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

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Vigilant1

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Everyone quit working on the Beetlemaster ?

Dan
Still here, but wrestling with my taxes and those of my MIL. Uggh. On the Beetlemaster, next on my list is to better understand the props/propulsion end of this. I have a copy of Jan Carlsson's software and have started doing some "what-ifs" with it. It appears to be very powerful, but we're asking it to do something that isn't straightforward ("Give us the best 150 KT props that will also give us 200 lbs of thrust each at 70 knots"). I think Jan has already been down this road for us, but I'll understand the compromises better if I see some examples.
Not unlike the taxes, in a way. Some people bring all their receipts to their tax guy and let him give them back the completed return. But by agonizing with the tax software myself I get to see how the system works and (maybe) how to do things smarter in the future.

Mark
 

Pops

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Still here, but wrestling with my taxes and those of my MIL. Uggh. On the Beetlemaster, next on my list is to better understand the props/propulsion end of this. I have a copy of Jan Carlsson's software and have started doing some "what-ifs" with it. It appears to be very powerful, but we're asking it to do something that isn't straightforward ("Give us the best 150 KT props that will also give us 200 lbs of thrust each at 70 knots"). I think Jan has already been down this road for us, but I'll understand the compromises better if I see some examples.
Not unlike the taxes, in a way. Some people bring all their receipts to their tax guy and let him give them back the completed return. But by agonizing with the tax software myself I get to see how the system works and (maybe) how to do things smarter in the future.

Mark

I'm not so worried about getting every Kt in cruise as much as a decent single engine climb so I'll sacrifice some speed to get the SE ROC that I can live with.

I would like to build an aluminum set of wings for the JMR after I get the phase 1 flying over so I need to get back on the design, but been having fun thinking about the Beetlemaster, (what ifs are fun). I think I have enough aluminum left over from the Bearhawks projects for most of the material for the JMR aluminum wings. Pounding out the ribs will be my late summer or fall project and working on assembling the wings next winter.

So many projects and not enough years.

A good tax person and a good CPA is worth every dollar. It's all over my head.

Dan
 

lr27

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Has anyone mentioned scimitar props in this thread? Might be handy to adjust pitch without mechanical complexity. I'm thinking composite materials could mean the ability to tailor the stiffness and have enough strength that fatigue needn't be a problem. Might be tricky to design, though I suppose, since a prop blade is small, you could fiddle around with an actual test piece to get the stiffness right. It's not intuitive to me how altitude would enter into this.
 

Vigilant1

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I'm not so worried about getting every Kt in cruise as much as a decent single engine climb so I'll sacrifice some speed to get the SE ROC that I can live with.
I can see the logic in that. I want to see, at least theoretically, what can be done with simple wood props and a clean design.

I know it is different in many ways from the Beetlemaster, but Rutan's Defiant used fixed-pitch wood props, 320 total HP, and got 300 FPM SE ROC as well as a 184 kt cruise. We're at 58% of the Defiant's weight (Beetlemaster 1700 lb MTOW, Defiant =2950 lb) and 50% of the Defiant's installed horsepower. Now, the Defiant's "per lb" profile drag will be lower than the Beetlemaster (skin area goes up slowly with additional volume/weight), so the Beetlemaster won't see 184 knots, but that plane was evidently making some good thrust at high speeds using props also capable of good thrust at SE climb speeds.

I've also been thinking about the fuselage construction. For a welded tube structure, I'm pretty sure CF/epoxy skins will be heavier than .028 AL. But, I wonder if the ease of incorporating stiffeners/edge treatments (for doorframes, etc) might make simple-curved CF/epoxy skins a good choice anyway.
 
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Vigilant1

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Has anyone mentioned scimitar props in this thread? Might be handy to adjust pitch without mechanical complexity.
We did mention them indirectly. Lonnie Prince's "Q-Tip" props behave similarly, and there's quite a bit of experience with them on VW engines, guys say they do what he claims: All the top speed promised by the pitch number, but better climb due to slightly lower pitch when heavily loaded.

