Very low aspect ratio planes?

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berridos

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I bet you can fly your rcmodel also as a helicopter with the propper landing gear.
 

Himat

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You are right.
You have more adverse yaw in a glider, but on the other hand, in a LAR you have less margin for aditional induced drag and in a LAR you cant use usually the rudder to get into the turn as it lies too close to the cg and hasnt got leverarm and therefor any effect.
When you give ailerons in the verhees (pilot experience)and similar cases described in "Schwanzlose Flugzeuge" (best aero book i have read) the plane banks to the opposite side. Also the nose point to the other side initially. It feels like a soft rocking that repeats itself a couple of times (induced drag resonances- damping effect because of lack of directionality in absence of a tail). Afterwards and progressively the plane starts banking correctly and turning to the desired side.
The Hortens werent satisfied with Frise ailerons, until they sharpened the lower edge, however Horten planes have large AR.
There are several chapters on this and i definitly need to give it a second read. But as a conclusion, you can make the turn inmediate without yaw and with minimal drag if you lower slightly 3 elevons (two per side) and just lift the outboard frise elevon on one side.
If the rudder is close to the centre of gravity the airplane does have a short tail arm. I do read that you by a low aspect ratio airplane imply a tail less airplane or a flying wing. If so, the reason for the short rudder lever arm is not that the plane has a low aspect ratio, but that it is a flying wing or tail less design.

Two good examples of low aspect ratio airplanes with delta wings are the Mig 21 and A4 Skyhawk. Note that both are conventional tailed airplanes.
 

Sockmonkey

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So the thing you're pointing out is how using the ailerons on a LAR with a lot of sweep gives some unexpected effects because the ailerons are also your elevators yes?
 

Vigilant1

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The Zimmerman designs, the F-104, the Facetmobile-- all are low AR designs but they are more different than alike in their attributes and their aerodynamics. Unless we are more specific about what type of design we talking about, it seems to me that not much of interest can be said about "low AR" by itself.
 

Mavigogun

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Moreover, when proclamations lacking specificity are presented in an authoritative tone, they obfuscate my understanding, may construct a limited, false impression.

You have more adverse yaw in a glider, but on the other hand, in a LAR you have less margin for aditional induced drag and in a LAR you cant use usually the rudder to get into the turn as it lies too close to the cg and hasnt got leverarm and therefor any effect.
Regarding the Facetmobile: I’ve reviewed just about every available published recollection by Wainfan, a couple video lectures, and a radio interview. I don’t recall him ever mentioning the FMX-4 afflicted with remarkable adverse yaw, or impotent rudders; he said at very low speed a light Dutch roll may develop, dispelled in a moment by tapping both rudders simultaneously. The FMX-4 features a single elevator flanked by a single elevon on either side; Wainfan made no mention of any need for a second set of elevons to ward off tip stalls/spins- on the contrary, he claimed the platform was spin-avers and remarkably stable.
 
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Mavigogun

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I havent got any feedback on the facet, but if you have large endpalte rudders and therefor good directional stability you also have tons of drag (however endplates reduce induce drag if designed properly).
The facet is really a very interesting design, however the cruise performance has been willingly neglected.
The FMX-4 was relatively under powered by a Rotax 503; 90 mph at 75%, 115 mph wide open at 4K‘ elevation- and that with unfaired gear. So, not particularly slow for that power plant. Wainfan reckons those figures would be bettered by 5-10 knts with fairings. His next iteration, the FMX-5/Talon, greatly reduced (eliminated) washout, features smaller tip fins and improved top end performance in both wind tunnel testing and large scale models. I reckon “willingly” neglected cruse performance isn’t accurate; Wainfan reckons his latest work, appropriately appointed, to cruse around 170- so, faster than the most popular aircraft ever made, the Cesna 172... while stalling significantly slower.

I have heard no reports from David Rowe that his UFOs suffer from remarkable adverse yaw or impotent rudder, nor is adverse yaw apparent in flight recordings- so either the rudder is highly effective without inducing slip, or there is no remarkable adverse yaw in that low aspect craft either.
 

bwainfan

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Berridos's last post, at least as it relates to Facetmobile follows the sadly-common trend on HBA of someone who has no first-hand knowledge making authoritative-sounding statements that are completely wrong. When its just people talking "general principles" I generally refrain from jumping in but as it relates to my own work, it needs some correction. His statement that "but if you have large endpalte (sic) rudders and therefor good directional stability you also have tons of drag" Is incorrect. The actual drag increment for the fins wetted area is quite small and they do actually increase span efficiency.

Likewise "The facet is really a very interesting design, however the cruise performance has been willingly neglected." is pure masculine cattle exhaust. It presumes to know what I was thinking when I designed the airplane, and ignores the airplanes real performance. In fact Facetmobile was one of the fastest airplanes out there with its engine (Rotax 503). It was cruising at over 90 mph on 46 horsepower.

WRT adverse yaw: the FM required very little rudder to coordinate turns.

The adverse yaw phenomenon people are discussing above is usually caused by a combination of high dihedral effect and low directional stability. In such a configuration, the initial yaw caused by an aileron input causes sideslip. That, in turn, through the dihedral effect causes the airplane to initially roll opposite to the direction of the aileron input.
This is not specific to low ar configurations. Swept-back wings and some low ar configurations can have high dihedral effect even without geometric dihedral and exhibit the same characteristic. This is why some swept-wing airplanes, including all of the Rutan canards, and some miltary jets, ins particular those with a shoulder-wing configuration (like the Alpha Jet for example) have anhedral.

