Discussion in 'Hangar Flying' started by rtfm, Jan 14, 2014.
Actually I like this twin boom configuration and I think I've solved the CG problems for you. Turn the people around to face the other way and you have a twin boom tractor canard ...
Yep - I like the twin booms too. I am still waiting for a long overdue cheque to arrive for work I did back in November. So till it arrives, I'm reduced to doodling on the laptop which doresn't costy anything.
I'll doodle some more on the twin booms, and see what shakes out. Not that I'd build anything like this, mind you. This is all just to have a bit of fun, and to release some pent-up creative frustration.
Current RTFM-Aero VWF-18...
OK, back to "real" planes. I have two viable options which I am considering for the two-seater (the Hammerhead). which is looking more and more like a jellybaby...
Option 1 (mono wing) which requires a 15lb movable weight depending on if there is a passenger or not. No passenger, weight is at very rear of the fuse. Passenger, weight repositioned up to the firewall. This leaves the CG identical whether there is a passenger or not. I could put in two extra fuel tanks, and pump gas between the two as required. And use the gas in an emergency... The Wiz suggested that one.
The plane is clean looking, and apart from the slight forward sweep and inverted gull wing, pretty standard.
Option 2: (tandem wing). This solves the moving CG problem (as suggested by HITC), but adds more weight (I think) than the 15 lbs of ballast. It is an idiot-proof arrangement, however. The main (front) wing would be mounted on a cabane (like a Pitts Special). The rear wing mounted as low as possible just rear of the cockpit. Both wings to have ailerons/flaps. Standard rudder. No need for a separate h-stab?
Any comments, suggestions, criticisms etc gladly accepted. I'm thinking, of course, not only of possible flying qualities, but also visual appeal, and hence commercial appeal. Cost-wise, I don't think there will be a great deal in it either way.
I must hasten to add, however, that I don't need to make a decision any time soon. I am still working on the single seat (Razorback) plug, which I can only finish when I get some more disposable cash. And then I have to make the molds. And then I have to make the parts, fit it all out with bulkheads, firewall, engine, h-stab, rudder, seat electronics and hydraulics, and THEN in need to build the wing(s). And if there are any compelling reasons to use the one configuration over the other, I MAY use the selected configuration on the single seat Razorback also. But it's not happening any time in the near future.
Another option I have considered is a joined wing. But I have no technical background regarding this arrangement, and I'm too uncertain at the moment to consider it seriously.
I'm not sure about the weight increase either Duncan, it's likely that the weight would be the same or perhaps just a very little more. But it's also possible that it could be a lighter option, and the 15lbs ballast isn't part of the equation in the case of the tandem wing, so it could just possibly be quite a lot lighter. Keep in mind that in the case of the tandem wing both wings are lifting and carry the all-up-weight between them, so those wings need to have a total area of X. If you have a conventional monoplane the wing needs to have more area than X because it needs to carry the download created by the HS (Y), so the monoplane wing area needs to be X+Y.
If the tandem winged plane's wings' total is X then the span of the wings is very much less than the span of the mono for the same aspect ratio (of each wing) and since the weight of the wing is something like the square of the span (OK, OK but it's something like that!) then the wings should be proportionately much lighter than the mono wing.
Arguably the drag might be a little more but only because there would be four wingtips, but then that drag might actually be less too, if you take into account the HS tips ... Surface area is actually reduced for the tandem due to there not being any HS surface area, and induced drag should also be less since there is only positive lift being generated and no downward lift and extra mainplane upward lift due to having an HS.
So try making a sketch of your tandem wings at the same scale (and hence total combined area) as the sketch of the monoplane, you might be pleasantly surprised at how small (and potentially light) the tandem wings actually need to be.
Regarding the joined wing thing, I think it looks great on the Stratos but unfortunately that configuration doesn't seem to be a good thing, and on the Synergy. But on reverse joined wings (front wing high like the Sunny Boxwing) it doesn't look so good IMHO.
Thanks for the response. The idea of a tandem is certainly very appealing.Time to return to the SS and do some sums I think.
