The Razorback construction thread

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by rtfm, Jul 11, 2008.

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  1. Mar 28, 2014 #801

    Xanadrone

    Xanadrone

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    A logic assumption, confirmed by this demonstration (practical validation for big RC-scale at least):

    Hotwire demonstration - YouTube
     
  2. Mar 28, 2014 #802

    Hot Wings

    Hot Wings

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    It works for small parts but you are limited to the same airfoil at both tip and root with no twist, unless you warp the foam before cutting. Also note the much larger kerf of the cut at the tip due the slower wire speed. Good CNC software and well made hand templates on both ends compensate for this.

    I also prefer to cut from the top down. There is less distortion from core sag. It's not as important when cutting by hand with templates but if you are cutting a stack of cores from a large billet with CNC it can be a significant factor.

    It's hard to improve on the Rutan/EZ method of cutting cores, but it does take 2 people.
     
  3. Mar 28, 2014 #803

    Jay Kempf

    Jay Kempf

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    There are a bunch of ways to do tapered wing cores and other parts hands off. Gravity is the best tool so straight up is always best. I have a simple slicing jig that is hands free and it can follow simple templates on each end of a blank. In order to proportion you need differential pulleys or a differential stick with a movable pivot to pay out the feed cables. The old feather cut system was a bit finnicky but it worked.

    The single anchor method of cutting conics works great for things like turtledeck sorts of shapes.
     
  4. Apr 27, 2014 #804

    rtfm

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    Hi again,
    After a prolongued period of vascillating and indecision, I finally nailed the placement of the front wing, and committed myself to cutting out the wing recess. The plug now looks like a ship lost at sea after running onto rocks. A great big hole in her belly.

    But the surgery has been completed. Now all I have to do is to shape the recess perfectly to fit the wing. Easier said than done. But I'm working at it.

    When I finish this, I'll be flow-coating the entire plug with a nice layer of bog (Q-cell) and then sanding it back. It's a helluva job, but if one keeps at it, it comes to an end eventually. Thank goodness the airframe is so small.

    One of the decisions worrying me was airfoil choice. I had always thought to go with the Riblett foils, but when it came down to hard numbers, I just couldn't get them to line up. As it turns out, I decided on an Eppler e209 on the front, and the AS5045 (designed specifically for the KR2) on the rear. This combination gives me an exceptionally benign stall on both wings and an extra 2 degrees of lift on the rear wing once the Eppler has stalled. This, plus the 6 deg downsweep from the front wing puts the rear wing safely in an 8 degree AOA shadow.

    So while I sand, I'll be sourcing the foam for the wing plugs, and setting up the jigs for the wings. I plan to do each of the wings as a single piece, so the jigs need to be spot-on to tie together six separate pieces of foam. I haven't got the Graphlite rods for the spar caps yet, so I must chase that up also.

    Regards,
    Duncan
    PS Sorry, no pictures. Forgot to take any.
     
  5. Apr 27, 2014 #805

    autoreply

    autoreply

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    Downsweep?
     
  6. Apr 27, 2014 #806

    rtfm

    rtfm

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    DownWASH if you prefer... :)

    Duncan
     
  7. Apr 27, 2014 #807

    autoreply

    autoreply

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    Ah, okay. Did you run the various cases (no down wash, power on and off stall)?
     
  8. Apr 27, 2014 #808

    rtfm

    rtfm

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    Hi,
    Power on stall YES
    Power off stall YES
    No downwash NO

    It appears that even at slow speeds (ie 43kts - stall) there is considerable downwash from the front wing. At takeoff and landing, the rear wing produces more lift than during high speed cruise, due to less downwash, but the difference is still quite substantial. Essentially, even with no downwash, there are still 3 degrees difference in AoA (1 degree physical difference in incidence, and 2 degrees from the rear airfoil stalling at a higher AoA). But when would one experience zero downwash?

    Duncan
     
  9. Apr 27, 2014 #809

    autoreply

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    In ground effect ;-)
     
  10. Apr 27, 2014 #810

    rtfm

    rtfm

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    Ah. But the air is still deflected downwards off the wing so long as it is flowing over it, is it? The fact that it then almost immediately strikes the ground doesn't change the initial downwash? Or does it?

