. But the energy in the fall converts back to lift in the flare. It isn't difficult. It won't stall either, because the yoke is eased forward and the nose goes down below the horizon. The Cherokee won't stall unless the nose is pulled up with full back pressure almost.
    The Cherokee AR is about 5. Anything lower aspect ratio will fall even faster.
    I don't see a need to go to AR1.2 for steep glide to flare feature.

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    Re: Tailless Aircraft - Reflex and other design issues

    Quote Originally Posted by rotax618 View Post
    If you turn height into speed you need some way of slowing down quickly to land short, it is usually not possible to pull full flap unless your Vfe is very high.
    Airbreaks?

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    Re: Tailless Aircraft - Reflex and other design issues

    This is a silly argument.

    Glider landings with strong air brake/flap/split rudder fly "fast", well over max L/D speed, for glide path control and steep approaches, but they don't dive into the landing zone. It may appear that way on craft like The Shuttle, but it's energy management. No go around or last second throttle application to ease sink rate.

    Bush & Naval planes that land in high sink rate mode have long stroke tough landing gear. The Super STOL kit makers warn not to attempt the "plop" landings their plane is capable of, in planes without such gear. The rest of us try hard to achieve near zero sink rate on landing. Even the Super STOL guys usually use a blast of power on landing to ease sink rate when not showing off.

    So arguments about not even designed, much less flown, aircraft landing techniques seems premature.

    I'm more interested in opinions on V-173 style elevons vs. T tail Arup configurations.

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    Re: Tailless Aircraft - Reflex and other design issues

    Quote Originally Posted by ypsilon View Post
    Airbreaks?
    The long version of that comment is that there are quite a number of sailplanes with drag brakes (which the soaring community incorrectly calls "spoilers") that create enough drag when fully deployed that, even in a vertical dive, they can limit the aircraft's speed to below Vne. You could point the nose down vertically at the ground and never worry about destroying the airframe. Other than weight, there's absolutely no reason you couldn't have larger drag brakes that limit vertical dive speed to some relatively arbitrary lower airspeed. Of course, eventually you need to fold them when not in use, and call them a "parachute."

    Quote Originally Posted by Aesquire View Post
    This is a silly argument.
    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.

    Quote Originally Posted by Aesquire View Post
    So arguments about not even designed, much less flown, aircraft landing techniques seems premature.
    In context with everything else that's being discussed here and the way this thread has been going, I really don't see how it matters.
    Last edited by Topaz; February 19th, 2019 at 02:20 PM.
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    Re: Tailless Aircraft - Reflex and other design issues

    I have simply tried to bring this aviation type to notice for discussion, in support of my observations I have posted video evidence of the flight characteristics and some studies undertaken to investigate this genre of flying machine.
    LAR aircraft do fly and have flown, successfully and safely, contrary to the nay sayers, they offer a flight envelope that is not generally available from other types.
    I just cant understand why anyone could completely dismiss the video evidence and eye witness reports just because it goes against their pre-conceived ideas.
    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.

    Anyway the trolls have won, I have nothing more tos say.

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    Re: Tailless Aircraft - Reflex and other design issues

    Quote Originally Posted by rotax618 View Post
    ... 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.

    Quote Originally Posted by rotax618 View Post
    ...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."

    Quote Originally Posted by rotax618 View Post
    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.

    Quote Originally Posted by rotax618 View Post
    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.

    Quote Originally Posted by rotax618 View Post
    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.
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry David Thoreau

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    Re: Tailless Aircraft - Reflex and other design issues

    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.

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    Re: Tailless Aircraft - Reflex and other design issues

    “There’s a pony in here, somewhere ...”


    BJC

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    Re: Tailless Aircraft - Reflex and other design issues

    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.

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    Re: Tailless Aircraft - Reflex and other design issues

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

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    Re: Tailless Aircraft - Reflex and other design issues

    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.

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    Re: Tailless Aircraft - Reflex and other design issues

    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.


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    Re: Tailless Aircraft - Reflex and other design issues

    It will still flip over if the CG is higher. The only solution to high CG is huge tires.

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    Re: Tailless Aircraft - Reflex and other design issues

    Quote Originally Posted by BJC View Post
    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.
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    Re: Tailless Aircraft - Reflex and other design issues

    Now that makes me think of powering the landing gear, which might be practical depending on how you do it.
    Quote Originally Posted by BJC View Post
    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.

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