Twin-VW engine Push-Pull design idea (The "Beetlemaster")

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AdrianS

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After Christmas, when I have a bit of time, I will work up some rough sketches of my flat-panel volksplane ideas, which may be relevant to the beetlemaster.
I'm currently looking at honeycomb core rather than foam, as it looks like it would be lighter, and won't need shaping anyway. I have decided against moulded-in beams, as the weight saving would be minimal, and simple is best ;).

The only thing that is troubling me at the moment is access inside the tail section for
a) construction (glassing inside corners)
and
b) maintenance / inspection of the rudder & elevator controls.

I intend to design the rear fuselage as a unit, teminating in a ring frame that is bolted to the cabin / wing box section, which means it will be under 2.4m long : is this feasible ?
 

Vigilant1

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After Christmas, when I have a bit of time, I will work up some rough sketches of my flat-panel volksplane ideas, which may be relevant to the beetlemaster.
Great, it will be good to see what you come up with
I'm currently looking at honeycomb core rather than foam, as it looks like it would be lighter, and won't need shaping anyway. I have decided against moulded-in beams, as the weight saving would be minimal, and simple is best ;).
Hmm, the weight of the core material is such a small fraction of the weight of most finished sandwich components that I'm surprised you'll save appreciable weight with honeycomb over 4 lb/cu ft foam. And as this is a Volksplane "for the people", I know cost and availability of materials will be important, which would seem to argue for foam.

The only thing that is troubling me at the moment is access inside the tail section for
a) construction (glassing inside corners)
and
b) maintenance / inspection of the rudder & elevator controls.

I intend to design the rear fuselage as a unit, teminating in a ring frame that is bolted to the cabin / wing box section, which means it will be under 2.4m long : is this feasible ?
I'm not sure why it wouldn't be feasible. Making mating frames and bolt attach points is not the lightest way to make the attachment to the cabin, but it will work and allows easy independent fabrication of the front and back of the plane for those who are building in a small space. If you wanted to get fancy, having the ring frame offers a ready-made way to make the tail foldable which can do wonders to minimize the footprint in a hangar (if it's a trigear) or to allow transport on a trailer (assuming the cabin can be kept to less than about 7.5 feet in length). Fold the tail over the cabin, roll the plane onto the trailer, turn the plane 90 degrees to align the wings with the trailer's long axis using a turntable on the trailer.
Regarding the tail: You could go with a triangular cross section rather than rectangular to save weight and gain from the much greater inherent racking strength of that shape. Make one panel removable and you've solved your inside corner fabrication problem and access issues, though at some weight increase for reinforcement and fasteners.

All the above is offered at the risk of engaging in "plywood thinking".:)
 
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AdrianS

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To be honest, the idea of honeycomb rather than foam was partly for the "bling" factor : it sounds more high-tech ;).
The choice of core material is a fairly minor detail, to be honest.
 

Vigilant1

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An interesting piece written by one of the members of the design team for the Cessna 336 (Skymaster). (Link here) Unfortunately, the place where I found it didn't do a good job of documenting the original author or publication. Some points of interest and possible utility as we consider the Beetlemaster:

Layout: The Original Skymaster (Model 336) had fixed gear and not many were sold. The follow-up model (the 337) had retractable gear and was more popular.
Acording to the article, Bill Lear convinced the Cessna designers to go with the fixed gear, because only rich guys with not a lot of time to fly for proficiency would be buying a twin, and a fixed gear made more sense for them. Buyers disagreed, and so they later went with the retracts. The author believes that, if they'd started with the idea of retracts they probably would have made the design a low wing (to have an easier spot to stow the landing gear and for looks).


A low-wing, twin-boom Beetlemaster with retractable gear would be a sharp-looking plane—a bit like an Adam A500. But, the retracts are just too heavy for a Beetlemaster, the drag reduction at these speeds isn’t worth the cost in complexity, weight, or money. So the high-wing makes more sense. Dang!

SE Performance: As Dan (Pops) mentioned, significantly better performance on the rear engine than on the front. During the design and building of the prototype there were bets as to how this would turn out, and the chief engineer at Hartzell Propellers believed the rear engine would provide better climb performance because of "the lack of blockage behind it, the prop is 2 inches longer, and the inflow air velocity is favorably reduced, and when it is running it promotes better airflow attachment to the bluff afterbody cowl". As it turned out, the Cessna 336 climbs 24% better on the rear engine than on the front.

