Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Topaz, Sep 10, 2014.

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  1. Feb 3, 2017 #621

    pictsidhe

    pictsidhe

    pictsidhe

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    Finally got around to reading the 'build' thread and have dipped into this one. Suggestion, put a bigger fuel tank in, you don't have to fill it. But if you ever get an urge to attend Oshkosh or sun n'fun, you may appreciate it. For such rare use, I'd leave everything else as is and make allowances for longer TO and lower g's if you use it. 30lb baggage is unlikely with your case size.
     
  2. Feb 3, 2017 #622

    Topaz

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    Well, that's not strictly true. If you'll recall from post #37, the maximum allowable wing loading for this airplane to meet all my specifications is 7.6 pounds per square foot, and there are two mission requirements that are equally driving that number: Stall speed, and takeoff run. I can't arbitrarily decrease the wing size (increase the wing loading) without adversely affecting the takeoff distance - which is already pushing my threshold limit for this airplane. Best L/D glide and minimum sink glide start to become affected at 11.6 and 11.2 psf, respectively, so there are definite limits that I have to respect in terms of wing loading, or else I'll start to sacrifice key performance parameters.

    So while yes, there are higher-lift airfoils out there, they really don't gain me anything, in this particular case.

    I'm really happy with my choices so far. I appreciate discussing this with you, but I'm going to stick with my choices as-is, unless some later calculation invalidates them.

    Heh. Right now, I'm more worried that the fuel tank location I have will hold enough fuel - the whole 10 gallons. I haven't done the volume calculation on it yet. It's not hard, I've just been too danged busy.

    I see what you're saying, though. Yeah, within weight and balance limits, I could trade baggage for fuel. If the volume available proves to hold more than 10 gallons, I may just take all of that volume and do exactly what you say. The dude on the other shoulder is saying, however, "Do you really want to sit in a low-wing-loading motorglider for hours on end, being beaten to death (and distracted by) every thermal you cross?" Yikes!
     
  3. Feb 4, 2017 #623

    proppastie

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    Focus.....you can always have two aircraft.....sell the Porsche.
     
  4. Feb 4, 2017 #624

    Topaz

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    :eek:
     
  5. Feb 4, 2017 #625

    lr27

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    I'm having a bit of trouble figuring out how a high lift airfoil won't help you keep equivalent performance with less area, as long as you keep the span the same. Unless the shortened chord makes the Reynolds number too low, or the wing too thin. And some of the high lift airfoils are quite thick. One other possible problem might be that you'd need a higher angle of attack for max lift.

    An additional benefit of a high lift airfoil combined with a higher wing loading is that you'll bounce somewhat less, as the lift change per degree of change in angle of attack will be less. The lift coefficient change ought to be the same, but since, instead of going from, say, a Cl of 0.5 to a Cl of 0.7 because of a gust, you might go from a CL of 0.75 to 1.125, and the percentage change will be less.
     
  6. Feb 4, 2017 #626

    lr27

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    BTW, if you sell the Porsche, if you can find a 1976 Ford Fiesta, they are very economical and very fun to drive. If someone put brail messages on the road, you could read them with the steering wheel. ;-)

    But only if rust isn't much of a problem in your area, and you aren't addicted to low zero to 60 times. Unfortunately, mine kind of broke on the tire jack due to rust. I kept it for a while after that, but the driver's door never closed right again. Before that, there was the time I ran through a puddle at 40 mph, the repair in the floor let go, and a fountain appeared around the cut in the floor carpeting at the stick shift.
     
  7. Feb 4, 2017 #627

    TFF

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    I doubt his Porsche will yeald an airplane. My wife always says that about my cars, but they would not bring in enough. Cheaper to keep that fun.
     
  8. Nov 4, 2017 #628

    Jay Kempf

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    Looking at your revised drawing and I like. Just one question: roll over protection seems to have become a little skimpy, is that a result of the more reclined position?
     
