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Thread: "Micromaster"-- Centerline twin using small industrial engines

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    Re: "Micromaster"-- Centerline twin using small industrial engines

    Quote Originally Posted by blane.c View Post
    If you put the CG 100 feet above the plane, or below the plane and you put the plane in a bank the weight that far on a arm doesn't effect the stability?
    Nope. The lift acts through the CG, and gravity, by definition, acts through the CG. The forces both act on the same point, so there's no couple to create a torque around any axis.
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry David Thoreau

    Design Project: Conceptual Design of an "Inexpensive" Single-Seat Motorglider
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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    Re: "Micromaster"-- High or low wing? Tailboom?

    Tailboom: For now, I'm leaning toward a single low tailboom from the bottom of the fuselage pod. There's still room for a 47" prop above that. The only fly in the ointment might be the problem of getting the tail high enough to permit TO rotation/landing flare. I'll have to do more figuring on that. For drag reasons (and aesthetics) I'd like to avoid needing to slope the boom up in relation to the fuselage pod.

    I think the high or low wing choice for this plane will need to consider most of the familiar factors and a few unique ones.
    In-flight and boarding comfort/convenience: Edge goes to the high wing, IMO. It is nice to be able to able to get in/out and stay dry in the rain,
    boarding in the shade when it is sunny and hot. A low wing needs a wing walk, etc.
    Wing folding/removal: IMO, the low wing has the edge. If this is to be done by one person (important), it will be easier to lift and place/remove a 30 lb wing panel if it is at 18" above the ground than if it is at 5' above the ground. Either way will require some temporary fixtures/cleats to get things lined up and held temporarily.
    Weight: I dunno. I suspect having a low wing will slightly reduce the weight of the airframe, since the concentrated loads of the landing gear, wing spar carrythrough, the seat attachments and the rear boom attachment would all be low and can share much of the same structure, or at least be close together.
    Drag/aerodynamics: A mixed bag:
    -- Low wing planes tend to have lower, more compact fuselages. In a low wing, the spar carrythrough can sometimes be accommodated under the seat or under the knees. In a high wwe have the raised seat and then above the pilot's head we have to add the wing spar depth. Unfortunately, I don't know yet where the wing will need to go in relation to the pilot, I haven't figured W&B yet. It seems at least possible that the spar may wind up behind the seat, which would be great for visibility (high or low wing) and crush zone preservation (low wing)
    -- In theory, there's a higher penalty in drag and lost lift from a low wing than a high wing. Air over the top of the wing moves faster than underneath, so putting a fuselage there has (theoretically) more flow issues.
    -- Propulsion efficiency: I think a low wing has the edge. With a high wing, the air incoming to the rear engine will be "dirty" with the downwash from a wing that bisects the prop disk near its widest point. With a low wing, the rear prop disk will be entirely above the wing.
    -- Aft-body fuselage drag: This is going to be a big factor for this design, especially in single-engine flight using the front engine. In the case of the Cessna Skymaster, they found that the rear prop disk did a good job of cleaning up the flow at the back of the fuselage, the reduced pressure encourage the air to flow smoothly. When the rear prop is stopped, the flow around the back of the fuselage grows a lot more chaotic and drag increases (this is the primary reason the Cessna 337 performs better with the rear prop running/front stopped than the other way around). With a Micromaster design, we'll probably have a low tail boom at the bottom of the fuselage pod (below the rear prop). I think it might be easier to keep the flow around the rear cowling a little cleaner if it were not in the high-speed downwash of a high wing. With a low wing, the rear portion of the fuselage is above the wing downwash.
    Crash Safety: A slight advantage to the low wing. In a ditching situation, a low wing is better. Into rough terrain, structure below the occupant is better than structure above him/her. Chances of flipping over are reduced with a lower CG.
    Ground handling: A low wing has the edge: lower CG, less propensity to be tipped up/over in a cross breeze. But, low wings do increase the tendancy to float on landing due to stronger ground effect. This same thing can help them (a little) in getting weight off the wheels in a soft field TO.
    In-flight visibility: I don't know yet, much will depend on where the pilot's head ends up in relation to the wing LE.
    Fuel location: Not a factor in wing placement. The wings will be removable, and there's no good reason to put fuel in them. These engines don't burn much, a 12-15 gallon tank can be accommodated in the fuselage probably kept high enough to allow gravity flow to front and rear carbs (if they are kept low).

    Anything I missed? I confess to having little/no hard data upon which to base the aerodynamic comments above.
    Last edited by Vigilant1; March 12th, 2019 at 10:00 PM.

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    Re: "Micromaster"-- High or low wing? Tailboom?