Rear engine cooling
: Proved to be a challenge, and this would probably be info applicable to the Beetlemaster. The Cessna team wound up mounting a small 20-bladed fan with tip angles of 25 degrees onto the crank to push air out of the rear cowling. The fan absorbed just 3 HP on the Model 336 (a VW-powered plane would need an even smaller one, probably). The Model 336 intake air came primarily from the scoop atop the wing, but the article indicates that the underside of the wing where it meets the fuselage might be just as good or better.

Wing Struts/control cables: Struts were used in order to save weight (remember, designers didn't put high cruise speed high on their priority list), but also to provide a convenient way to route control cables from the cabin to the tail booms and then to the empennage. The rudder cables formed a complete loop (pedals, up the left strut, down the left boom, control horn on the left rudder, cable through the elevator to the right rudder, cable down the right boom, and down the right strut to the pedals). The elevator controls apparently started with something similar, but was redesigned to have all the cables go down a single side/boom, which eliminated 7 pulleys and reduced the original unacceptable amount of friction.

Wing Structure: From the article:
“The wing is of a conventional two-spar design with the front spar at 20% chord and the rear spar at 60% chord. The brace strut intersects the front spar at the boom attach point. Main fuel tanks are installed outboard of the booms (between the spars), and optional auxiliary tanks are installed inboard of the boom. The induced high torque from tail loads is transmitted through a cell structure consisting of the two spars, a large torque rib at the root, and the auxiliary fuel tank skin assemblies. Normal wing torque loads are carried by a torque rib at the outboard end of the main fuel cell area, an immediate torque rib about midway in the fuel tank region, the torque rib at the strut intersection, and the upper and lower skins.
As one can see, these torque-resisting cells had to be designed not only for carrying the design loads, but, also, to provide an acceptable amount of rigidity for the empennage.
I can even see where twin wing-to-lower-fuselage struts on each wing would do a good job of supporting and stiffening up the tail boom attachments in the Beetlemaster, but the cost in drag (and appearance) would be there. Best to avoid this, if possible.
[FONT=&amp]
[/FONT] Beetlemaster: If it uses a single wing spar, then obviously another way will need to be found to handle the tail torque loads.

Airfoil/wing planform:
From the article:
“Since high speed was not a top priority, we used a large wing area of 202 square feet and on aspect ratio of 7.2. A NACA 2412 airfoil was used at the wing root and boom, tapering to a 2409 section of the tip. The wing was fitted with powerful flaps located outboard of the twin booms. These were 30% of wing chord slotted flaps that were 8 feet in length on each side. In contrast, the Frise type ailerons were only 4.75 feet in length with a 25% chord width. This was to be an airplane capable of operating from rather small and rough grass fields.”
Although the prototype design had no flaps inboard of the booms due to concerns that flaps there might disrupt air to the rear prop and cause problems, this concern proved to be unfounded. They added flaps inboard of the booms later because, without them, there was no flap-induced downwash over the tail to counter the normal pitching-down tendency of the main airfoil when flaps are deployed, and this caused some significant control pressures/issues. This is probably a lesson applicable to the Beetlemaster: Flaps inboard of the booms should be considered. As previously mentioned, I think split flaps would be worth trying in this application for several reasons.

Other: The article contains other observations on elevator control and pitch trim issues that are applicable to the Beetlemaster. Also, in a crash of considerable forward velocity, the rear engine mount was designed so that the engine would swing down rather than smash forward through the firewall—and actual results in crashes showed that this worked well. That’s a zero-cost, low-weight feature that could find its way into the Beetlemaster.


Overall, a pretty interesting article and a rare view inside the design process of this unusual airplane.
Happy 2015 to all!
 
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Pops

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Good to see the Beetlemaster was not forgot about. I like the high wing Beetlemaster over a low wing version. The low wing version would look like the Ion Aircraft with a front engine added.

Ion Aircraft

Friend of mine has a local company that is building a Ion carbon fiber prototype and productions molds. I stopped by his place about 4 weeks ago. Beautiful work.

Ion Project | Carbon Fiber Composites

I agree, the air intake for the rear engine should be under the trailing edge of the wing at the high pressure area. Also would use wing struts.


About the Cessna 337. In doing the 100 hr, you get to thinking that it's a flying inspection panel, but they all are needed.

Definitely flaps inboard of the tail booms.

When are we going to start the test flying :)

Dan
 

Vigilant1

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Good to see the Beetlemaster was not forgot about. I like the high wing Beetlemaster over a low wing version. The low wing version would look like the Ion Aircraft with a front engine added.
Yep. The Adam A500 has the push-pull twin layout, twin booms, and a low wing. I think it looks good, but practicality argues for a high wing on the Beetlemaster.

