Ultimate slow-flier: Micro and Macro

[Kinotoko]

Alright, so I want a slow flying plane. Since I haven't seen one that flies slow enough to fit my fancy I'll have to design it. Problem is, I've never actually designed a plane before (and built it), and have only an academic understanding of aerodynamic principles gained through vicarious absorption of aviation and modelling history etc. Key here is that a VTOL is not a slow flier; it is a different class of aircraft. A slow flier must have mostly horizontal thrust (preferably something close to 'normal' but definitely having no more than 45 degrees of vertical). Note that autogyros and hybrids (LTA gas-assisted) are fine here.

The things I know of that have been used for slow flying are:

~Flaps, for slowing down an aircraft to shorten takeoff/landing distance. Effect seems to be reduction in fuel economy/increase in drag (else all wings would have static flaps built in, right?).

~Slots near trailing or leading edge of wing; specific purpose/effect unknown (to me).

~Undercamber increases lift at slow speeds.

~Turbulators: A ridge running lengthwise on the LE of the wing to create turbulence acrose the wing's top(?? not sure about this).

~Certain rotary devices such as the Fanwing can increase lift over normal wings.

~The Kline-Fogleman airfoil might have generated extra lift at lower speeds (I can't remember for sure).

~Obviously, low weight is a critical factor. But even living room fliers are uncomfortably fast...

~Using a grotesquely thick airfoil (or just a lifting body) filled with helium or hydrogen can vastly improve lifting power.

Most notably, these two:
~Extremely low mean camber, as in single-surface micro wings, increases lift.
~Extremely low aspect ratio increases wing efficiency, which reduces power requirement and presumably speed.

...seem to run contrary to this one:
~In certain slow flying models such as the Lazy Bee, extremely high aspect ratio combined with extremely thick flat-bottom(?) airfoil seems to increase lift while decreasing speed.

Of course, after reading what I could find using the search function on this (modelling science) forum, I realize that a tiny-scale slow flier might indeed be far different from a large-scale flier. So I ask you: First, how accurate is my understanding of what affects aircraft speed? And of course, what methods are best for a large scale slow flyer, and how do they differ from the methods that are best for a micro slow flier?

If left to my own devices, I can only conclude that I should build an undercambered lifting body filled with hydrogen with an auto-rotating fanwing attachment and flaps made entirely from carbon fiber and covered in microfilm... (you can see why I make sure not to resort to my own devices)

[NX-687]

Give us a steer on whether indoor or out and what sort of gear , motor type you are thinking , just straight and level or mild aero's, just depron will do it , its not expensive and easy , to make changes :)

[Brandano]

There's no contradiction. High AR wings have lower drag, but tend to stall at a lower AOA. Low AR wings have higher drag but stall at extremely high AOA. This coupled with a lot of power allows you to fly much slower. Gliders use high AR wings because they need to make use of what little power is available to them in the most efficient way possible, not to lower stall speed.

[Kinotoko]

Well, NX, a micro size would be for indoor and the macro would be for outdoor and/or indoor if it was slow enough. Tiny size for perfect calm, and then a normal sized model designed to cope with non-ideal conditions. As for type of engine, I have no idea, other than that I would use electric. I honestly don't know what's out there. Wouldn't it be better to figure out what type of power is needed for a design based on its aerodynamics before choosing a motor?

Brandano, that makes a lot of sense. I take it that this means that low AR is better than high AR for slow flight, and that the Lazy Bee's slow flight ability is based more on the AR than the flat-bottom airfoil itself. But what about airfoil thickness? If I understand correctly, this implies that a single-surface wing with a very low AR and an extreme degree of curve (making an effect similar to a thick airfoil) would be best for slow flight, no matter what the size.

Another idea I had from watching a video of an aerobatic autogyro was that taking two autogyro rotors and mounting them at a 35-45 degree angle (or a variable angle for faster flight) might provide an extreme degree of lift...

Or perhaps mounting the engine at a 35-45 degree angle with a Kline-Fogleman type wing behind and below it , as if you had tilted the whole fuselage 35-45 degrees. A cockpit for a pseudo-scale look would be mounted below the prop.

The real idea I had behind all this was that there has to be a minimum maintainable speed for forward-thrust flight. I imagine it would be similar for large models and slow models (proportional to the model's size of course). The question is what it is and how to attain it.

[slipstick]

You obviously have a target speed in mind otherwise you wouldn't be saying things like "even living room flyers are uncomfortably fast". Why not share your target speed with us ?

BTW it's not specifically light WEIGHT that's important for a very slow conventional plane it's more very low WING LOADING. Hence the man-powered aircraft which, with pilot, weighed a lot more than most models but flew a lot slower.

I think you've also got your idea of aspect ratio the wrong way round. The Lazy Bee has a very LOW aspect ratio (span/chord is a low number), sailplanes have a very HIGH aspect ratio (span/chord is a high number).

Also wings with single curved surfaces do not have low camber, they usually have high camber associated with low thickness.

Steve

[MarkusN]

Look at the designs of classic indoor rubber powered microfilm coated planes. Those are about as slow as you can go.

[Kinotoko]

Well, slip, my target speed I suppose would be walking speed or less, at least for the micro and preferably for the macro too. Did I call the Lazy Bee high AR? I always get them mixed up... As long as you know what I mean >_>

Anyway, I already know I can make a very slow flier by doing what I said: low AR single surface wings with a high degree of curve and high AoA. I guess what I want to know is whether there is another technique that would go slower still. One thing that I'm thinking of now (partly due to your mentioning wing loading and the human powered craft) is a plane with 2-6 extremely high AR wings (higher than sailplanes) and a very low geared prop (or high geared? anyway I mean slow). Since the wings are more efficient they require less power to fly, and therefore less speed, like the human powered planes.

Also, another thing I forgot to mention that is supposed to increase efficiency is the internal wing configuration (http://www.rexresearch.com/carrcoan/carrcoan.htm).

I actually have a picture somewhere in one of my magazines of a super-efficient design that has given me some ideas; I'll post it if I can find it...

[MarkusN]

Sounds like snake oil to me.

Wing grids (multiple-decker wings, venetian blind wings) are notoriously inefficient (because the individual wings are influenced by the flow field of the others) and have high drag.