Prince also has a reputation for being willing to work with new designs until everyone is happy.

A reliable, economical prop that provides even a bit of pitch change could be useful for this design.
 
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Pops

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IF I was making a kit of the Beetlemaster I believe I would use CF skins on the fuselage over the tube frame. 30" wide firewall front and rear with straight sides. The only curve would be the curve wrap around the bottom of the fuselage and the top at the windshield, no compound curves. Also it would be easy to do in aluminum. But it would be nice to have the door frames and windshield lip molded in the skins. When using aluminum the frames would be riveted to the skins. On a one off build, I would use aluminum and in a kit CF. I would also make CF front and rear cowls from CF in a kit but one off I would just build from foam and fiberglass as I have done in the past.

The designer of the Bearhawks , Bob Barrows has made a composite prop that he is using on the Bearhawk LSA with the stroked C-85 engine. He told me that the performance is about the same as the aluminum prop but with a good weight saving. I don't think he tried to design the prop for any twist at different rpm's.
 

Vigilant1

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1588822524557.png
(Real Skymaster above, not the Beetlemaster!)

What? Resurrecting this thread after a 3 year break?! Apologies in advance, but since the idea/concept is still rattling around in my head, and because I have some more questions/ideas--here it is again. :)

Summary: For those who missed/forgot it, here's a quick catch-up/summary:
The OP for the thread hit the major points: Explore the idea of using two (relatively cheap) VW-based engines (fixed-pitch props) in a centerline thrust configuration to build a practical, inexpensive traveling airplane—one that that can continue to fly safely after an engine failure. I think the general consensus we came to is that:
1) Such a plane could have good utility (payload, climb rate, cruise speed) and be economical to operate (chart below).
2) Achieving acceptable single-engine performance is the most challenging aspect of the idea, and drives major design choices (e.g. max gross weight, prop pitch to give acceptable SE climb capability, etc). Achieving safe single engine climb on 75HP and at a max gross weight sufficient for two passengers appears to be practical, but would require careful attention to structural efficiency (to keep empty weight in check) and aerodynamic efficiency (to reduce parasite drag and especially to reduce induced drag at climb airspeed).

Three sub-variants of the Beetlemaster were proposed by Pops, Autoreply, and me. Jan Carlsson also provided a lot of very useful assistance, and proposed a design similar to the one Pops came up with. Jan is missed a lot, and his many helpful (and funny) contributions to this long thread helped answer a lot of questions.