  9. Nov 4, 2017 #629

    Topaz

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    Good question. Seat recline hasn't actually changed - it just looks like it because of the new "swoopy" fuselage. Addressing your question, the bulkhead behind the head-rest is intended to be fairly robust and may include a "roll bar" tube, but the primary roll-over protection is provided by the vertical tail. The latter is now a lot beefier because of the T-tail, and it would take a fair amount of energy to break/fold up the aft fuselage such that the aft edge of the canopy hits the ground. It's only easy to see on the full three-view drawing. The 5'10" sized pilot, and shorter, actually have pretty good head protection in a flip-over accident, and even the 6'3" pilot likely would never get a "thump" in all but the nastiest flip-overs.
     
  10. Nov 4, 2017 #630

    cluttonfred

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    I like where this is going, it has a very Rutanesque, almost Quickie, vibe about it while remaining quite conventional. I am not so sure about the rollover protection, my eyeball sees a line from the engine top front, and definitely from the top of the firewall, to the top of the fin still intersecting the pilot's head. With the recline angle, even a fairly low bar located about at the front of the panel, maybe pushing up the canopy line a little, would add a lot more head clearance in a rollover.
     
  11. Nov 4, 2017 #631

    Topaz

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    Okay, that's two in a row with the same comment. When I draw a line on the actual airframe from top of the tail to the engine inside the cowling, there's still a few inches clearance between the pilot's head (5'10") and the ground. The 6'3" pilot almost makes contact. I think this is all okay, but I'll make a note to revisit the issue during the next re-draw. Fair enough?
     
  12. Nov 4, 2017 #632

    cluttonfred

    cluttonfred

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    You don't have to do anything, free advice is often worth what you paid for it, just making an observation. I do like it. ;-)
     
  13. Nov 5, 2017 #633

    Hot Wings

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    2 things look like potential problems with the pilot fit.

    Can you find a wheel/tire of that diameter that is narrow enough that it will still fit between the pilots legs and leave room for comfortable rudder pedals?

    The fuel tank being that far back may make entry/exit difficult even if the panel swings out with the canopy. Some Quickie builders (I'm one) opted to attache the panel to the canopy rather than to the fuselage for that reason. It's still hard to get out due to the reclined position. If I had it to do over the canopy would start right at the firewall. A little hump on the floor a couple of inches aft of the heals makes a nice something to push against during the exit. The Quickies fuel tank gets in the way of that but you seem to have plenty of room on the floor for a heal chock. Entry should be much easier than the Quickie. You can step on the wing.
     
  14. Nov 5, 2017 #634

    BBerson

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  15. Nov 5, 2017 #635

    Topaz

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    The tire drawn is the one used on the Fourier RF-4, in exactly the same type installation. I don't recall the exact tire off-hand, but chose it because the airplanes are roughly the same weight and would be used on similar airports. The rudder pedals on this airplane are going to look more like a Kitfox or Avid (simple welded tube) than the massive things you'd find in a Cessna. As-drawn, there's plenty of room right now. The top-view shows this better, but I didn't post that. Sorry. I'll model an exact US-spec tire when the time is right, and widen the fuselage a couple of inches if it proves necessary. Right now I don't see any fit issues at all.

    Yeah, the instrument cluster is going to have to be attached to the canopy frame. I'm normally loathe to subject instruments to that pounding but, in this case, there won't be any gyro instruments at all - just AS, Altimeter, VSI (Vario), Tach, Compass, and probably a dual CHT. It'll still be a "point your toes" entry and, as I alluded to before, this is an airplane you "wear" instead of "climb into". It's not as if this is a long-range cross-country ship where I'd be sitting in that "office" for hours on end and need to be able to move around.

    The fuel tank is what the fuel tank is. It's the right volume, in the right position, to do the job I need it to do. Fuel is not going in the wings because the wings are removable and it's my personal opinion that putting fuel in removable wings is a far higher risk than putting fuel in the fuselage. Too many potential failure modes introduced, especially over time.

    Good tip on the "heel chock". If this plane ultimately gets built, I'll look into that. As this is all moldless composite, adding one would be a day's work at most, and can be done without affecting the structure.