At model size wing design is always a compromise between AR and chord to keep Re in a sensible range.

For very slow fliers low AR can have an advantage, as the tip- / LE-vortices energize flow over the top surface and prevent separation, or keep up lift despite separation. These concepts demand relatively high power to overcome inherent drag, though.

(Energizing top surface boundary layer is the effect of slats and slots at the nose, BTW; it lets the BL keep attached at higher AsOA)


What also can help are instationary flow approaches such as ornithopters. We have all heard the story of the bumblebee, which according to aerodynamicists can't fly because of too high wing loading and too low Re.

I read an article some years back, which suggested that for very low Re a design with a short horizontal ledge in front of a conventional thin section cambered wing promised advantages in lift. Apparently some insect wings are designed this way.

[Brandano]

Well, the high cambered low AR wing coupled with a powerful motor means that at high AOA's a good portion of the weight of the plane is actually supported by the prop, unloading the wing and reducing the stall speed even further. Also, a bigger proportion of the wing is in the propwash, and gets its own localized high energy airflow, while the wingtips benefit from the high energy flow due to the wingtip vortices. Essentially, think about the Convair XFY "Pogo" http://en.wikipedia.org/wiki/Convair_XFY or the Vought XF5U "Flying Pancake" http://en.wikipedia.org/wiki/Flying_Pancake as examples of overpowered low AR wings. But even with marginal power a light design with a low AR wing can achieve some amazing slow flight: check these 1920-40 vintages: http://www.youtube.com/watch?v=Nxz1UF67EQI
http://www.youtube.com/watch?v=i8y5suPs1uE

[vintage1]

I recall from my younger says that indoor microfilm models flew at a slow walking pace. It was possible to snap the wings if one walked too fast carrying them, and they would disintegate outside in any breeze at all.

[slipstick]

Even I have a couple of simple indoor rubber models that fly at no more than walking pace. That's not really much of a challenge....I thought you meant really slow ;). The serious indoor competition models will often fly at less than 1 mph. Obviously they only fly in completely still air (the World Championships are typically held underground in huge disused mine caverns with no air movement and very even temperatures).

Of course if you want controlled rather than FF you'll need something a bit bigger purely to handle the additional weight of the control gear. Then you'll discover that most conventional controls like rudders and elevators are not really effective at very slow speeds.

Steve

[JetPlaneFlyer]

As others have said... Look at indoor microfilm covered duration models if you want to fly really slow, but you have to accept that such extremes of design come with a price i.e. they are so fragile that they will fall apart if there is even a draught in the room... outdoors forget it!
Don't get concerned with aspect ratio, optimum camber, airfoil thickness etc, etc.. At such low Re numbers 90% is about having low wing loading, so wing area as large as possible and weight as low as possible... simple as that.

If you want to go the route of deflecting thrust downward to produce lift rather than relying on lift from the forward movement of a wing then the solution is simple... get a helicopter :rolleyes:

[JetPlaneFlyer]

There's no contradiction. High AR wings have lower drag, but tend to stall at a lower AOA. Low AR wings have higher drag but stall at extremely high AOA. This coupled with a lot of power allows you to fly much slower.

Brandano i'm not sure this is entirely true. Low aspect artio wings do stall at a higher AoA but the lift slope for a low AR wing as flatter than that for a high AR. Bottom line is that despite stalling at a higher AoA a low aspect wing produces less lift (lower Cl value) at stall than a high AR wing (Re number variations aside)

You can see an illustration of this here: http://www.aerospaceweb.org/question/aerodynamics/q0167.shtml where the low AR wing stalls at 30 Deg and the High AR wing stalls at 17 Deg, but despite this the high AR wing still produces a better Cl value (i.e. more lift) by quite a wide margin.

For a practical example ask yourself why swing wing aircraft land and take off with thir wings 'straight' (i.e. high AR mode)? If they could fly as slow with the wings swept ( low AR) as they could with them straight then there would be no point in the swing wings in the first place. ;)

Your later point about thrust taking some of the load off the wings at very high AoA is of course true... but go far enough down the route of exchanging 'prop thrust' for 'wing lift' and you end up at a helicopter, which I feel is missing the point. If you want a model that supports itself directly from the thrust of it's propeller then get a helicopter and be done with it.

Steve

[Brandano]

Steve, there's other considerations when landing a plane other than the actual lowest possible speed, like how long your landing gear has to be and whether the pilot can actually see the runway. Take for example the facetmobile: it's capable of a lower speed than its landing speed, but doesn't have a long enough landing gear to take advantage of it during takeoff and landing. Also, variable sweep planes in the fully swept configurations usually have their COG too far forward to allow really slow flight. A pure delta wing is a better example. Here again, most of these designs are limited in their landing speed by the fact that they can't keep a high enough AOA due to the landing gear being too short. The Rafale can land on a carrier, but it's really a task that has been forced onto it, and I wonder if it can do it in other than perfect weather.

[Kinotoko]

Markus, the multi wing design I had in mind (6 wings I'm thinking) is not like the old Venetian blind planes that we've all seen falling apart in old silent films (I may think unconventionally but I'm not that dumb!). The super efficient design I found a picture of was similar in arrangement to the IWA design, i.e. one wing in front swept back slightly toward two wings in the back, a sort of staggered triplane configuration, with all three wings being extremely high AR. The idea I had from this was something similar, but instead of one set of three wings composing the entire plane, you would have one set in front and one in back taking the place of what would normally be a tail. One might have to arrange for the propulsion to be located either in the back or in the middle of the fuse, and the fuse itself might have to be made flexible to allow efficient control, but anyway that was my idea.

Also, I'm not clear on 'Re". This is Reynolds numbers, correct? In which case, I'm also not clear on what Reynolds numbers are or specifically how they affect model aircraft. I see from readin up on them that they have to do with the viscosity of air and the drag of an airfoil itself, which seems clear enough, but I'm not sure how this affects the aerodynamics of the plane.

Brandano, thanks for those links; they're awesome.

Vintage, slipstick, JPF; I've actually come to a slightly different conclusion than you. There seem to be two basic ways to achieve slow flight.