The chart below covers the (last heard) dimensions, weights, and other attributes of the three proposed planes and some estimated performance parameters based on rudimentary drag estimates and thrust estimates produced by Jan’s software.
Vigilant1 (Mark)
Pops
AutoReply
Span​
35’​
34’​
45”​
Wing Area​
126 sft​
140 sft​
126 sft​
Length​
Approx 23’​
24.55 ft​
Approx 24’​
Target empty weight (lb)​
900​
850​
840​
Target Max TO weight (MTOW, lb)​
1600​
1500​
1890​
Max Fuel (Gal/lbs)​
50 gal / 300 lbs​
33 gal / 200 lbs​
43 gal / 260 lbs​
Payload with full fuel​
400 lbs​
450 lbs​
790 lbs​
Seating​
2 (?SbS, 43”?)​
2, tandem​
2 + 2​
Engine make/displacement​
2 x Casler 2387cc (94mm bore, 86mm stroke)​
2 x Revmaster 2331cc (94mm bore, 84mm stroke)​
VW​
Installed Power (Continuous HP)​
2 x 80 HP @ 3400​
2 x 80 HP@ 3200​
80 HP​
Propellers​
57”x 48.5” wood, fixed​
57” x 48.5” wood, fixed​
Cabin construction​
4130 + CF​
4130 + AL skin (CF?)​
composite​
Wing construction​
Composite​
AL, strut​
composite​
Boom/tail construction​
Composite​
AL​
composite​
Estimated Values:
Stall Speed (clean, assumed 1.26 cL, @ MTOW). Sea Level​
55 kts​
52 kts​
59 kts​
Fuselage pod wetted area​
125 sft​
138 sft​
155 sft​
Wing wetted area​
238 sft​
262 sft​
230 sft​
Boom and tail wetted area​
98 sft​
116 sft​
106 sft​
Skin friction coefficient used​
0.0058​
0.006​
0.005​
Landing gear, struts, etc (sft)​
0.41​
0.47​
0.41​
Acft Equiv flat plate drag area (sq ft)​
3.1​
3.5​
2.9​
Acft Equiv flat plate drag area w/1 stopped prop (sq ft)​
4.0​
4.5​
3.8​
Drag (sea level): (lbs)
(note MTOW)
60 Kt, Induced at MTOW (lbs)​
72​
63​
76​
60kt, parasite, 2 eng​
38​
38​
35​
60 kt, parasite, 1 stopped propeller​
49​
49​
47​
Total drag at SL, 60 KT
60 kt, 2 engines​
109​
101​
111​
60 kt, one stopped prop​
120​
112​
123​
Drag (at 6000’ MSL)
130 kt induced drag at MTOW​
18​
16​
19​
130 kt parasite, 2 eng​
148​
169​
137​
130kt parasite, 1 stopped prop​
194​
215​
156​
130 kt total (2 engine)​
166​
185​
156​
130 kt total, 1 stopped prop​
212​
231​
175​
60Kt, Induced at MTOW (lbs)​
82​
73​
87​
60 Kt, parasite, 2 engines​
31​
36​
29​
60 Kt parasite, 1 stopped prop​
41​
46​
39​
60 kt total, (2 engines)​
113​
109​
116​
60 kt total, (1 stopped prop)​
123​
119​
126​
Estimated Rate of Climb (at MTOW) (FPM) (see below conditions and climb rates)
Thrust assumptions -->>
Each engine 75 HP@3300.
Prop 57”D x 48.4”Pitch SL, 60 KTAS: 210 lbs thrust SL, 130 KTAS: 154 lbs thrust 6000’ MSL (60.3 HP), 60 KTAS: 170 lbs thrust
6000’ MSL (60.3HP), 130 KTAS: 121 lbs thrust
SL, 2 engines, 60 KTAS​
1180​
1291​
993​
SL, 1 engine, 60 KTAS​
342​
397​
280​
6000’ MSL, 2 eng, 60 KTAS​
861​
935​
720​
6000’ MSL, 1 eng, 60 KTAS​
178​
206​
141​
6000’ MSL, 2 eng, 130 KT (150 MPH)​
288​
231​
276​
Estimated econ cruise performance (400 lb payload, full fuel, 130 MPH TAS at 6000’ MSL)​
Power required (HP, % of installed power)​
59 HP, 37%​
63 HP, 39%​
54 HP, 34%​
Fuel burn rate (gal/hr @ 0.5 lbs/hp)​
4.9 GPH​
5.2 GPH​
4.5 GPH​
Range (statute miles)​
760 sm​
705 sm​
1125 sm​

A lot to like in that chart. Good normal climb rate (note 75HP assumed), and low fuel flow at econ cruise. At an altitude of 6K MSL and moving at 150 MPH, Jan's software estimates each engine will need to put out just 65% of rated power (52 HP). They'll live cool, happy, long lives . . .

Note: There are lots of assumptions in the cals behind the chart. The biggest "gaps" may be in off-design propeller performance (more on that in later posts) and aerodynamic drag. The calculations assume aerodynamically clean airframes, but nothing unachievable (see above. The equivalent flat plate drag areas of 2.9 (AutoReply) to 3.5 sq ft (Pops) compare to published figures of 2.32 sqft for an RV-6A and 4.4 sq ft for a Cessna 150).