A better approximation of the tandem design yields a very attractive plane. As far as I am able to determine, the current wing sizes/positioning are substantially correct. It looks like this may well be the way to go.
Thanks again HITC.
Does one have flaps on both wings? Ailerons? I've assumed both flaps and ailerons on both.
Duncan, I don't think you can have flaps on one wing of a canard or a tandem. If there are flaps on both they would need a differential ratio and be mechanically linked. Usually you would have aleirons on the largest wing area and elevators on the smaller wing. I sort of remember elevons on the prototype EZ canard but the plans build was elevators on the canard. In any event the front wing needs more angle of attack than the rear wing (2 degrees?) to prevent deep stall and that makes the Cl
different on the wings asuming the same airfoil is on both. I would factor in the Cl difference when determining NP. One
last thing is I would look at having 2 percent more camber on the front wing tips than the root an a couple degrees of
down twist so the wing has a progressive stall over a reasonable AOA change to lessen altitude loss during a stall. This
wouldn't do much for an intentional stall but more for one from being low and slow on landing.
Your twin boom iteration looked to be approximately right in terms of balance the other two with rear end engine utterly impossible . Double check your moment signs or something and run a first check on tail volume --also obviously way too small . A forward swept wing IS destabilizing in yaw (for the same reason that rear sweep is stabilizing --more drag is presented by the 'trailing ' wing if yawed and so it diverges . If you were to stall it also with that much sweep and taper the root would stall first and the whole show pitch up -unstable Forward sweep is 'tricky' and best avoided if at all possible (in your case it is ) there was a quite highly forward swept Akaflieg glider (Darmstadt possibly) dubbed "quo vadis" (wither goest thou ?) because of it's wanting to head off in whatever direction it liked --can be googled .
On tandem wings -
Look up the Quickie two specs and wing geometry as a first cut -- it was a very nicely done design but a bit hot -the Viking dragonfly is a better overall example -both reverted to standard landing gear from being canard tipwheels but otherwise worked well . Cluttonfred (Matthew) has good links to parasol tandem designs that you could study .
That's also the thinking behind canards. Unfortunately, the down wash means you'll need far more wing area for any reasonable stall.
Tandem wings are one of the least efficient configurations, with the conventional being the most (!) efficient one. Unless your configuration forces you (Scaled Proteus for example), I personally wouldn't bother with layouts without a tail in the back.
The boomed pusher though has a lot of potential. Add some luggage space behind the seats and your wing stops needing the sweep too. I've always liked the Ion (not Icon), though I'm not a fan of the tandem seating arrangement. A longer tail would have solved a lot of their issues too...
Not sure whether inboard stalling with fwd sweep is such a problem. Different profiles and/or flaps do just fine for most gliders that have significant sweep.
See Miles Aircraft since 1925 by Don Brown, my favorite of the Putnam "aircraft since..." series, for some notes on the adventures with CG and flaps with the early Miles tandem wing designs which included both high wing forward and low wing forward models. In short, yes, flaps on both wings is one option if you are going to have flaps at all, though you must be careful that no possible flap setting would cause the rear wing to stall first.
Personally, I would go with much simpler, constant-chord wing planforms and full-span control surfaces (one per wing) to make the build that much easier. Conjugating the elevator function between the two wings (front up rear down or vice versa) seems like a good idea in terms of low-speed handling but not so much in terms of maximum lift coefficient. Ideally, I'd like to go with flaps on the front wing and elevons/flaperons on the rear wing, though that does require a torsionally-stiff fuselage and powerful control sufaces so the tail can wag the dog. Separating the functions also makes the mechanics a bit simpler. Some sort of compromise would likely be best, and experimenting in X-Plane and/or with RC models would help you find the right balance.
The Mauboussin M.40, for example, essentially used flaperons on the front wing and elevons on the rear wing.