One, you try to generate extra lift with the wing. The simplest way to do this is by giving yourself a low AR wing so you can angle the motor up; this is what the lifting bodies and flying pancakes do, as well as models such as the Lazy Bee and (I think) IFO, when they are flying slow. This allows you to tip forward and resume a more normal flying speed. This is not used in any real aircraft that have gone into production, because it is very inefficient and real airplanes are concerned with useful payloads and speed.

The other way to increase lift is with mechanisms such as flaps, which allow a higher AoA, slots in the wings (don't know the proper name for those), and rotary wings, which use effects such as 'energizing the boundary layer' or simply shunting a higher volume of air downward to increase downwash and therefore lift. These methods are used as STOL devices for real aircraft and are seen periodically in experimental craft.

The second way to achieve slow flight is simply to modify the aerodynamics so that you require less speed to get the required lift.

The most common method of doing this is going for the lowest wing loading possible. This is achieved by maximizing wing surface and minimizing weight. It is employed mostly by indoor modellers, who can afford to eliminate virtually all weight from their models; RC modellers need hardware, outdoor modellers need rugged structure, real aircraft engineers work with useful payloads, etc.

The other method also involves reducing wing loading as much as possible, but also involves making the wing itself more efficient. Instead of making the wing broader and stripping off material to reduce weight, you make make the wing longer and add additional wings. Both methods reduce wing loading, but the second method doesn't require stripping weight beyond simple optimization, and also increases wing efficiency. If one can get the IWA vortex effect with super-high AR wings then the ideal number of wings becomes some multiple of three.

This second method will still result in a fragile model, but also a much more aerodynamic one, albeit possibly with reduced maneuverability.

So... basically... I'm thinking of going with the last method there, which AFAIK hasn't really been explored for making slow-flight models.

JPF, as for saying one should just get a helicopter and get it over with rather than have a plane flying at a ridiculous angle: I disagree that these are as similar as you seem to imply. The reason I don't want a helicopter is because the control dynamics are entirely different. While one gets more or less the same range of maneuvers with a helicopter as with a 3D airplane - meaning a helicopter can do anything a plane can do - flying a helicopter is a constant balancing act, whereas a typical plane (though, perhaps, not a 3D plane) can at least be allowed to fly straight with the confidence that it won't tip over and fall sideways out of the sky. Indeed, a plane with sufficient dihedral can be allowed to fly straight and level without any interference from the pilot save occasional course and pitch correction.

Even if a plane is getting a large portion of lift from its propeller by flying at some ridiculous angle, it will (if properly designed) still handle like an airplane, and that is the difference between a slow flying plane and a helicopter (or other hovering craft). Craft like the flying pancake, lazy bee, etc. have a certain clumsy charm that can't be matched by a helicopter. I don't, however, think that their minimum speed is the slowest that can be achieved, which is why for now at least I'll be focusing on a different design.

Also, I'm pretty sure that the designs for indoor and outdoor versions of what I have in mind would be mostly similar, except for ruggedness... I've also discounted the use of LTA gases for this design, as LTA hybrids require thick airfoils or lifting bodies, as well as all of the high-lift tricks and gadgets like flaps, because they operate in a more or less contradictory manner (type 1 slow flight as opposed to type 2).

What do you guys think? Am I crazy? Completely confused? Simply wrong?

[JetPlaneFlyer]

A quote from a professional:

a high aspect ratio wing produces much more lift than a low aspect ratio wing. At its stall angle, the Cessna 172 generates a lift coefficient of about 1.7 whereas the Lightning only generates a coefficient of about 1.3 at its stall angle. The lift equation tells us that the higher the lift coefficient is, the slower the plane needs to fly. It is much safer to fly as slow as possible during landing, so a high aspect ratio wing is far more desirable during this stage of flight.

read it here: http://www.aerospaceweb.org/question/aerodynamics/q0281c.shtml

Take a look at virtually any recognised STOL aircraft and it will have relativly high AR wings... The best example I can think of is the Feisler Storch. A few years ago i was lucky enough to see a genuine WWII example flying down the runway at low altitude at little more than a walking pace (admittedly into a moderate headwind)... the landing roll was about 10ft! the pilot said to me that in a little more wind he could have performed a hover!

This page on Wikipedia lists a bunch of STOL aircraft http://en.wikipedia.org/wiki/STOL ... none of them are what I'd class as low AR, most are definitely in the high AR bracket... has every designer of these aircraft over the last 70 years got it wrong?

[Brandano]

going for multiple wings has its drawbacks. Each wing comes with its wingtip, its wingtip vortex and the induced drag that goes with it. this has to be counteracted by the motor, and so you need a bigger motor, which means more weight, which means more wing.... and do on and so forth. If you are going for a high AR wing, you will probably get the best results with a single wing and the most streamlined design possible. Go for high efficiency and the lowest weight for the onboard equipment that you can achieve.

[EDIT] Steve, they didn't get it wrong, at the same wing loading a high AR wing is more efficient and allows for a lower stall speed. And allows for a low enough drag at high speed to make the aircraft viable to move from A to B. But you can't compare the cessna and the lightning, without comparing also their wing loading, and their mission profile. A low AR wing can be built to have a lower wing loading than a high AR wing, if you are ready to have a plane that is mostly empty. the real problem with a high AOA approach is that you need long complex gear legs, and if you let the plane's nose rise too far up the drag increases so much that you might not have enough engine or time to avoid a stall. On the other hand, if the power is not an issue, low AR wings have much better behaviour at low speeds than highly efficient high AR wings. Having less overall lift might be balanced by having a wing that doesn't depart automatically in a spin and is lighter to build

[JetPlaneFlyer]

Even if a plane is getting a large portion of lift from its propeller by flying at some ridiculous angle, it will (if properly designed) still handle like an airplane, and that is the difference between a slow flying plane and a helicopter (or other hovering craft).

Sorry but I disagree.. any aircraft that is generating a large amount of it's lift from it's prop (aka rotary wing) is going to handle much like a helicopter; specifically if you cut the power it will fall out of the sky rather than glide like a conventional aircraft that generates it's lift from it's wings.
It's perfectly possible to design a inherently stable helicopter that will fly 'hands off' I know of a guy that builds successfull rubber power free flight helicopters so if all that is putting you off 'choppers' is stability think again.