    Low wing is better on floats, it shields the rear prop from water spray. On the other hand, the low wing might hit the dock.
    So most float planes are high wing. But a pusher is a different beast.

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    Re: "Micromaster"-- Centerline twin using small industrial engines

    For folding, it depends on if you take the whole thing off, or just fold the outer panels. High wing is just as easy to fold if they fold back and rotate. If it's a strut-braced wing, High is also better as the struts won't mess up the flow over the wing.
    If you go with a single boom, an "H" tail keeps the rudders out of the propwash.

    There are blueprints online for this guy.

    So if you felt lazy they could be scaled down.
    Last edited by Sockmonkey; March 13th, 2019 at 11:34 AM.

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    Re: "Micromaster"-- Centerline twin using small industrial engines

    Quote Originally Posted by Sockmonkey View Post
    For folding, it depends on if you take the whole thing off, or just fold the outer panels. High wing is just as easy to fold if they fold back and rotate. If it's a strut-braced wing, High is also better as the struts won't mess up the flow over the wing.
    I think we'll do okay without struts. If using CF rods, cantilever wing spars for a plane this small are ridiculously light. Per some earlier discussion, 3 lbs of parasite drag will cost us almost one full HP of thrust and will cost us about 47 FPM in single-engine climb rate (the equivalent of adding over 100 lbs of weight), so keeping things "clean" will be important.

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    Re: "Micromaster"-- High or low wing? Tailboom?

    Quote Originally Posted by Vigilant1 View Post
    Tailboom: For now, I'm leaning toward a single low tailboom from the bottom of the fuselage pod. There's still room for a 47" prop above that. The only fly in the ointment might be the problem of getting the tail high enough to permit TO rotation/landing flare. I'll have to do more figuring on that. For drag reasons (and aesthetics) I'd like to avoid needing to slope the boom up in relation to the fuselage pod.

    .
    The CGS Hawk curved the boom.

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    Re: "Micromaster"-- Centerline twin using small industrial engines

    This design should have great visibility up. Like the Corsair doing carrier landings, you may prefer a slight turn on approach versus straight in. You could also just be a tad high so you have room to slip and see what is in front of you.

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    Re: "Micromaster"-- Centerline twin using small industrial engines

    Quote Originally Posted by Sockmonkey View Post
    There are blueprints online for this guy.
    BTW, IMO, that is a good looking plane, probably a hoot to fly. Unfortunately, I don't think the layout would work well for the Micromaster mainly due to the drag of twin booms, wing detach complexity, increased build weight and complexity of the twin booms, and control run complexity. On the control runs: Cessna engineers had a devil of a time reducing the friction and slop in the Cessna 336 control setup. It's all solvable, but the small size of the Micromaster prop (approx 48" dia, so 24" radius) relative to the fuselage pod height (approx 43") makes a single low boom possible, where it wouldn't work with the Skymaster (or the Fokker D XXIII in the picture)

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    Re: "Micromaster"-- Centerline twin using small industrial engines


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    Re: "Micromaster"-- Centerline twin using small industrial engines

    Quote Originally Posted by Sockmonkey View Post
    There are blueprints online for this guy.
    Go on...

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    Re: "Micromaster"-- Centerline twin using small industrial engines

    Quote Originally Posted by Tiger Tim View Post
    Go on...



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    Re: "Micromaster"-- Centerline twin using small industrial engines

    Just because.

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    Re: "Micromaster"-- Centerline twin using small industrial engines

    Quote Originally Posted by Sockmonkey View Post
    Just because.
    Hey, cool! Looks lighter and simpler already.
    I wish the Micromaster's pod could be as nicely tapered as that in front and back, but the approx 20" width of these blocky V-twins is gonna hurt on that score. The prop will only be sticking out approx 14" on each side of the cowling. Well, the Luciole, SD-1 and others are making it work, maybe the Micromaster could do the same.

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    Re: "Micromaster"-- Centerline twin using small industrial engines

    Looks cool. But of course the vertical tail needs to be double size with just the one.

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    Re: "Micromaster"-- Centerline twin using small industrial engines

    Quote Originally Posted by Vigilant1 View Post
    Hey, cool! Looks lighter and simpler already.
    I wish the Micromaster's pod could be as nicely tapered as that in front and back, but the approx 20" width of these blocky V-twins is gonna hurt on that score. The prop will only be sticking out approx 14" on each side of the cowling. Well, the Luciole, SD-1 and others are making it work, maybe the Micromaster could do the same.
    Yeah, you would either need cutouts for the cylinders to poke through, or an extension shaft about a foot long. Are there no cheap inline industrial twins?
    Last edited by Sockmonkey; March 14th, 2019 at 11:58 PM.

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