Characteristics of a straight-wing, tailless model in the Langley free-flight tunnel

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nickec

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

... Reducing fuel capacity doesn't reduce structural weight. ...

With all due respect, reducing fuel capacity reduces gross weight, which can potentially reduce structure.

... a good design, either composite or alu would end up weighing over 300 lbs empty. ...

Empty weight goal is 155 pounds. DA-11 achieved 175 pounds - with structure carrying 36 pounds of fuel.

Flight manuals regularly quote never exceed G figures with various gross weights. At lower grosses allowable G's are higher.

Maximum fuel weight always enters into aircraft design calculations.

I understand your thinking is likely that more fuel means more safety margin and more range.

At a fuel burn rate of one gallon per hour, which is what Leeon Davis reported for the DA-11, 3 gallons would give 2.5 hours with a reserve.
 

Aircar

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

The thread name refers to a hypothetical design not so unlike the Northrop 'black bullet' --XP something I forget right now 'possibly XP56 (linked before on the Synergy thread and others ) --it was not a success, to say the least and it might pay to take a look at why and think again about what happens when just slicing the tail off something like the DA11 .
 

nickec

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

northrop_xp-56.jpg p_006.jpg 800d1148538424-jerry-blumenthals-raspberry-ds41012.jpg

--------Look familiar?----------------------Compare.---------------------Contrast.

I like the middle option. :)
 
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autoreply

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

With all due respect, reducing fuel capacity reduces gross weight, which can potentially reduce structure.
Barely in such light airframes. Most is sized for skin buckling, handling etc. Save the landing gear, you'd be hard pressed to save any weight by reducing MTOW.
But then, fuel, luggage and pilot weight are exchangeable. Coming from a heavy pilot with a bit of luggage and a bit of fuel, if you're like me (sub 140 lbs), you can simply fill up the tanks, take a lot of luggage and stay below MTOW. No weight penalty to enlarge the tanks.
Notably fuel in the wing (within reason) won't have any impact on structure, save on the landing gear. Usually, performance, not strength is the bigger problem then...
Empty weight goal is 155 pounds. DA-11 achieved 175 pounds - with structure carrying 36 pounds of fuel.
Good luck with that. Even the Cri-Cri (with lighter engines) is hard-pressed to make that. Maximizing one parameter will cause grave penalties in other area. And just for the record, let's stipulate what I actually said:
I'd be surprised if - even with a 90 lbs engine - a good design, either composite or alu would end up weighing over 300 lbs empty. 250 lbs or so seems reasonable (without instruments, upholstery and a BRS)
 

nickec

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

Post 23 shows the Black Bullet. Note that it is a swept wing with diffuser tips, or if you prefer, anhedral outboard. Such a design requires twist. How much twist has long been debated and continues to be hashed over. In short, the Black Bullet planform is not a plank - not a straight wing.

My proposition is only to size very close to the NACA model, a plank with constant chord, and see what happens.

All our opinions remain opinions until tested. I appreciate all viewpoints including those opposed to my thinking.
 

nickec

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

Thanks for reminding us ultralajt.

I have long wondered about your "Plankie" - mentioned on your website but never pictured there.

Are you willing to share more about the Plankie?
 

autoreply

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

Everything is "barely" at these very light weights. ;)

OK, some hard numbers then. From what I'm working on (engineered=>tested), a MTOW increase of 10%, all of it in the non-lifting mass, would result in less then 1% more empty weight (all of it obviously being structural), so a 50 lbs MTOW increase adds about 2 lbs of structural weight. In these very light airplanes, most stuff is very strongly non-linear. Save the landing gear, wing spar and tail boom (not for you obviously ;)), there's zero change in empty weight.
 

nickec

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

... Save the landing gear, wing spar and tail boom (not for you obviously ;)), there's zero change in empty weight.

I see that I have again failed to convey my meaning. Sorry about that.

I was trying to convey two things.

1. Spar and landing gear might lose some weight.

2. Moment arms are shorter.

Number two needs to be fleshed out: Consider that the engine is closer to the spar. In the DA-11 the engine was beyond the pilot's feet. Engine mount, propeller, and engine are all closer to the CG in the NACA model.

I might be wrong, yet it seems clear to me that pusher revamp of a DA-11 size airframe will weigh less than the original DA-11 --- if configured like the Facet Opal, or like the NACA model outlined in the report.