Where to start? Well the CAFE races have been dominated by Klaus Savier (light speed enginering) in a Vari EZ with the overall win. This race factors speed and fuel burn per passenger weight with the result being the most efficient design. Then there is the Quickie which had a 18 HP industrial engine and a lot better speed than planes with greater HP. The downfall
of canard thinking is NOT DOWNWASH which can be countered by appropriate AOA rigging of the following wing. The
problem is the canard vortex hitting the following main wing. If this vortex goes above the leading edge stagnation point
then the wing looses lift. If the vertical separation is done favorably to land on the rear or the upper surface or only intersects the lower surface then lift is enhanced. The design sketch Duncan shows above has a longer span on the front wing
which should shed the vortex flow outboard of the rear tandem wing making it a non issue. Second thing that seems odd
is the suggestion that the tandem wing solves a variable weight issue when the variable weight is not over the Cg. There
may be a possibility of maintaining balance at the expense of trim drag on the rear wing which could widen the CG
envelope with ever larger trim drag. This approach does not have favorable impact on pitch stability.
Since this thread is "design musings" I suppose that twin booms and pusher aircraft are in range of the discussion
but to be relevent I think examples of the type that have been produced could be presented. And finally while mounted atop a soap box I might mention that adding the variable load of a baggage compartment behind the seats offset from the Cg will do little to improve weight and balance. Ok climbing back into my cave again.
nothing to improve
The point was that the variable weight is actually quite close to the desired CG position of a tandem wing configuration. A lot closer than to the CG of the monoplane configuration. I thought the CG of the tandem would be about where the knees of the crew are and since the craft would be set up to be balanced with at least one crew then the variable weight off CG would be one crew about 18" aft of optimal which would still be within acceptable CG range for a tandem wing. Perhaps I missed something though.
John Roncz helped design the Eagle 150 (then based in Perth, Australia). He spoke very highly of the three surface plane (he would...) but other testers have corroborated his initial claims.
The Eagle has a 39ft^2 front wing, a 56ft^2 main wing, and a 16ft^2 h-stab giving it a 12.71 lbs/ft^2 wing loading. The front wing has a flap running the length of the wing, and the main wing is equipped with both flaps and ailerons.
I remember seeing one of these beauties at Ardmore Airfield back when I was just getting interested in planes, and I remember thinking it was the most beautiful plane on the field.
So I spent most of today working in X-Plane to equip the Razorback with a tandem wing configuration, and testing various arrangements. I then compared the flying qualities of these concepts with the well-tried inverted Gull Wing version I already had.
To be honest, the tandem configuration flies very smoothly. The gull wing is more "zippy" and has about double the roll rate. Both fly very nicely. But the tandem really looks sleek and sporty.
I ended up with a smaller fore wing, set about mid-fuselage height. The main wing is right up on the roof immediately behind the cockpit. And the h-stab is on an 18-inch fuse extension (which I have now completed on the actual plug. It looks very cool. I don't know how to add textures, so my models are just grey - pity. Fore wing: 14ft span, main wing 16ft span. MAUW is only 600lbs, and she really motors. X-plane shows 150kts S&L. I'll be happy with less than that...
Not being able to work at the shop is proving to have its benefits. Quite an enjoyable and productive past few days...
It is not helping Duncan to be complimentary when major flaws exist in the thing posted for comment (the VERY highly forward swept rear engine pusher drawings)
--consider the consequences before the diplomacy perhaps. Duncan's sketches show nearly 45 degree (at MAC) forward sweep --possibly a record amount and combined with a high taper (plus inverted gull wing --an unknown additional complication at the wing root critical area ) -having the engine at the extreme aft end is asking for trouble from simple balance and the polar moment of inertia (you have created a 'dumbbell" in effect --it will be hard to start but then stop rotating in pitch and yaw . If the prop was bolted directly to the crank then the landing gear would have to be enormously long and weigh a great deal - mass also aft of the proper CG -if it was retracted (somehow) into the gulled wing then the structure would need more beefing up and fuel volume lost in about the only feasible place for it. the tail surfaces are tiny and somewhat 'shaded' by the wing (they overlap slightly even ) --at least twice the tail area would be needed and ditto for fin area to overcome the wing's inherent directional instability --the list could go on but that much is clear.