Thoughts? We've covered a lot in 320 posts (so far), but the floor is still open. Here are some areas I've been thinking about and maybe we can kick around :
1) Tail Configuration (twin booms vs single low boom). I love the twin booms (signature look of the Skymaster!), but the load paths, construction details, and especially the long and tortuous control linkage runs from stick/ruder to the tail surfaces deserve a lot of thought (in my opinion). We might be really glad to have a single low boom under the prop--there appears to be room, but it's not without its own challenges.
2) Factors pro-con for side-by-side seating. I know Pops wants a tandem layout, Autoreply wants 2+2. I'm on the fence between a tandem (sporty!) and side-by-side (it's a date!) 2 seater. Frontal area is greater, but wetted area is less, weight will probably be less, and it appears divergence/convergence angles of the airflow need not be very great. It will likely boil down to where the wing needs to go (and resultant visibility restrictions). Which brings us to . . .
3) Weight and balance, component weights. I've got some >very< ugly sketches, but it's hard to be sure exactly where the seats, etc need to go. As discussed previously, it would be best if the pilot's eyes are in front of the leading edge.
4) Factors in the choice of a suitable VW engine (Revmaster, Casler, Great Plains, etc). Any unique factors to consider if there will be two aboard?
5) Construction type and methods
– Composite wings? Impact on weight, drag, performance, cost, and build time
– Speeding the build: Adapting parts from other designs? Cowlings, etc.

Sorry to intrude again with this topic. Designing and building the Beetlemaster would be huge task, but it seems like it might be a practical, unique, and fun airplane.

Mark
 

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tdfsks

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A twin push pull VW design has been built - the Ganzer Gemini from 1982.

Whilst not in the configuration of a C337 but it might provide some idea of the likely performance - especially on one engine.

This aircraft used 2 x Type IV VW engines.

Here is an article on the Gemini from Homebuilt Aircraft magazine way back in December 1982.

IMG_8603.JPGIMG_8604.JPGIMG_8605.JPGIMG_8606.JPGIMG_8607.JPGIMG_8608.JPG
 

BJC

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I’m pleased that the thoughts in this thread still live.

Some random comments, which, as usual, depend on the intent of the design:

While there are aero benefits to a 45 foot span, that is problematic for most hangarage options. I like the idea of being able to utilize a standard T hangar without having to remove wing tips.

I don’t need more than about 4 hours plus VFR reserve of fuel.

I know that the focus is on the VW engine, but If I were designing, I would try to have a design that would accommodate the Rotax 912 and the Corvair.

If designing with the thought of selling plans, I would opt for 2 seats, side by side.

I might have missed it, but what is the TO distance over a 50 foot obstacle?


BJC
 

Vigilant1

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I might have missed it, but what is the TO distance over a 50 foot obstacle?
Nope, you didn't miss it, I was just too lazy to calculate it. :) Actually, I don't have good numbers for the expected static/very low speed thrust, and that will affect the result a lot. But, lacking real info, here's a wag:

What's the thrust from zero to rotation speed (approx 70 MPH)? Assume 150 installed HP for the Beetlemaster.
- A rough rule of thumb for static thrust of small airplanes is 3-4 lb/hp. That would be 450-600 lbs. I think that's high, FWIW.
- Like a lot of calculators, Jan's software doesn't try to provide thrust info at very low speeds (in his case, his estimates start at 40 MPH). At 40 MPH with these props, his software gives us 242 lbs. It is 570 lbs at 70 MPH.

So, as a very rough WAG, I'll use 325 lbs of average total thrust over the span of 0 to 70 MPH. For a 1600 lb airplane, that gives us an acceleration of 6.5 f/s^2 (or 1.99 m/s^2, or .2 G). So, 840 feet to reach 70 MPH.