I have made several chuck gliders to explore rudder placement because the basic difference between the NACA model and the Facet Opal is fin/rudder location. While such tiny unpowered models are far from conclusive, they suggest that both fin planforms - NACA and Opal - are workable.

There are many successful Radio Control models using the Facet Opal planform.
Below is a picture of the smallest RC model I have seen of the Opal.

a1780239-57-opal.jpg

And below is the largest:

a1780238-76-DSCN3577.JPG

Only full size testing will fully convince me of the value of either planform. By this I mean the NACA and the Opal.
 
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Aircar

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

Nickec - the salient point about the Northrop black bullet ,the Langley model tested the FACET Opal and some others of this ilk is that they need to be jacked up to clear the rear prop plus some allowance for rotation angle -this puts them on fairly stalkey landing gear and a high Cg with very LITTLE polar moment of inertia in pitch ; ergo they tend to 'trip over themselves' (like someone stepping on their shoe laces ..) There was a low wing ducted fan tailless ultralight built here by David Betteridge in the early 80s --the Hornet - that showed this characteristic by virtue of it's much repaired nose --it has a habit of nosing over (it was a ''tail' wheel despite having no tail but the basic fault exists as a consequence of the physics involved) The FACET also suffered from this trait and another video -possibly linked to the old FACET thread shows a slow motion demolition of it's landing gear --luckily it all wiped off before being able to flip on it's back like the XP 56 did .

I tried to get a flight in an AV22 Fauvel at Fayence in 1973 but nobody wanted to fly it and it had the characteristic recently doped (repaired )nose and fin top indicating again the perils of very short coupling in pitch --an extended nose and longer wheelbase will help (eg Genesis) but for a glider landing downwind and a little downhill in a rough field is the acid test . Thousands of control line "planks' have been flown and later RC models of the same --have you seen ONE flying wing of any kind NOT "work" in fact ? --it seems that almost any variation can be successful provided the Cg is far enough forward -two seconds with a sheet of A4 paper can 'prove' that almost anything can fly without a tail and look great. But what else is "characteristic' of such designs that might not show up in a chuck glider or RC .....?
 

nickec

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

... very LITTLE polar moment of inertia in pitch ...

How great a difference do you suppose there is between the Colomban Cri Cri polar moment of inertia and the polar moment of inertia of what I propose?
 

danmoser

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

The low moment of inertia of a flying plank means it is very responsive to pitch control input, but is also just as responsive to gusts.
I've heard pilot reports that this characteristic is at first unsettling, but that pilots may adapt to it and learn to love it.. and in fact, sometimes make them start disliking conventional aircraft that are less responsive.

One thing to watch out for with planks is their inherent lack of pitch damping .. often confused with, but not the same thing as pitch stability.

For pitch stability on a plank, you need a positive pitching moment airfoil, usually done with a reflexed trailing edge .. but without a horizontal tail surface, it is under-damped in pitch.

Since it is stable, it tends to seek the equilibrium angle of attack when upset .. but since it is under-damped, it will overshoot past the equilibrium point and oscillate around it for a few cycles at a relatively high frequency.

Trying to correct for every bump of turbulence will heighten pilot anxiety .. perhaps best to just get used to letting it oscillate.

The narrower the chord -in other words, the higher the aspect ratio- the less pitch damping it will have... exponentially less.
By the way, sweep can add some pitch damping without adding a tail.
 
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Topaz

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

How great a difference do you suppose there is between the Colomban Cri Cri polar moment of inertia and the polar moment of inertia of what I propose?

It's not really a matter of comparisons. It's a matter of calculating what you need. The general paucity of pitch control power of "flying plank" style aircraft means that the effect Aircar is talking about is real. They have enough pitch control power to fly, certainly, but even in "tailed" aircraft, the elevator power is set by some takeoff or landing situation connected with rotating the aircraft.

The "fix" for tailless aircraft that seems to work the best, AFAIK, is to use tricycle gear adjusted to set the aircraft in takeoff attitude, and then provide enough clearance angle at the tail to protect the prop in a landing at stalled condition. There will be, very likely, some difficulty keeping the nosewheel off the ground once the mains are solidly down, due to the relative lack of control power. A long-stroke nose-wheel leg can help that a bit.

Tailless gliders have it relatively easy here - simply mount a single main wheel at or just aft of the CG, and then employ a nose skid. You don't want to keep the nose up on landing in that case, so the natural tendency to drop the nose actually helps you here. The skid could, perhaps, be replaced with a nose-wheel, but the latter would have to be highly damped or the nose is likely to bounce back into the air.

