Very low aspect ratio planes?

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rotax618

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The most successful of the large models had very small tip fins, these models were built and flown many years ago, I thought the addition of small tip fins would increase the span efficiency, but in reality they actually disturbed the stability of the vortex, I discovered this much later while testing various planform shapes of sheet foam models.
 

lr27

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I have a delta planform model made of flat foam that has a central fin and rudder. The rudder is effective. Adding it made a significant improvement n slow flight at high angles of attack.
 

Riggerrob

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Dear Rotax 618,
Full-sized Saab Drakens also had nasty stall characteristics.
They could get into a deep stall where wing tips were stalled, but forward LERX were still lifting. The
Swedish Air Force eventually developed a stall recovery technique that depended upon momentum to depress that nose and many thousands of feet of altitiude!
 

cluttonfred

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I think that's what Rotax 618 was implying when saying that SAAB spend millions to learn what an inexpensive model also demonstrated.

I have no idea how it would impact vortex generation and other issues, but one thing that has occurred to me using all-moving outer wing sections as elevons to provide docile stalls because the elevons would be nose-down as the wing goes nose-up. Such elevons were used in some non-delta tailless designs like the Short Sherpa and Granger Archaeopteryx.
 

danmoser

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I have a delta planform model made of flat foam that has a central fin and rudder. The rudder is effective. Adding it made a significant improvement n slow flight at high angles of attack.
That parallels my experience as well.. mine was essentially a flat plate equilateral triangle delta with a central rudder, AR~2. I was able to turn it without using ANY rudder input (elevons only), but a modest amount of rudder allowed it to carve cleaner, tighter turns.. I could also turn it with rudder only, but not quite as easy. It would maintain lateral control, even at very high AoA, nearly vertical descent.... next step is to build an improved version with motor & prop.
 

Johan Fleischer

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Pinterest is a time wasting site, as it doesn't distinguish between delusions, concepts, plans, aeromodels, prototypes, or production machines. Terrible!
hehe, I agree. Pinterest is not to be regarded as a useful tool. Just for entertainment and fun. But sometimes one of the things view'ed as entertainmant, gives a little spark of interest 🙂
 

Arfang

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Since this thread is specifically oriented towards very low aspect ratio airplanes, I hope it's OK if I post here intsead of creating a new thread.

In an attempt to better understand the hows and whys of the low AR formula and hopefully help others understand the subject I made a summary trying to put together what has been written on this forum. It has been 'dumbed-down' to be able to be digested by a non-engineer like me. The following is mostly focused on tailless delta airplanes. I also included some questions at the end, everybody is more than welcome to chime in and correct me if I'm wrong.

Some pro's of low AR airplanes:

-large internal volume
-benign stall characteristics
-potentially simpler construction using tube-and-gusset or flat panels
-high load fraction
-performance in the same range as more conventional airplanes

Some tailless swept-wing low-AR airplanes and their aspect ratio:

-Moskalev SAM-9 (0.96)
-FMX-4 (1.06)
-proposed PAV by B.Wainfan (1.86)
-Gtex09 (1.44)
-Piana-Canova PC.500 (1.94)
-BICh-1 (3.0)
-Dyke Delta (2.94)
-Baker MB-1 (3.32)
-Verhees Delta Mk.1 (2.03)

Vortex lift

A leading edge vortex (LEV) is generated above the wing upper surface when the LE sweep angle is large enough, about 55 to 60°. On a curved leading edge planform, the vortex will start at this 55° section and move in- and outboard. This vortex delays flow separation and allows the wing to operate at a larger angle of attack at the cost of higher drag.

The vortex 'generation' and stability increase as the LE becomes sharper, the larger the radius the least predictable and less stable the LEV becomes. This leads to an unsteady and asymmetrical LEV formation that causes an uncontrolled rolling moment. Altough, only the root section needs to be sharp, by having a small leading-edge radius or using stall strips, the wingtip airfoil can have a more conventional round leading edge airfoil.

Double delta, 'inverse Zimmerman' and gothic planform

Since the leading-edge vortex will start at the wing station closest to 55-60° of sweep, an aircraft with a compound delta or 'double-delta' like, for instance, a leading-edge root extention (LERX), will experience an increase of lift forward of the CG, thus creating a nose-up moment. A more 'gothic' planform, like on the FMX-4 will trip the vortex more aft, thus creating a nose-down moment that tend to bring back the aircraft to a more level flight attitude.

The 'superstall' of the Draken might be attributed to the combination of a large highly-swept wing located in front of the CG, creating a large nose-up moment, and a large radius airfoil destabilizing the vortex.

Delta wing and induced washout

On a conventional wing without taper or sweep, the tip portion experiences an increase of upwash, meaning the tip flies at a higher AOA and the tip vortex delays the stall in that region. At AR 2 and below, the entire wing is affected by the tip vortices.

Sweep make every wing section fly at a higher AOA than the inboard section because of the distorded airflow moving spanwise.

Taper increases the magnitude of both effects.