I agree, the air intake for the rear engine should be under the trailing edge of the wing at the high pressure area. Also would use wing struts.
So, given the need to support the tail booms and prevent their racking, maybe go with twin spars (at least out that far)? Also, about those struts: Two per wing (to a single point under the fuselage) would provide support and rigidity to the tail booms, and the struts would provide a convenient route for the control cables (like on the 336). I'm not sure about the drag penalty and how much they will reduce the weight of the spars. If possible, think it would still be advantageous to be able to fold the wings back just outboard of the tail booms (hangar space is getting to be pretty dear in many places), and that limits the outside of the booms to 7'6" apart if it is to go on a trailer or inside a standard shipping container. Possible? With a 24-30" wide cabin, that means about 30" of wing per side to the fold point/struts, which isn't very far, and there will be about 14 feet of wing outboard of that point, so the support offered by the lift struts is limited (compared to what it would be if the struts attached farther out). If the flaps stop there (which would simplify the wing folding), they'll need to be a fairly large % of the wing cord to have an impact on descent angle, etc.

About the Cessna 337. In doing the 100 hr, you get to thinking that it's a flying inspection panel, but they all are needed.
Yes, just the bellcranks and pulleys would be a lot, and I've heard stories about access to the rear engine and the landing gear components. It will be interesting to see how the height of the rear engine on a Beetlemaster would impact maintenance chores--getting to the stuff on top would require a step stool or short ladder. It would be handy if as many ancillaries as possible could be moved underneath.

When are we going to start the test flying :)
There are big and small Skymaster R/C models. I wonder if useful information on power requirements, trim issues, weight, SE climb, etc could be gleaned from a Beetlemaster model of the right size and power. I know it has been done with other airplane design efforts, but the Reynolds/scale effects sometimes swamp the info of interest. I'll bet EAA has some info on this.
 
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autoreply

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Interesting read Vigilant, thanks for sharing.

I didn't think of the effect of not having inboard flaps on induced flow over the tail. For my design (single-engine pusher) I so far ditched them, given their small span (15" or so) and the effect on drag/cruise since the flap hinge line will trigger a thicker boundary layer going directly in the rear prop.

I would also still fancy a high/shoulder wing. Getting the controls to get to the mid-wing is certainly more complex. From there however, I don't see the complexity. Have a torque tube in the stub wing and a conventional pushrod out to the tail surface. With an inverted V-tail (yep, there it is again), you actually have less complexity.

Don't agree about a low wing having better looks either. See many sailplanes and the Extra 400/500, though opinions there certainly vary alot. A high wing also makes the rear prop inflow WAY better. A high wing can have a neat flow field, a low wing will be a messy inflow in the rear prop, hurting cruise speed, climb and noise.
 

cluttonfred

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I am going to buck the trend and say that I ***love*** the low-wing alternative suggested (against his will) by Pops. ;-) Side-by-side seating right on the CG, a nice canopy, push-me-pull-you VW engines, fixed tricycle gear, would be great fun. If you keep the tail booms above the wing bottom surface (keeping them as narrow as possible to minimize the effect on airflow over the wing) then you could use continuous split flaps across the whole wing and fuselage bottom from aileron to aileron.

As an aside, that Ion Aircraft design looks great and I like the generous vertical tail surfaces and there are some interesting design choices being made to keep the cost down, notably the bonded aluminum wing. Composites are not usually my thing, but I could be tempted by something like that.
 

Pops

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A low wing BeetleMaster, Parish the thought. We would have to change the name, and you know we can't do that, because it's written in stone already. Think I'm getting sick. :)

Dan
 

Teogib

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Interesting thread. I've not posted to a homebuilt forum since the RAH days.

Every few years I kick around a VW push-pull design in my head. I'd offer the following ideas for the record:

High wing, push-pull, two-seat, side by side.
-Wings: RV-7 or RV-9 wings/centersection/tanks. Not braced.
-Booms: Aluminum, stressed skin. How the #$@# to attach to the wings? Attach to a side-by-side fuselage?
-Fuselage: Crude aluminum box structure similar to BD-4.
-Horizontal stab: Aluminum. Constant chord.
-Vertical stabs/rudders: Aluminum. Similar to Aircoupe.
-Gear: Fixed Grove mains. Nose similar to Zenith 801.
-Engines: Two Corvair, VW, Subaru, Viking, ?