All airplanes are compromises. Tailless airplanes have their own set, just like every other kind.
 

autoreply

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

I see that I have again failed to convey my meaning. Sorry about that.
No need to apologize. It's hard enough to convey these kind of things face to face, let alone by means of a few electrons zooming by ;)
1. Spar and landing gear might lose some weight.

2. Moment arms are shorter.

Number two needs to be fleshed out: Consider that the engine is closer to the spar. In the DA-11 the engine was beyond the pilot's feet. Engine mount, propeller, and engine are all closer to the CG in the NACA model.

I might be wrong, yet it seems clear to me that pusher revamp of a DA-11 size airframe will weigh less than the original DA-11 --- if configured like the Facet Opal, or like the NACA model outlined in the report.
Tails are often underestimated in weight and can be pretty heavy. (Not that they need to be, ours (just the tail boom) is 10' and 3 lbs..)
But then you need a fair bit more wing area for the same lift (stall speed), about 50% more or so? Not sure whether in the end it'll be an equal draw or one would be noticeably lighter.
I have made several chuck gliders to explore rudder placement because the basic difference between the NACA model and the Facet Opal is fin/rudder location. While such tiny unpowered models are far from conclusive, they suggest that both fin planforms - NACA and Opal - are workable.
Extending it below the TE for some (free) extra certainty?

If you have fins anyway, why not extend them a bit and add a small horizontal? *poof*, all the issues are gone. Yeah, I know :gig:

yhst-17210252890263_2272_212197671

sadler-vamp-small.jpg
 

nickec

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

... One thing to watch out for with planks is their inherent lack of pitch damping ...

I am familiar with the damping issues. Part of the reason I want to explore the planform stems from Bill Daniels' comment below, which I read many years ago.

"... This experience led me to the counterintuitive belief that the mass
distribution about the pitch axis and the airfoil reflex
should be at the
absolute minimum for best flight characteristics. This permits the wing
chord to be reduced for a higher aspect ratio."

Bill Daniels

I want to investigate, using a full size testbed, if he is right. By using DA-11 size testing will cost little in material.

If an appropriate test pilot appears, a full roll cage similar to that in a top fuel dragster could protect the pilot while flight was restricted to altitudes under ten feet. If such testing was always on grass with a full face helmet, injury would be minimized and maybe even eliminated, if the worst case scenario arrived - say a hard groundstrike followed by a tumble decelerating from about 65 mph.

I have also considered a protective cell like that required by Formula One racing rules. By building such a cell half as strong as their rules require, and by using fiberglass instead of carbon, a pilot would likely always walk away from a 65 mph mishap. You might ask then: why not just build a composite airframe? I admit that is a valid option.
 
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Aircar

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

That two seat Vampire is, I think, the one developed in Australia under a federal grant (by the local Vampire agent) --the whole firm was sold off.

The thing about flipping over is that nobody MEANS to --the worst case is during landing finding the surface is too soft and applying power to go round (all split second realizations -- the drag from below coupled with the thrust from above and suddenly de cambering the wing with negative flaps (up elevon) might be just enough to do the trick and over you go. (it takes less of these factors if you have no wind, hot ,high, a tailwind or any downslope, long or wet grass --even conventional aircraft flip over in this situation. (see the RV6 photo in the process on an adjacent thread)

Also to note - with very low powered aircraft the landing gear is not a length problem but as the power increases (and hence prop diameter) then the CG height issue gets more of a problem (eg the 2000HP or so of the XP56 and several other tailless fighter proposals ) -the wings own mass and pilot are the main culprits for small aircraft.
 
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nickec

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Re: Characteristics of a straight-wing, tailless model in the Langley free-flight tu

... fairly stalkey landing gear and a high CG with very LITTLE polar moment of inertia in pitch ...

Just finished comparing the CG heights of Colomban's Cri Cri and a scaled up version of the NACA model to DA-11 wing dimensions.

A 15 degree rotation allowance results in surprisingly short landing gear. The CG is closer to the ground than the Cri Cri.

When I superimposed the side view of the Cri Cri over the scaled up NACA model I found the landing gear height equal on both aircraft.

The explanation for this result is that the Cri Cri locates the engines quite high and although the flying tail weighs 2 kg (4.4 lbs.) - it is mounted quite high. With these three masses located high - in comparison to the NACA model - the Cri Cri CG ends up higher than the NACA model CG.

So "stalkey" gear issue might be avoided here. Inertia in pitch remains a possible gotcha.
 
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