Regarding Re numbers... you are correct in that re is a ratio of 'inertial force' over 'viscous force'... What it means in practice is that small wings operating at very low speed do not function in the same way as larger/faster wings. The airflow over the upper wing surface of a very slow moving wing wont stay 'attached' to the surface therefore heavily cambered and thick airfoils just dont work well at small scales and slow speeds. An ideal low Re airfoil is something resembling a slightly cambered flat plate.. flaps and slats just wont work in the same way as they would on larger and faster aircraft, they would be unlikely to 'pay back' the extra weight incurred.
As I said earlier... if you want VERY slow flight then concentrate on building light and providing plenty of wing, there is no substitute.
If however you want to exploit lift direct from the prop then build a helicopter or a model with tilting rotors like the V-22 Osprey.... or perhaps a lightweight 3D type model that is capable of 'hanging on its prop' (like a helicopter)... But funadamentally i'd not consider any of these aircraft in the same bracket as a true 'slow flyer'... maybe thats just me?

Steve

[BMatthews]

Some of your ideas are OK but some are faulty.

First off, low wingloading is paramount for low flying speed. Without an extremely low weight and thus wing loading your slow flyer is doomed from the outset. So extreme strutural design will be required. This doesn't mean it needs to be weak or lack character. I'm thinking that for the larger version you could use a lot of the techniques used by free flight rubber scale modelers to achieve a nice looking model with suitable strength while still being very light.

Flaps are just a camber altering device. For a model intended to fly at the slow speed end of the spectrum all the time you should just "build in the flaps" in the first place by using a very high camber airfoil.

For low airspeeds testing over decades of competition free flight models has shown that "thin is in". Indoor models gain from this in two ways by their use of single surface panels in terms of both aerodynamic efficiency as well as minimal structural weight. For an outdoor model this would work too but you may want to look at struts and/or functional bracing lines to be able to use minimal materials for spars that are then locked together structurally much like the old WW1 biplanes that locked their upper wing and lower wing spars together via the interplane struts and rigging wires.

Ailerons at very low speeds on high camber airfoils generate huge amounts of adverse yaw and as such don't work all that well. However rudder and elevator controls work just fine and that's the way to go.

You can gain some additional lifting area by aiming at a tandem wing planform along with the use of biplane wing arrangements at both ends. The use of a 100% size stabiliizer means that the rear wing set is lifting as well as the front set but not quite as hard as the front set. This reduction in lift at the rear is needed for the sake of pitch stability. However you're still gaining overall. The other planform that would work would be the same tandem setup but with monowings as used by the Langely Aerodrome. Here's a link to the general arrangement for you....
http://home.att.net/~dannysoar2/Langley.htm
Now nothing says the props and motors need to be located between the wings but since the CG for a tandem wing of this sort would be located between the wings (actually slightly in front of the mid point between the 25% chord points of the two wings) it may be prudent to mount the motors and props where shown in the Langley design. If using a single motor then perhaps some arrangement could be made for belt drive or use a single prop but then go for twin booms. If mounting two motors on the wing some extra bracing would be needed at those locations.

Note that for the lightest possible weight on such a monoplane tandem configuration you'd want to pretty much duplicate the rigging of the wings as shown. I'm guessing that you'll want landing gear and the fixed landing gear would double as the lower rigging attachment point.

If you go with a biplane tandem configuration twin motors could be mounted at the first set of interplane struts and rigging points which would be a structural hard point.

Normally I'd say that all those oddball wing arrangements are just gimmicks and the vast majority of them are. Certainly your description of the staggered tri wing setup is not going to serve the best purpose of allowing for the lightest self supporting structure or provide the best wing area with minimal aerodynamic interplane interference.

Depending on the sizes involved your airfoil could be something as complex as those found on free flight FAI F1A gliders with their carbon box leading edges or as simple as medium density balsa of something like 1/8 x 1/2 at the leading and trailing edges with no spars. The leading and trailing edges forming the spars and the interplane struts and bracing lines adding the needed support to keep them in line and withstanding the loads.

At the sort of speeds you're after this would definetly NOT be an outdoor model at all except for those rare times when there's no detectable breezes at all.

The next challenge would be to see if you can build a 3 foot model of this tandem biplane configuration complete with radio and batteries and have it come out weighing no more than about 2 or 3 oz. which is about what you need to fly as slowly as you're after.

Angling the model into a very high angle of attack similar to 3D models that do the harrier maneuver isn't really flying slow. Yes the model is traversing slowly but it's prop hanging at least as much as it's flying and if that's all you want then just learn to fly a 3D flat foamie. True slow speef flying implies that the model is flying in a stable and trimmed manner that requires no intervention from the pilot except for turns and corrections for any gusts. A model in a semi prop hanging harrier style of flight will not be able to do that.

If you're after a micro sized model that will fly in this manner then you're going to have to figure out how to achieve building a 25 inch indoor style model that weighs no more than about 5 grams ready to fly. Any more than that and it'll be flying faster than you can walk. The FAI indoor models that fly at a slow walking pace that everyone is describing are mandated by the rules to weigh a minimum of 1 gram. They used to be able to make the models much lighter than that but they were too fragile for shipping so the FAI introduced a 1 gram rule some 25 or 30 years back. I think there's been a wing area rule since then since the 1 gram models were getting so extreme that they were as fragile or moreso than the previous models.

If you're truly after slow flight where a normal walking pace is the goal then you've got your work cut out for you.

One option for control that would result in less weight would be to go for two motors and use mixing to mix the throttle signals for each with the elevator and aileron controls. Then you don't use any rudder or elevator at all. Just trim the model so it flies pretty much as a free flight model. Adjust the trim so the model flies level at around 1/2 throttle and has enough pitch stability that it naturally noses up and climbs when you add more throttle. Reducing throttle would allow it to glide downwards. For turning the aileron stick would be mixed into the two throttle signals so power is cut back on one side and increased on the other and the differential thrust would turn the model.

I've got a biplane flying wing that flies with this setup and it was very surprising just how capable it can be.

At least this is how I'd do what you're after.

[Kinotoko]

JPF, I never said high AR wings were bad for slow flight; in fact, I came to the conclusion they would be best for slow flight. I was simply disagreeing with you that traditional indoor models are the way to go. I intend to lower wing loading by increasing the AR (making them longer, increasing surface area), using light construction and electronics and adding multiple wing surfaces.