The comments about tandems and canards are correct as regards achieved performance --the saving in wetted area and undercarriage weight in the Quickies more than makes up for any induced drag (but splitting the wing can REDUCE induced drag -ask PRANDTL about why. ) the stability and wing interaction aspects are fairly advanced stuff and still subject to surprises --ALL canard and tandem wings had big enough problems to keep them out of production until Rutan "tamed" the basic configuration as an expert in flight test and stability. Some tandems still crashed (flat spin in -eg the PAT 1 built by an ex scaled employee --others just copy the Rutan 'formula' . CERTIFIED canards or tandems are as rare as Hen's teeth still. Really though these things are outside Duncan's present level of understanding -harsh to say but obviously true --he would be better advised to stick with a very conventional and not 'hot' design -not short coupled for example ,even to build a Corby Starlet maybe to get something in less time and pick up some builder experience (the ONLY Australian homebuilt was a single seat low wing for many years -the Starlet, so it is not breaking new ground to do the same ) It is in the DETAILS that the real devil lies and we are not even near them yet. Cruel maybe but perhaps this advice is also kind.
Your #134 just appeared Dunc AFTER my post (just appeared as I sent --yet to read it but unlikely to change my advice)
OK just scanned you latest iterations -- before I comment, the Eagle X Wing (last version after the Graham's 'ripped off' the Quickie 1 adding an untapered wing , I knew their test pilot when he was based at Essendon with Goldwings aviation(flying DC3 casino..) he HAD a Starlet actually and vastly preferred it to the "eagle one" (single seater) I also know Graham Swannel and David Betteridge who both worked on contract on the Eagles (not name dropping but to reinforce the 'more than magazine report' nature of my familiarity with the aircraft and their -unpublished- histories ) I in fact nominated David as the first AUF president --but another story.
There are a heap of them gathering dust in a hangar in Malaysia I gather -- I thought it was bog ugly and one of the least elegant airplanes built in a long time --to get to the certification stall speed it sprouted vortex generators, cuffs and strakes --and was quite slow as a result. It was a fiasco financially.
Just look where you landing gear is in #1 -- implies that the canard carries ALL the load or ?? The rear tail has quite a short moment arm if it is being used for control (compare to the Eagle X wing for example) --the directional stability would be marginal at best for the same reason. #2 is again not consistent --the gear position suits a tail dragger (why not just build a scaled down Corsair ..?) anyway that is 2cents worth.
Thanks for the post. Just to be clear, however, my design 'musings' were just that. Doodles and having a bit of fun. It actually started with my wondering how to sketch a homebuilt F-18 look-alike. Then "How far forward can a wing be swept and still look OK"? Does it fly? Will it be stable? Dunno. For the sake of sketching and having a bit of fun are concerned, it doesn't matter. I was hoping to spark some discussion, elicit a few other people's daydream designs. That's all.
HITC suggested a tandem wing, and I thought it was interesting, but felt unsure of how it all hangs together. At least a single wing is in familiar territory. I have spent some time sketching, playing with x-plane, running numbers through the spread sheets. I hope no-one thinks I am seriously considering building one of these F-18 daydreams, or thinks I feel confident designing a tandem wing.
My actual project is comparatively mundane. A bit short-coupled, true, but I've extended the tail by 18 inches. The wing a very conservative, having been pulled back from a quite aggressive forward sweep to something under 4 degrees. Nothing too startling.
Perhaps I had better stop posting daydream designs. I don't want to give the impression that I'm going to chuck in what I've done so far, and build a fantasy plane. Just trying to have some fun guys, and throw out some ideas...
Just in case nobody recalls the Eagle XTS - STARGAZER - A unique database on Burt Rutan and his projects! beauty in the eye of the beholder..
It seems a little strange that everyone and his dog copies just about anything which flies and lauds it as the next best thing - but there seems to be a reluctance to afford the Quickie clones the same indulgence?
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