This is why an airplane like the FMX-4 has reflex and a lot of twist. This is not a stability issue but it allows to trim the airplane for a given airspeed.

Delta wing airfoils and double-wedge/faceted airfoils

It has been mentionned in an article by Bill Husa that 'precise' airfoils are less important for a delta-wing planform. This could be explained by the mostly vortex-nature of lift for this planform. The same article shares the story of the A-4 Skyhawk airfoil design: a rounded-nose flat plate was evaluated as been 'almost as good' (I'm paraphrasing here) while been much simpler in design than a conventional airfoil.

Low AR with long root chord means the airfoil is operating at higher Reynolds numbers. It has been suggested that this high Re number allows for a less than perfect airfoil shape but, again, almost as good and simpler to build.

A list of questions I can't currently answer

In order to have some sort of 'scope statement' let's assume I'm trying to apply the above theory to create a french UL regulation compliant single-seater. (330 kg MTOW (733 lb), 70kph (38 kt) stall speed or less, single engine with a max power output of 80hp or less)

How important is the 'polygonal-gothic' planform of the FMX-4, Gtex-09 and other projects?

Is the goal, as mentionned above, to start the vortex aft of the CG to create a nose-down moment? Would a simpler delta planform with a double wedge airfoil at least not create a nose-up moment? What makes the gothic planform more suitable than a delta one?

At what point does the airfoil shape start to really (not) matter?

My understanding is that as long as LEV is acting, airfoil shape loses some of its importance (see reference to Bill Husa article above).

Now, below that 7-8° threshold where LEV start to act, when does a faceted airfoil stops acting like an airfoil outside of vortex lift? I modelled several faceted models in CAD which created some very angular airfoils while others are more smooth, biconvex-like, depending on the number of facets and their distribution. I'm wondering when an airfoil becomes 'good enough' outside of vortex lift? (see question below)

Norman mentionned the fact that a wing below AR=2 will be entirely engulfed in tip vortices. At this point why even taking 2D models and values into account?

How do you figure out airfoil characteristics?

This question is directly related to the one above. Let's say I have modeled a faceted airplane, the airfoil sections are good enough but how do I actually know that? How do you extract Cl, Cd and Cm from such a airfoil/entire airplane to do a trim analysis for instance? There are several reports giving details on Cl, Cd and Cm for symetrical double-wedge and biconvex airfoils, but what about a more 'complicated' one?

Constant-chord or constant-percentage elevons?

The FMX-4 use what seems to be flat-plate contant chord control surfaces while the Verhees Delta Mk1 has tapered elevons. Which is more suitable?
 

rotax618

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As I have said before designing even the simplest of aircraft is a non trivial task. With that said, it is possible for a non professional to design a successful aeroplane, many have done it including myself.
Your summary is spot on, obviously you have an idea of what your dream will look like, if you are still open minded, consider the flyability of your dream, things like vision can be a deal breaker, the FMX4 had clear panels in the floor.
I have built a number of LAR faceted models and simple flying wing glider models to study planform effect, the faceted models taught me that LAR behaves in a very different flight regime than conventional wings, even a flat plate will fly well if the planform is correct and the aspect ratio is low.
even faceted airfoils can provide a good L/D, if you study the engine out glide of one of my models You will see that the L/D is not directly related to span.
 

rotax618

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As to the question of the elevons, all of the LAR aircraft have very wide chord control surfaces (FMX4, Verhees Delta, Dyke Delta, Rowe’s UFO, Arups) the width of the elevators/elevons is proportional to the root chord, my experiments suggest that the control surfaces behind such a large chord are in a very turbulent flow and appear to have a “dead” point where they are ineffective for small movements - the S3 Arups put the elevators high up on the fin above the wing, and I believe a pitch trim was added to the fin of the Dyke delta, Zimmerman’s pancake had tailplane/elevators protruding from the sides. You can see the large size of the surfaces on the UFO.
B3A8DE16-272E-4966-98ED-BE352ECDAFA6.jpeg
 

berridos

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Nice refresher.
You only commented on the pros. The biggest drawback i see, is that in approach attitude you wont be able to get out of vortex mode unless you have huge power or time to drop the nose significantly or both.


My biggest questionmark is how the vortices will behave in a double delta configuration in wich the outer section has more sweep than the root section assuming the root section has 50-55º sweep. Will the root vortex mix with the outer vortex and strengthen the lift together? Norman stated that possibly the root vortex wouldnt form under that planform. Watching the cfd simulation on the saab draken i noticed the root vortex travells spanwise along the leading edge and there could be a chance of reinforcing the outer vortex.
Guess only openfoam has the answers, but that black ms-dos style gui is very intimidating)))
 

cluttonfred

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Personally, I would love to see more experimentation with the Piana-Canova rhomboidal planform just because its straight lines make it so suitable for quick and easy construction. It would be great to see a basic VW-powered PC-style single-seater to compare with other single-seat, VW-powered designs out there to get a concrete comparison of a straightforward LAR designs with other more conventional designs of similar weight using similar engines.
 