A thorough airing of ideas and thoughts in this board is a good thing, but ts counterproductive to have people citing suppositions, assumptions and misconceptions here to retroactively dismiss the success of a few who have actually conducted successful flight testing of a concept. John Dyke, David Rowe, Bart Verhees (and I) have all produced low AR airplanes that fly well and have demonstrated them repeatedly and publicly. As with all airplanes each has its configuration-specific quirks I'm sure but they all fly quite well.

The question isn't whether a successful low AR light airplane is possible: The answer is yes... Its already been done more than once.
The really interesting discussion is what specific configurations and concepts have merit for what we want to do with an airplane.
 

bwainfan

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Best you open up a fan club website on the facetmobile.
I truly dont care about your obsessive facet benchmarking. Seems you cant understand that a 3ft distance between rudder and cg makes the rudder worthless because there is no leverarm.
maybe on your obsessive facettick, the leverarm up to the wingtip is enough to turn by drag
170 what? Are you kidding?
[/QUOT
Best you open up a fan club website on the facetmobile.
I truly dont care about your obsessive facet benchmarking. Seems you cant understand that a 3ft distance between rudder and cg makes the rudder worthless because there is no leverarm.
maybe on your obsessive facettick, the leverarm up to the wingtip is enough to turn by drag
170 what? Are you kidding?
Your website shows no progress since 2016 on your (very conventional) project. Perhaps yout ime would be better spent on finishing your own project instead of snarking here about others work....
 

Mavigogun

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Barnaby, I wonder if you might speak to the design of your “tip fins” on the FMX-5 and Talon?

With the angle of the below-wing portion, what is the primary consideration? On a Talon landing recording, it appeared the tip fins ejected on touch-down- this seemed to be as-designed.

Do the tip fins help retain vortices stability at high angles of attack? Do tip fins prevent, retard, or redirect vortex contribution along the attached sections of wing? (were the wing not truncated, the tip a point at the trailing edge, there would be that much more leading edge to spawn vortices, I reckon) How do these musings relate to the tip-finless, single central stabilizer concept depicted on the Feasibility Report?

Lastly, the FMX-4 and 5 both feature tip fins that project forward of the leading edge- why is that?
 
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bwainfan

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Fins on Talon Topper are permanently fixed. You may have found a video of a minor mishap but I'm not familiar with it. The fins on both airplanes are positioned so the fin structure can hook into the mounts to the main hull at a point near the fin aerodynamic center to minimize moments on the fin attach. The aerodynamic effect of having the fin project forward of the LE is trivial, and was not a major consideration.

The interaction between leading-edge vorticies and fins is a very large topic..too much to discuss here. Some configs work well, some don't. In general you don't want to put a fin in the path of the vortex core. On Facetmobile, the vortices pass inboard of the fins. On a central-fin config, they say outboard of the fin. I have lots of unpublished (an never-to-be-published) data on configs with bad fin/vortex interactions.

check out this report for some good wind-tunnel test info on the subject:

 

cluttonfred

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From the link Barnaby posted and othe articles by D.H. Peckam available on the Aerade archives, it looks like we should forget about all this facet stuff and just built the wing flat. ;-) I am only half joking. The flat plate truncated delta (an equilateral triangle sitting on a rectangle) managed a CL of 1.2 at an AOA of 30 degrees pretty much irrespective of Reynolds number. Hmmm....
 

Mavigogun

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A digression, but for those interested, the linked-to video is a promo for Near Space Corp that includes the Talon Topper; while I've cued the link to the moment they bounce it in, blowing off the tip fins, there are other images of interest to that craft- so rewind:

 

cluttonfred

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That tracks with the paper (and related research) in Barnaby's link above. The endplate fins hurt flow over the "gothic delta" wing and reduced max lift coefficient substantially compared to the single central fin. It does seem like the central fin would be blanked out at high AOA, reducing it's effectiveness, but that could be overcome by moving the fin far enough aft for the major portion of the rudder area to project below the trailing edge.
 
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Sockmonkey

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That tracks with the paper (and related research) in Barnaby's link above. The endplate fins hurt flow over the "gothic delta" wing and reduced max lift coefficient substantially compared to the single central fin. It does seem like the central fin would be blanked out at high AOA, reducing it's effectiveness, but that could be overcome by moving the fin far enough aft for the major portion of the rudder area to project below the trailing edge.
IIRC there was a LAR model posted here that mentioned how a slight belly fin helped a lot. Wasn't a delta though. In any case, having your central rudder stick out the bottom a bit looks like it would help a lot at high AOA as the lower bit would get some improved authority from directing air that's been squeezed under the fuselage to make up for blanketing of the upper part.
 

rotax618

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I found that the central rudder on my models was very effective at very high alpha. You have to change the mindset that the airflow over the surface of a deltoid planform of AR<2 is the same as a conventional rectangular planform. The upper surface of a rectangular wing at alpha>12deg is completely stalled and any vertical control surface in that stalled airflow will be ineffective, conversely, the airflow on the upper surface of the deltoid planform consists of two inward counter rotating longitudinal vortices, the airflow is not chaotic but is stable, inducing airflow over the central vertical surface.
This is the same reason that the choice of airfoil on deltoid planforms is non critical, a flat plate or faceted section works as well as a carefully chosen conventional airfoil, don‘t take my word, do your own experiments - get a sheet of low density styrofoam, put a brushless motor on the front and some simple RC gear, sharpen the leading edge and try it.
This article is worth a read.
 
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