    The AR-5 is optimized for high-speed flight, whereas this little motorglider is optimized more for minimum drag at low- to middle-range speeds. I agree that there is a short segment just aft of the trailing edge where the fuselage "camber line" isn't aligned with the downwash off the wing, but the rest of the aft fuselage is, and I feel the pressure-recovery shape is more important to drag reduction in this airplane's performance range than in something like the AR-5. Operating at lower airspeeds/higher lift coefficients mean a bigger downwash angle off the wing, so aligning the after fuselage with that flow definitely reduces drag. Plus I simply like the look. That's important, too.
     
    Last edited: Nov 5, 2017
  16. Nov 5, 2017 #636

    Jay Kempf

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    Being 6' 2" myself that was what I was focusing on. I personally never use the fin as a rollover but that is the choice of the designer. I imagine someone riding in the craft hanging from the straps in a roll over and the amount of vertical movement allowed. Then I put something like 4" over the top of the tallest head just behind the head. Plus a strong hoop in front of the panel. For a glider I never rely on the tail because if you catch a wingtip it is breaking off in front of the fin. Been to to many crash sites where that was true to overlook the possibility. No human can take any sort of hit or be in close proximity to Plexiglas and come out of a high force event unscathed. Would protect my upper cranial region over anything else. Makes for more frontal area but not much. Lately I have been isolating what I call the crash capsule in design. This is a separate area in the center of structure that has different specifications for load paths and G ratings around the pilot. The rest is classic 1.5 yield or 2.0 if composite math. Anyway, just perspective. I really like the revised layout. You are designing this for you. For a 5'10" human it all looks right about on the mark.
     
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  17. Nov 5, 2017 #637

    Hot Wings

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    Yes, they are that. My Quickie is a 44 regular. Took a look at one of your older illustrations and it does seem there is room for that wide a wheel, but it's going to be tight.

    [​IMG]

    I also noted that your CG for the 1/2 VW might be about 2 or 3 inches further forward than it actually will be. The last one I checked the CG on came out right between the rods. Depending on the actual weight of the engine that looks to be only about 150 to 175 inch pounds so it may not make much difference at this stage?
     
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  18. Nov 5, 2017 #638

    Topaz

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    Heh. Love the RF-4 picture! :gig:

    The engine drawing is a recreation of the rather basic one I got from Scott Casler at Hummel Engines. I have to admit that, as an "installation drawing", it's not really what I'd hoped for. I wish I could get a better one, but that appears not to be in the offing. What looks like an extension shaft in the drawing doesn't show in photos I've seen of the engine, but instead is case and forward bearing and so on, so there's a lot of metal forward of the cylinders. That's why I dropped the CG where I did. I'm eager for better information if you've got it. As you say, I don't think it'll make a huge difference, but I'm about to launch into the real aeros, stability, and trim analysis, and I'd like to at least have that in as small a ballpark as I can. It's a tiny little airplane.
     
  19. Aug 20, 2018 #639

    Rienk

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    Is this thread/project still moving forward?
    I recall reading elsewhere that another design was taking precedence, but I've been out of the loop for a while, and not sure what's going on?
     
  20. Aug 20, 2018 #640

    Topaz

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    Hi Rienk,

    Nope, no other project has pre-empted this one. I have a little distraction going on regarding an ultra-low-cost "experiment", but that's another matter that doesn't really affect this project, even if that other one ends up building first. It may well be that way. To phrase this as has been said on the forum here a number of times, I'm tired of being a "dreamer" and not a "doer", and the time for that to change is now. Between a new wife, new home, and taking care of an 89 year-old father with Parkinson's, my finances may not even allow this (DS54) project to be built in a reasonable timeframe, so it may have to wait a little bit in favor of something that, while it has a lot less capability, at least exists and flies.

    This project also needs to have some adjustment made to it, because the economics of flying has changed in my little corner of the woods. It's now less expensive for me to get a (much more practical) Sport Pilot certificate than it is to get a self-launch endorsement for gliders, as incredible as that may seem. Doing so would open up the entire world of LSA's to me. But the DS54 has retractable gear, and that would have to be changed to make it qualify as an LSA. I've been turning that over in my head for a while now. I think that's the way it's going to go. Shouldn't affect the performance very much. A very clean, faired, mono-wheel main gear is about as low-drag as you can get and still have a tractor prop.

    So "please stand by".
     
    Last edited: Aug 20, 2018

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