As for slow flight by high AoA... I'm not going to use that method, but I still maintain that they behave more like planes than copters, from the standpoint of how one controls them, not how they act when you cut power (although I would think they would level out if they had enough room to build up speed in a dive). If you have to maintain forward motion to stay in the air and steer with control surfaces rather than by changing the prop pitch, then it's a plane, not a heli, that's my point.

I actually don't have much interest in 3D planes either, incidentally; I should point out that the reason I prefer planes is not necessarily that they are more of a challenge to fly, but that... well, it seems like there's more to it. It's like a horse; you're never completely in control of an airplane. With a helicopter, you go up, down, left, right, stop... but with an airplane it's like being strapped to the back of a rocket that you have to steer. You can't just say, 'Ok, stop the ride, I want to get off'.

However, I have never flown or ridden in a helicopter, model or full scale, so maybe I'm wrong, I don't know. But it seems to me that as long as the craft has to keep moving forward or risk falling, and has to be steered using control surfaces, one maintains that feeling.

Anyway...

Bruce, thanks a lot for the input, you have a lot of good ideas. But I don't think I've given an appropriate description of the design I have in mind right now... This is a diagram of the 'internal wing' configuration, which is the three wings I was talking about:

http://www.iwatoyco.com/technics.asp

And I've attached a pic of the model that uses it, the XStream from Sharkjaw Toys. In post #4 I posted a link to more info on the internal wing, but here it is again:

http://www.rexresearch.com/carrcoan/carrcoan.htm

My idea was to adapt the internal wing arrangement to a high AR version, and use one such arrangement at the front of the plane and one at the back. Note that having three wings attached to each other also increases the strength of the wings. The reason I was thinking of putting the prop in the center of the fuse was mostly to avoid interrupting the vortex effect of the IW. For steering I was thinking of making the entire fuse flexible and flexing it to steer, using all the wing surfaces for control. If one has ever seen the movie "Nausicaa of the Valley of the Wind", there are insects in it which move like this and somewhat influenced this idea...

[JetPlaneFlyer]

low AR wings have much better behaviour at low speeds than highly efficient high AR wings. Having less overall lift might be balanced by having a wing that doesn't depart automatically in a spin

I really dont think this is always the case either. Very low AR wings potentially have a dangerous tendancy that at high AoA the drag produced is greater than engine thrust available. The scenario is the aircraft gets into a high AoA final landing approach... the pilot finds he is dropping short of the runway.... he increases throttle to max but the aircraft drag is too great and the plane continues to sink. He cant push the nose down as he is too low. A crash is inevitable, there is no mode of recovery. This very scenario caused the death of many pilots in the days of the F4D Skyray when the aircraft fell short on carrier landing. Modern jet fighters get over this problem only by having engines of enormous thrust.
There is absolutely no reason that higher AR wings must have unforgiving stall charecteristics, most do not. Perhaps if designing a high (or low) AR wing for ultimate performance then safe stall charecteristics may take a back seat, but this is the designers choice. It is however quite possible to design a high AR wing with safe stall charecteristics that still far outperforms a low AR wing in virtually all respects appart perhaps from flight at high mach numbers.

[JetPlaneFlyer]

JPF, I never said high AR wings were bad for slow flight; in fact, I came to the conclusion they would be best for slow flight. I was simply disagreeing with you that traditional indoor models are the way to go. I intend to lower wing loading by increasing the AR (making them longer, increasing surface area), using light construction and electronics and adding multiple wing surfaces.



Good luck with your project however I'm not sure you are on the right lines... Despite my promotion of high AR wings there is a problem at small scale and very low speed. High AR wings have a narrow chord and this reduces Re number, therefore at very slow speed it becomes a balancing act between AR and Re. Also the point Brandano raised about low AR designs being potentially lighter is very true, at small scale/slow speed there is no point increasing AR if the weight increases as a result. Again I would look at indoor models for an idea of the AR compromise that works best.
Multiple wings are potentially handicapped by the same issue... due to Re number (amoung other issues) one large wing will be more effective than multiple smaller ones. The only real advantage of the multi wing approach is good structural strength for minimum weight, especially if bracing wires are used, like everything it's a compromise.

As for the pseudo airflow diagram on the Shark Jaw Toy web site... forget it.. it's pure marketing BS, no scientific basis whatsoever that I can see. Generally in fact a canard arrangement such as the one photographed reduces the lift potential of the main wings because a) the main wing or wings operate in the downwash of the fore wing and b) for stability the main wing must be trimmed at a lower incidence than the fore wing and therefore the main wing can never achieve it's maximum Cl value (maximum lift) because the fore wing will stall before it gets there.

I'm just repeating myself hear but I'll say it again because it's maybe not sinking in... for slow speed you need to have maximum wing area and minimum weight, no aerodynamic 'tricks' will be a substitute. 'Simplify and add lightness' as some famous aircraft designer once said.

Steve

[BMatthews]

That's a cute looking little gizmo but it's not some sort of "secret" to achieving super slow flying speeds. There's no secrets to be had for this. Just lots of wing area and light weight for as low a wing loading as you can achieve. There's a very good reason that the kings of the ultra light duration modelers (FAI indoor microfilm duration) use a more or less standard planform with long'ish tail moments and oversized stabilizers. It's to allow the stabilizer to not only perform the pitch stabilizing duty but to also contribute to lift.

Your oddball "contained" wing configuration won't be able to copy this format since it ends up being sort of like a flying wing but with added drag from the gaps.

[Kinotoko]

First, JPF, I realized a few minutes ago that I was completely contradicting myself talking about helicopters versus planes; first I was talking about plane's inherent stability, then about their inherent uncontrollability. This is because the first time I tried to state my case I wasn't sure what I was talking about (I wasn't sure what it was about planes that I liked over helis, mostly) and I was surprised to hear that self-stabilizing helis were easily made; I had read that flying a heli was like balancing at the top of a hemisphere, where the slightest tip any direction starts you sliding down that hemisphere... Not sure where I read that now, actually, but that's why I was talking about helis being a balancing act.