Arfang

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As I have said before designing even the simplest of aircraft is a non trivial task. With that said, it is possible for a non professional to design a successful aeroplane, many have done it including myself.
Your summary is spot on, obviously you have an idea of what your dream will look like, if you are still open minded, consider the flyability of your dream, things like vision can be a deal breaker, the FMX4 had clear panels in the floor.
I have built a number of LAR faceted models and simple flying wing glider models to study planform effect, the faceted models taught me that LAR behaves in a very different flight regime than conventional wings, even a flat plate will fly well if the planform is correct and the aspect ratio is low.
even faceted airfoils can provide a good L/D, if you study the engine out glide of one of my models You will see that the L/D is not directly related to span.
Thank you for your comments. I watched your videos several times over the years. Always liked your low aspect designs. Since you experimented with scale models and given your building experience, I'd like to pick your brain a bit if you don't mind:

Are you still working on LAR designs and do you plan on building one in the future? When experimenting with models, did you noticed a difference between different scaling factors, in other words: where does scale modelling starts providing information you can work with?

Nice refresher.
You only commented on the pros. The biggest drawback i see, is that in approach attitude you wont be able to get out of vortex mode unless you have huge power or time to drop the nose significantly or both.
Thank you. Vortex lift starts at around 7-8° AOA, you would need 'huge power' to counter the 'huge' drag at high alpha. Sure a low aspect design could fly at a 30° AOA or more but does it needs to? The Useless Flying Object, for instance, lands at a pretty 'normal' AOA from what we see on the videos.

Personally, I would love to see more experimentation with the Piana-Canova rhomboidal planform just because its straight lines make it so suitable for quick and easy construction. It would be great to see a basic VW-powered PC-style single-seater to compare with other single-seat, VW-powered designs out there to get a concrete comparison of a straightforward LAR designs with other more conventional designs of similar weight using similar engines.
I too would like to see a Piana-Canova inspired design being built and flown. My point is that we should be able to have a good enough idea about how the airplane is going to perform before it flies. Obviously it should be easier to do so with a Piana-Canova/Payen/Arup type of airplane with conventional airfoils and not relying on vortex lift. You're probably right, maybe the simplest way to LAR is by using conventional airfoils, straight lines and not rely on LEV.

Still, I like the 'faceted' LAR formula and I'd like to know how to properly design one.
 

Norman

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Norman mentionned the fact that a wing below AR=2 will be entirely engulfed in tip vortices. At this point why even taking 2D models and values into account?
Because the best lift to drag ratio (and thus cruise) of most airplanes is at a CL of about 0.2 which will be at an AoA well below where the vortices are big enough to have much effect so an efficient cross section would be nice for cruise efficiency.
 

cluttonfred

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OK, well here's where the rubber hits the road and where I don't have the math skills or engineering knowledge to answer the question. What sort of CL max is a reasonable estimate for something like a Piana-Canova design at a AR of just under 2.0? With some sort of basic rule of thumb I can estimate minimum speed and size the wing accordingly. Since Part 103/SSDR and microlight/LSA categories all have minimum speed and maximum weight requirements, that wing area required for stall/landing speed is the key factor that drives most of my conceptual designs.
 

Arfang

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Because the best lift to drag ratio (and thus cruise) of most airplanes is at a CL of about 0.2 which will be at an AoA well below where the vortices are big enough to have much effect so an efficient cross section would be nice for cruise efficiency.
I probably misread your post. What I understood from your earlier post is that wingtip vortices, on a straight wing without taper for instance, will affect the entire wing at or below AR=2. If I understand correctly wingtip vortices are always there as long as the wing generates lift.
 

pictsidhe

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OK, well here's where the rubber hits the road and where I don't have the math skills or engineering knowledge to answer the question. What sort of CL max is a reasonable estimate for something like a Piana-Canova design at a AR of just under 2.0? With some sort of basic rule of thumb I can estimate minimum speed and size the wing accordingly. Since Part 103/SSDR and microlight/LSA categories all have minimum speed and maximum weight requirements, that wing area required for stall/landing speed is the key factor that drives most of my conceptual designs.
I think the FMX4, with vortex lift, managed a CL around 1.0. So start there. I extracted numbers from Barnaby's PAV report. That will need sharp leading edges. I am highly suspicious of claims of high CL for low AR. Barnaby is a very talented guy. He designs 'real' aircraft at his day job... If he could only get around 1.0, that only confirms my suspicion of experimental error in the old low AR designs. Measuring speed accurately at high AOA is not a trivial matter. A pitot-static will read very low.
From various comments, he did a lot of wind tunnel and model testing to get the final shape of the FMX4.
At low aspect ratios, span efficiency tends towards 1.0, so use that to estimate induced drag. Again, I am somewhat skeptical of claims far above that. A little is certainly possible with the right tip shape.
Threads like these would really, really benefit from him chipping in!
 
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