Climb rate is about 1200 FPM, so 20 FPS, so about another 2.5 seconds (250 feet) to reach 50 feet altitude. That gives us about 1100 total feet from brake release to 50 feet.

Sanity check: I think this WAG is probably too pessimistic, likely a problem with my prop thrust estimate. A Cessna 152 weighing 1670 lb and having 108 HP installed takes 1390 ft at 20 deg C. (according to the POH). The C-152 has about the same wing loading and span loading as the described Beetlemaster, and weight to HP is about 15 lb/HP compared to about 10.5 lb/HP for the Beetlemaster.
 
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Vigilant1

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A twin push pull VW design has been built - the Ganzer Gemini from 1982.

Whilst not in the configuration of a C337 but it might provide some idea of the likely performance - especially on one engine.

This aircraft used 2 x Type IV VW engines.

Here is an article on the Gemini from Homebuilt Aircraft magazine way back in December 1982.
Thanks much for the article on Ganzer's Gemini. We'd touched on it earlier (this post), but didn't have anything nearly as detailed as the information you provided in that article.
He claims each of the 2000cc Tp IV engines was giving him 65 HP at 3100 RPM and 75 HP at 3600 RPM. At his 1700 lb weight, the fact that he was able maintain level flight at full gross weight at 6000'MSL is something. I'd love to know what props he used--clearly he had some fairly coarse pitch (favoring speed over climb) if he wasn't able to reach 3000 RPM under static conditions with a 75 HP engine.
FWIW, the Rutan Defiant (two 160 HP O-320s) used a 60" dia x 70" pitch prop in front and a 61" dia x 72" pitch prop on the rear. With a 216 MPH top speed and the ability to climb at almost 400 FPM on one engine, it is concrete evidence that fixed pitch props can give capable (not optimal) service over a wide speed range. But it takes the right airframe. And I'm certainly no Burt Rutan!
 
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Pops

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The BeetleMaster is always in the back of my mind, think about it often. When you get a power loading of in the 10's or less, I start getting a smile on my face.
I still like 2 Revmaster engines. Lighter than the Subs. Can buy 2 engines for about the cost of one R-912.
I still like the C-337 airframe layout. Still like the use of 4130 steel tube with CF shells for the fuselage. I would like to use CF wings with fuel in the outer wing panels outboard of the booms. Just not enough room in the wings between the fuselage and booms for all the control cables for ailerons, elevators, rudders and flap cables or pushrods and any usable size of fuel tanks.
Designing the CF wings is over my pay scale. That is why I was think of aluminum wings and booms and tail surfaces. I would rather all the airframe be CF except for the crash protection of the steel tube fuselage frame. Bob's use of simple molds has made me realize the CF fuselage shell molds would be very easy.
Thinking my previous 850 lbs EW would never happen, but we can dream. Changing that to 900 lbs and still have to watch every oz.
Thank you for starting this thread up again.

Dan
 
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Vigilant1

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I still like 2 Revmaster engines. Lighter than the Subs. Can buy 2 engines for about the cost of one R-912.
Yes all the (uncertified) VW engines are a bargain by "airplane" standards--to buy and to maintain.
I do like a lot of things about Revmaster and their R2300 (2331cc actual). They've been at this a long time and they have a well refined product. They check their suppliers and don't sell substandard stuff. Dave Thatcher likes 'em, it's what he specifies for the CX5 .
But they want $9,400 for the engine (maybe--the page hasn't been updated since 2012). The engine (built, test run, and checked) comes with the Revflow carb, which wouldn't be my choice. So, if I'm willing to spend that kind of money, I'd ask Revmaster to put a Zenith on it and test it that way. And we'd still have a lot of "if they break, got to get 'em from Revmaster" parts (alternator, ignition, heads with the crazy porthole, etc).
Scott Casler's price is $7,200 for a 2387cc engine (with electric system, starter, dual ignition). It's got a magneto (can be hand propped). My impression is that the Force One hub and bearing along with the cranks he uses (from/same as Great Plains?) has been as reliable in service as the Revmaster. The engines weigh about the same. The $4000 difference (for $2000 x 2 engines) is significant. I sure don't think the Revmaster is a bad choice, though.
And, of course, a builder on a tight budget can still build a VW engine up from parts.