At any rate, I think that in the second case I was a little closer to explaining the way I see planes versus helis... *sigh* I'm not too good at interpersonal communication.

From what you're saying about Re, I'm getting that you think this design would work at a bigger scale then, for the most part? It occurs to me now that I've never actually seen a tiny sailplane (granted there are no thermals or slopes indoors, but hey, you could use fans); I take it this has to do with Re?

If this is the case, then the 'micro' of 'micro and macro' will indeed end up being far different from the latter.

I should have posted this earlier, given its relevance, but here it is: Sonofagun's 70" (I think) Air Surfer.

http://www.rcgroups.com/forums/showatt.php?attachmentid=780179

I was thinking that high AR wings should allows slower speeds than indoor techniques while still retaining a useful payload ability and some ruggedness (i.e. RC and transport capability) but if I understand you correctly, this is only the case at larger scales; the Air Surfer would make a better template for an indoor design.

Incidentally, how big do you have to build to avoid the low Re blues? Meaning how big would my design have to be to work like I expect it to?

And... apparently Carr worked in the Internal Wing concept for over a decade; you really thing he just got tired of it and made up some gibberish and a cool-looking model design to sell to kids? Placing wings that close together does affect their performance; there's no reason to think that you couldn't figure out a way to affect said performance positively... He's even (supposedly) caught the interest of the military for UAV development.

[JetPlaneFlyer]

The Airsurfer looks cool, though it probably falls a little short of your 'below walking speed' objective. This model does follow very closely the mantra of 'Simplify and add lightness' that I quoted earlier, no special tricks on this model, just a big wing and very light construction. A conventional tailed design would be able to generate more lift but the weight would inevitably increase so it's hard to say what would ultimately work best.

At the speeds your aiming at I’m afraid it would be impossible to avoid the issues of low Re number.

If you want extreme slow flight there is no alternative to extreme weight reduction. If it were possible to build robustly constructed fixed wing models that would fly at an airspeed below walking pace (that dont rely on prop thrust lift aka 3D) then someone would have done it long ago…

Steve

[vintage1]

Some of your ideas are OK but some are faulty.

First off, low wingloading is paramount for low flying speed. Without an extremely low weight and thus wing loading your slow flyer is doomed from the outset. So extreme strutural design will be required. This doesn't mean it needs to be weak or lack character. I'm thinking that for the larger version you could use a lot of the techniques used by free flight rubber scale modelers to achieve a nice looking model with suitable strength while still being very light.



This is pertinent and accurate.

Low sped means low wing loading. Nothing else affects speed more than this.
Large arae low weight mens structural design is paramount. A long thin light large wing is a LOT more fragile than a short thick large wing. Unless you use rigging. a long wing NEEDS depth to not snap: This drives section far more than aerodynamics, around the root at least.


Flaps are just a camber altering device. For a model intended to fly at the slow speed end of the spectrum all the time you should just "build in the flaps" in the first place by using a very high camber airfoil.
[
Absolutely Thin undercambered wings will net you maybe another 25% of speed reduction, all other things being equal.

For low airspeeds testing over decades of competition free flight models has shown that "thin is in". Indoor models gain from this in two ways by their use of single surface panels in terms of both aerodynamic efficiency as well as minimal structural weight. For an outdoor model this would work too but you may want to look at struts and/or functional bracing lines to be able to use minimal materials for spars that are then locked together structurally much like the old WW1 biplanes that locked their upper wing and lower wing spars together via the interplane struts and rigging wires.

Ailerons at very low speeds on high camber airfoils generate huge amounts of adverse yaw and as such don't work all that well. However rudder and elevator controls work just fine and that's the way to go.



Yes. Absolutely agree on all counts. large light and thin is what you want, BUT structural issues rear their ugly head. Bracing wires are find at ultra low speeds.

You can gain some additional lifting area by aiming at a tandem wing planform along with the use of biplane wing arrangements at both ends. The use of a 100% size stabiliizer means that the rear wing set is lifting as well as the front set but not quite as hard as the front set. This reduction in lift at the rear is needed for the sake of pitch stability. However you're still gaining overall. The other planform that would work would be the same tandem setup but with monowings as used by the Langely Aerodrome. Here's a link to the general arrangement for you....
http://home.att.net/~dannysoar2/Langley.htm
Now nothing says the props and motors need to be located between the wings but since the CG for a tandem wing of this sort would be located between the wings (actually slightly in front of the mid point between the 25% chord points of the two wings) it may be prudent to mount the motors and props where shown in the Langley design. If using a single motor then perhaps some arrangement could be made for belt drive or use a single prop but then go for twin booms. If mounting two motors on the wing some extra bracing would be needed at those locations.


That is good thinking all round. Canard or tandems give overall better L/D ratios.

Note that for the lightest possible weight on such a monoplane tandem configuration you'd want to pretty much duplicate the rigging of the wings as shown. I'm guessing that you'll want landing gear and the fixed landing gear would double as the lower rigging attachment point.

If you go with a biplane tandem configuration twin motors could be mounted at the first set of interplane struts and rigging points which would be a structural hard point.

Normally I'd say that all those oddball wing arrangements are just gimmicks and the vast majority of them are. Certainly your description of the staggered tri wing setup is not going to serve the best purpose of allowing for the lightest self supporting structure or provide the best wing area with minimal aerodynamic interplane interference.

Depending on the sizes involved your airfoil could be something as complex as those found on free flight FAI F1A gliders with their carbon box leading edges or as simple as medium density balsa of something like 1/8 x 1/2 at the leading and trailing edges with no spars. The leading and trailing edges forming the spars and the interplane struts and bracing lines adding the needed support to keep them in line and withstanding the loads.


Don't forget teh butterfly type models made of a hoop of carbon with a bit of film, covering the hole..MINI UFO stuiff. Worth a look..

At the sort of speeds you're after this would definetly NOT be an outdoor model at all except for those rare times when there's no detectable breezes at all.

The next challenge would be to see if you can build a 3 foot model of this tandem biplane configuration complete with radio and batteries and have it come out weighing no more than about 2 or 3 oz. which is about what you need to fly as slowly as you're after.