Performance hopes: I would be satisfied with cruise of 125mph, stall under 60mph, and ability to maintain altitude to a density altitude of 6000 feet on a single engine.

One can dream....
 

Pops

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Interesting thread. I've not posted to a homebuilt forum since the RAH days.

Every few years I kick around a VW push-pull design in my head. I'd offer the following ideas for the record:

High wing, push-pull, two-seat, side by side.
-Wings: RV-7 or RV-9 wings/centersection/tanks. Not braced.
-Booms: Aluminum, stressed skin. How the #$@# to attach to the wings? Attach to a side-by-side fuselage?
-Fuselage: Crude aluminum box structure similar to BD-4.
-Horizontal stab: Aluminum. Constant chord.
-Vertical stabs/rudders: Aluminum. Similar to Aircoupe.
-Gear: Fixed Grove mains. Nose similar to Zenith 801.
-Engines: Two Corvair, VW, Subaru, Viking, ?

Performance hopes: I would be satisfied with cruise of 125mph, stall under 60mph, and ability to maintain altitude to a density altitude of 6000 feet on a single engine.

One can dream....
Thanks for jumping in with your ideas. Whats life without dreaming? But I think this dream is possible. Dan
 
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Vigilant1

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Teogib,
Welcome to HBA, thanks for chiming in.

Every few years I kick around a VW push-pull design in my head. I'd offer the following ideas for the record:

High wing, push-pull, two-seat, side by side.
I'd like to see the performance comparison of a well-designed side-by-side compared with tandem. The side-by-side is the more popular configuration in general (if Van's sales numbers are an indication), it might save some money (one set of instruments), and it would offer a chance for adding a rear seat >IF< the SE performance allowed the weight. But it takes away from the tandem sportiness (think OV-10) and would need to have a carefully designed "tailcone" fairing to reduce possible low pressures back there and get air to that engine.

-Wings: RV-7 or RV-9 wings/centersection/tanks. Not braced.
-Booms: Aluminum, stressed skin. How the #$@# to attach to the wings? Attach to a side-by-side fuselage?
If our preliminary estimates are right, a single VW trying to keep this design aloft and be able to manage a climb will need approx 140-150 sq ft of wing and considerable span (36-38', to reduce induced drag). The RV-9 has a 28' span and 124 sq ft, so it probably wouldn't do the trick. But I like the idea of using someone else's work (and maybe parts/jigs). Even if the RV-9 (or Thatcher CX5--same span and area, probably built a little lighter) wing were "plugged in" to 24" stub wings, it would still probably leave us a little short of span. But just a little--maybe it would work.

Tail booms on the fuselage sides would need to be low (at the bottom corners) to make room for the rear prop disc. At that point, it would probably save weight and complexity to use a single larger boom from the center (bottom) instead.

Mark
 

etard

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AAARRRGGHHH!!!

13 pages and not one simple napkin sketch! I must admit that I have learned a few things reading this thread, but dang can we get a simple CAD drawing or something?

I like the idea of a the push/pull, but I was hoping that it would look more like an actual VW Beetle, I'll take mine in seafoam green with a ragtop like my 63'.
 

Pops

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AAARRRGGHHH!!!

13 pages and not one simple napkin sketch! I must admit that I have learned a few things reading this thread, but dang can we get a simple CAD drawing or something?

I like the idea of a the push/pull, but I was hoping that it would look more like an actual VW Beetle, I'll take mine in seafoam green with a ragtop like my 63'.

Or the red 1956 that I used to have. Also had a tan 56.

I did some drawing about 10 years ago, but if they are still here, I could never find them.
Dan
 

Himat

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AAARRRGGHHH!!!

13 pages and not one simple napkin sketch! I must admit that I have learned a few things reading this thread, but dang can we get a simple CAD drawing or something?
The best I could do with MS Paint in a hurry, The WaterBeetlemaster:
WaterBeetleMAster.jpg
 

Pops

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Vigilant1-- I just came to realize a certain element on this site is just not going to be of any help on designing the BeetleMaster :)

Dan




Added -- Can I defect and give them a "Like" ?
 

Vigilant1

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Vigilant1-- I just came to realize a certain element on this site is just not going to be of any help on designing the BeetleMaster :)
Hey, Madison Avenue says any publicity is good publicity. Delta's designs look cool--with eyes and a happy face smile, and some ladybug spots I think they'd be even better. And, if earth's gravity is ever reduced by 60% and if our atmosphere ever "thickened up" appreciably to a rho of about .004 slugs/ft3, I think they'd produce good climb on one engine.:)

Mark
 
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