    Cheers,
    Duncan
     
  11. Apr 29, 2014 #811

    Aircar

    Aircar

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    It changes the RELATIVE angles Dunc --the front wing being closer to the ground also has a higher lift slope curve (higher apparent aspect ratio) and a lower stall angle . The Ligeti Stratos was initially unable to keep it's nose up after rising out of ground effect despite full back stick, after several hours of talk by phone to persuade Charles to get SOME air experience before launching off on the first flight of his aircraft and putting him in touch with a professional instructor and test pilot for other homebuilts etc by the name of Bruce Northeast he realized that he did not have some god given ability to fly without any training and relented to let Bruce do the first flights (both Charles and Bruce were small blokes and fitted into the Stratos-just, (look at any photos of Charles in it and note his knees jacknifed up to get them behind the front wing carry through) Bruce did some high speed taxiing and then opened it up to do a long low run but found that he could not kep the nose from dropping down as soon as it was a few feet up -he wiped off the nose gear in the subsequent 'nose in' that was a gentle 'parabola' with full back stick --maybe if it had been from a lower height the ground effect might have raised the nose again but it could not ,he declared it unflyable. From then on the aircraft needed negative flap on the rear wing in order to be trimmable (look at any photos and note the rear wing flap angle) --with a lightweight like Charles on board. I could not even come close to closing the canopy and I am just under 6 feet and a lot lighter back then (101/2 stone)

    Charles was dogmatic and built in both wings with no chance to adjust either one -he was also dogmatic about not having a strut on his aircraft (doing so removes the root bending moment allowing a simple adjustable fitting and also removes the need for any spar to go across the nose giving foot space but he would not listen. (even just a beefed up rear wing and a pivotting lift pick up would have allowed to alter the basic settings . With the reflex on the rear wing the vast majority of the lift came only from the canard making it more like the Synergy interms of relative lifts . Some quickies had reflex on the rear wing ailerons to allow more trim range as well. (and to counter nose drop in rain ) Give yourself some outs when planning wing and canard attachment.
     
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  12. Aug 19, 2014 #812

    AVI

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  13. Oct 12, 2014 #813

    skier

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    Any updates? It's been a couple months since we last heard of progress from you, Duncan.
     
  14. Oct 12, 2014 #814

    rtfm

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  15. Oct 12, 2014 #815

    rtfm

    rtfm

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    Hi,
    I have made very little progress in the past months, mainly because I have run out of funding. (One of the inevitable side-effects of not having a job, I guess). But there has been SOME progress. I've cut the rough recesses for the wings, and have made the central wing sections so that I can accurately finish the wing seating and get the plug complete. But they are renovating the entire complex where I have my workshop, and I have had to (temporarily) store all my tools, plugs etc in a 40ft container. A real pain in the arse...

    So probably there will be minimal progress for the rest of the year unfortunately.

    Thanks for asking,
    Duncan
     
  16. Jan 19, 2015 #816

    rtfm

    rtfm

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    Hi,
    My good mate HITC is busy working away on his new project, and every time I think of him in his workshop, I get more frustrated at not being able to work on the Razorback. Yesterday I drove out to the industrial site where my workshop is stored in a container, and the plug is under some trees with a tarp over it. Everything looks OK, but I'd like to get the plug into a new workshop ASAP. And get it finished.

    In the meantime, I've been checking the design calculations again. It has proved to be quite a challenge to find reliable formulae which apply to tandem wing aircraft. But over the months I have ferreted out some really useful information. I have two sanity checks for the design. Inputting the data into X-Plane, and building a 1/4 scale RC model. X-Plane agrees almost exactly with my formulae in the spreadsheets. I'm still working on the RC model.

    With the re-calculations, I have come to realise that with two equal sized wings, some of the traditional rules of thumb which apply to pure canard configurations don't necessarily apply. In a traditional canard design, for example, the main wing is aft, with the canard way up front and carrying significantly less load. All the grunt lifting is done by the aft wing. So it makes sense to work with the aft wing as the baseline, and simply increase the canard by a few degrees, ensuring it will stall first. Also, the downwash produced by the much smaller canard is significantly less than that produced by a full sized fore wing.

    So, with a design which essentially has two equal wings fore and aft, it is not simple to get it right. First, calculating the angle of the fore wing downwash is critical. And for someone who is calculus-challenged, it was not easy to find a way to calculate this. But I finally did, with lots of help from the guys in the sailing world as it turns out... and surprisingly, the downwash angle at the aft wing is nowhere near as severe as I at first thought.

    This angle is a function of both Cl and AR, so is greater at or near the stall than it is in cruise. Also it decreases with vertical separation and with distance aft of the fore wing TE. In the landing scenario, my rear wing is in a 2 degree downwash "shadow". I have chosen my airfoils carefully, so that the aft wing stalls at a full 2 deg later than the fore wing, giving me 4 degrees of safety in the final flare.

    The next critical thing to calculate is the placement of the wings. The front wing actually carries 25% more load than the aft wing - due mainly to the downwash angle it induces. With my CG pretty much a given, the only way to ensure that the fore/aft wings' centre of lift falls suitably rearward of the CG was to move the wings fore and aft until the plane was balanced. Fortunately, my original calculations seem to have been reasonably in agreement with the new formulae. In fact, while there was an agreement between my spreadsheet predictions and X-Plane simulations previously, the degree of agreement was not always satisfactory. Now it is nearly perfect. The next step is to complete the 1/4 scale model and try it out.

    So with the calculations re-done, minor adjustments made, and the planned move to a house with a shed out back, I may just be able to get the plug finished and start building the actual plane before too long.

    Warm regards,
    Duncan
     
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