Angling the model into a very high angle of attack similar to 3D models that do the harrier maneuver isn't really flying slow. Yes the model is traversing slowly but it's prop hanging at least as much as it's flying and if that's all you want then just learn to fly a 3D flat foamie. True slow speef flying implies that the model is flying in a stable and trimmed manner that requires no intervention from the pilot except for turns and corrections for any gusts. A model in a semi prop hanging harrier style of flight will not be able to do that.



High alpha is NOT just prop hanging. Its as much using a wing that does not stall completely at very high angles of attack - you trade good L/D ratio for "best lift" and "never-mind-the-drag". The issue here is that a traditional wing optimised for efficiency tends to stall rapidly and completely giving a rapid transition from 'good' to 'bad' a wing that is 'bad' already, does not show that abrupt transition, and so can be run at higher angles of attack under semi-turbulent flow using a lot of power - in this context you have to differentiate between e.g. indoor competition models that fly slow because thats the way to get the very best sink rates, and a high-alpha model whose purpose is to fly slowly irrespective of efficiency. They are two different approaches and both fly slow, but for different reasons..



If you're after a micro sized model that will fly in this manner then you're going to have to figure out how to achieve building a 25 inch indoor style model that weighs no more than about 5 grams ready to fly. Any more than that and it'll be flying faster than you can walk. The FAI indoor models that fly at a slow walking pace that everyone is describing are mandated by the rules to weigh a minimum of 1 gram. They used to be able to make the models much lighter than that but they were too fragile for shipping so the FAI introduced a 1 gram rule some 25 or 30 years back. I think there's been a wing area rule since then since the 1 gram models were getting so extreme that they were as fragile or moreso than the previous models.

If you're truly after slow flight where a normal walking pace is the goal then you've got your work cut out for you.

One option for control that would result in less weight would be to go for two motors and use mixing to mix the throttle signals for each with the elevator and aileron controls. Then you don't use any rudder or elevator at all. Just trim the model so it flies pretty much as a free flight model. Adjust the trim so the model flies level at around 1/2 throttle and has enough pitch stability that it naturally noses up and climbs when you add more throttle. Reducing throttle would allow it to glide downwards. For turning the aileron stick would be mixed into the two throttle signals so power is cut back on one side and increased on the other and the differential thrust would turn the model.

I've got a biplane flying wing that flies with this setup and it was very surprising just how capable it can be.

At least this is how I'd do what you're after.


I am not clear as to what the final goal is.

Slow speed for WHAT? As has been pointed out a rotary wing model like an autogyro is capable of very slow flight. So is a parachute!.

[JetPlaneFlyer]

And... apparently Carr worked in the Internal Wing concept for over a decade; you really thing he just got tired of it and made up some gibberish and a cool-looking model design to sell to kids?

Yeah.. this pretty much sums it up... The claims he makes for the concept are ridiculous and just make no sense at all even a basic physics level. For instance:

Therefore, the IWA design doubles the mass of the air or water

How can the mass of a given fluid be doubled?... Some form of nuclear fission process perhaps? It cant be explained by increased velocity because he makes a seperate (smaller) claim for velocity increase. As he claims it works on water then he also can’t be basing the doubling of mass on compression of the fluid. This does not make sense at any level.

Also and even more laughable:

All of this adds up to a design that, incredibly, creates its own lift and thrust.

Come on give me a break!... If it makes it's own lift and thrust then why bother with an engine at all :rolleyes: He has discovered eternal motion! He even claims his glider will continue to accelerate after being thrown due to it's 'self generated' thrust!!... so why does it ever come down?.. Why don’t I see these models winning hand launch glider contests?

He patented this in 1986 so ask yourself why, if even half of what he claims is true, has no aircraft manufacturer taken up this technology?... Do you not think Boeing or Airbus would be interested in a design that creates its own thrust... zero polution, zero fuel costs... just think of it?... Why is there absolutely no scientific testing to back up his fantastic claims?...

The answer to this conundrum is simple; it's complete BS.

Steve

[BMatthews]

Look at the claims made for the Manta homebuilt design. A while back an electric model of the Manta was flown and video of it posted on Youtube or some other host. It's ability to fly was laughable.

This isn't to say that this style of linked wing will be as bad. It may very well fly quite well in some form. But it's not a planform that presents it's wing area in a way that will allow the surface area to work at it's peak. Snake oil promises generally turn out to be just that. There may well be some advantages to this planform but super slow flight will not be one of them unless you can produce the model in at least as light a form as some other more conventional planforms.

Another advantage of a tandem planform would be that the fore and aft wing(s) are working at the same Reynolds numbers while supplying their lift.

[JetPlaneFlyer]

Bruce,
Could not agree more.. The models actually look quite eyecatching and the wing design is robust and compact, ideal in many ways for a knock-about RTF foamy 'toy'. The electric power RC powered versions fly fine judging by the video clips on the web site(though nothing that a conventional model could not do just as well or better)... It’s the ridiculous claims made by Carr that gets my back up. Guess he thinks it will help him sell the models to the gullible :rolleyes:

Getting back on topic... There is absolutely nothing to recommend this design as a slow flier. I don’t think even Carr's fanciful work of fiction makes any claim for super low flying speed... There again maybe it's in the small print, he does claim the design has miraculous properties in pretty much every other area, so perhaps I just missed it :rolleyes:

Steve

[Kinotoko]

Well, you could double the mass of air flowing over a surface in a given period of time just by speeding it up... I figured that's what he meant. As for producing its own thrust, I kinda figured he meant it negates a lot of the wing's drag (since obviously it couldn't be producing thrust)... The reason I was thinking of using it for a slow flyer was that I was hoping it would increase wing efficiency (which is all I ever really thought it did).

I might still use a similar arrangement, because it does look pretty cool, and could be used to brace the wings against each other, just as long as I could arrange it so they weren't interfering with each other.

Incidentally, what kinds of aerodynamic tradeoffs come with bi-and tri-wing designs like the old flying coffins, assuming they're made properly?

[BMatthews]

Well the only way to double the air passing over the wings is to double the flying speed. Remember that the air is still and it's the model that passes through the air when flying. Grand claims to the contrary there's not much you can do to vastly increase the air flowing over a wing at one speed. Something that CAN help is to seal off the tips so the air at the wing tips is forced to stay as much as possible on the wing itself. Tip loses account for a lot of both drag and loss of lift. One way to do this with a biplane configuration would be to have interplane struts at the tip and then cover it so the two wings and tips form a sort of box. Santos Dumont did this pre WW1 with his canard biplane.

I'm not sure what you mean by flying coffins unless it's the nickname given to WW1 fighter planes. But if you do then yeah, biplanes offer a way to pack a lot of wing area into a small volume and in your application if they use the full on functional bracing offer a lot of structural advantages. However nothing comes for free. The two wings interact and act on the air slightly differently than a monoplane wing does. The key is to use an interplane gap (the vertical spacing) that is closer to between 1.3 to 1.5 times the wing chord than it is to 1 times the wing chord. Under no circumstances for a model where efficiency is key would you want to use a gap of less than 1 chord. And gaps over 1.5 offer only minimal gains. There's not really anything I know of to be gained by staggering the wings of a biplane other than the old fighters did it so the pilot had a better forward and up view of attackers.

Triplanes have the same restrictions as to interplane distances so unless it'll be a hugely tall structure it's hard to use wider chords and keep the better large value interplane distances. This is why the Sopwith Triplane had fairly narrow chord wings. The other biplanes of it's time really had pretty much the same wing area with less structure required. Think about it. A triplane will need all the same wing parts for each panel but the triplane will end up with the same area as a biplane but with 1.5 times the number of parts.

Truly a monoplane has the most area for least parts but you'll need to rig it much like shown in the Aerodrome info or like the old Bleriot style monoplanes of the times. Using the rigging will allow the use of much smaller and lighter spars since they will only be resisting pretty much pure compressive loading instead of pure bending loads.

EDIT; we have a couple of indoor RC events each winter up this way. You're giving me the idea to put typing into practice and have a go at a super slow flyer of my own. Stand by for a basic drawing of what I'm proposing in my stuff from above.

[Kinotoko]

Sorry for the delay; I was in Chicago visiting family.

I was thinking that staggering would have an effect similar to increasing the inter-wing gap; i.e. having them only 1.3 chords apart but staggered by half the chord would offset some of the interference and give an effect of more than a 1.3 chord gap... because a diagonal is longer than a straight line (so to speak... you know what I mean).

If you had a wing on bottom and a wing on top, with the top having its leading edge aligned with the trailing edge of the bottom, would you still get the same interference?

Also, how far apart would you have to locate two wings on the same (geometric) plane to avoid interference (ala the previously mentioned Langley Aerodrome)?

I actually have a design in mind that would appear similar to a snake with wings... Basically just space the wings out with an ultra-lite, long fuselage (for instance, one to four carbon riber rods) and steer by flexing said fuselage.

[BMatthews]

But the air flowing through the wings only sees the vertical dimension. There may be some advantage to using the right type and amount of stagger but you're best to just not bother since without some actual references to wind tunnel work you're guessing at best and using wishful thinking at worst.

The flexy fuselage sounds interesting and with carbon fiber so readily available these days it becomes a definite possibility. Serious thought and care would be needed to ensure that it's stiff in the required axis and flexible in the movement axis. Also you won't want it to be flexible over the whole length since that raises unknown issues with the wing's and tail's lift centers moving around with respect to the center of gravity.

You seem to be quite taken with the art value of this exercise which is fine and with care could result in a really interesting craft. But first and foremost you need to consider the aerodynamic factors and only then look for the artistic and unique way to achieve the basic aerodynamic ends. Anything else results in a compromise at best and a failure at worst.

But if you're willing to try and fail a few times to achieve what you're after and end up with the early trials as decorative cieling hangers then I say " full speed ahead" and will be the first to complement you.

Just remember that truly light weight is the key to very slow flying. Keep it light enough and give it enough lifting surface and even the oddest looking craft will fly.

Look at the minimalism of the rather clever indoor RC flyer that uses not much other than a carbon outline covered with flexible plastic film. The stretching up of the outline forms the arc of the airfoil witout the need for ribs. It's known as the UFO but when I try to search for the darn thing way too many other links come up so I can't find a pic at the moment.

[Kinotoko]

You mean the IFO? I know the design. Actually, one of the factors I didn't mention is that more or less any design I come up with is going to be at least pseudo-realistic: no profile or stick models for me, unless the full scale version would also be profile or stick (flying bicycle anyone?). But that really just requires having a built-up fuse, so no biggie.

I actually hadn't thought about the CG moving around with a flexy design - that would have been an unpleasant surprise...

Anyway, thanks a lot to everyone who posted info - I'm going to keep this thread marked to come back to, because I learned a lot in this discussion. Sad truth is that I have one complete Delta Dart (with a pretty custom design printed on the tissue covering, at least) and one Guillow's Javelin frame still in need of covering. I've got that $200 Plantraco 1.1 gram Rx/Tx combo kit with accessories, but I still haven't finished a plane for it... been tearing up foam and making prototype wings (I even used my dad's power jigsaw and belt sander on 5/8" pink insulation foam). I just got excited about this concept - I'll probably shelve it for a while though. Gotta focus...

[BMatthews]

IFO.... that would explain why I couldn't find what I wanted.... :D

You may want to look at the idea of an indoor PennyPlane class model but with a little bit of added wing bracing for ideas on something to put that Plantraco into. A pager motor and gear drive with some sort of super small battery pack would likely fly it in fine form.

[vintage1]

well, just for a laugfh, a friend brought a toy round yesterday. It has about a 12" span made of foam, and was a semi scale cessna: Came with three channel 2.4Ghz radio and a teeny LIPO that you recharged from dry cells.

Flew about 10mph flat out and he could catch it out of the air on the glide - maybe around 6mph stall or so. Outdoor conditions were perfect: zero wind and hazy sunshine. I f;lew it out of range at around 20 meters and crashed it. No damage. Ceiling appeared to be around 50 ft but it took some getting up there. not much power.

Another well known poster here remarked that the price - about $150 - was 'worth it for the radio gear alone' and he had visions of getting one and stripping out the micro gear and doing a 15" span spitire or somesuch :D

I see there s a thread on this little gem


http://www.rcgroups.com/forums/showthread.php?t=637399

Definitely worth getting one and having a play with it as basic research.