Still like the use of 4130 steel tube with CF shells for the fuselage. I would like to use CF wings with fuel in the outer wing panels outboard of the booms. Just not enough room in the wings between the fuselage and booms for all the control cables for ailerons, elevators, rudders and flap cables or pushrods and any usable size of fuel tanks.
Designing the CF wings is over my pay scale. That is why I was think of aluminum wings and booms and tail surfaces. I would rather all the airframe be CF except for the crash protection of the steel tube fuselage frame.
What if...there was a "save 9 months" quick-build kit for the main Beetlemaster wing panels (outboard of the booms)? The Thatcher CX5 wing panels are almost 13' long with a wing area of about 60 square feet. They (with the control surfaces) have a Vne of 155 MPH, and are stressed for a 1320 lb airplane (it might work fine for a 1600 lb plane with 3 more feet of inboard wing on each side). If you built the Beetlemaster wing inboard of the booms (to get your 34' span and 140 sq ft of wing are) and the booms themselves, these could plug in outboard of that. The complete CX5 wing kit is $6000 (more here, with the info on the source). Oh--they are built to accommodate a 10 gallon fuel tank in each one. One possible complication--the CX5 has dihedral that a high-wing Beetlemaster wouldn't need. From looking at the wing CX5 spars, it looks like this may not be a problem as the dihedral is introduced in the short central section between the spars.

Bob's use of simple molds has made me realize the CF fuselage shell molds would be very easy.
Me, too. Maybe not "easy" for me, but something I could accomplish.

I'll bang on later about the tail boom(s), composite wings, etc. No post should be so long that it needs a table of contents.

Thinking my previous 850 lbs EW would never happen, but we can dream. Changing that to 900 lbs and still have to watch every oz.
I made the change in the chart, I'm sure there will be more. Weight is important, for sure. At these weights, if we add 100 lbs of weight we lose about 10 lbs of excess thrust (due to the increase in induced drag). That, plus the need to lift more lbs, decreases our SE climb rate by about 60 FPM. But that might be made up by increasing excess thrust in other ways:
1) Adjust the prop pitch to get 10 more lbs of thrust at climb airspeed. Probably reduces high-airspeed thrust by some amount.
2) Reduce induced drag. More wingspan is the most obvious approach. Increasing the span of your Beetlemaster by 2' at these weights and speeds would increase excess thrust by about 7 lbs. (I'm not advocating for that, just crunching the numbers).
3) Reduce parasite drag. At these climb speeds, parasite drag isn't a lot. But, if you went to a smooth, all-composite Beetlemaster (same size, etc), you could expect to gain 9 lbs of additional excess thrust at this airspeed. And a bigger difference at high aispeed (reduced fuel burn, etc).

Just to mention it--if we could jettison the stopped prop, we'd gain 15 lbs of excess thrust. Woo-hoo! Probably get a letter from the FAA, too.
Thank you for starting this thread up again.
Having some fun! Thanks again for your thoughts.
 
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Pops

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What a great idea ! Hadn't thought about anything but scratch designing and building the wings. Do you know what airfoil is used on the CX5? My drawing has the tail booms at 8' apart. So CX-5, 13' outer wing panels and 4' to the center of the fuselage = 17' for 1/2 span, gives a span of 34' + wing tips of 12" each for a total of 36 ' span.
What is the cord of the CX5 wings. If they have more cord than my wings means more wing area, but I think they are close to what I have on my drawing. My drawing are out in the hanger at this time and its dark outside, and I'm afraid of the dark :) I'll look tomorrow .

Building the steel tube fuselage would be a quick and easy job, (nothing is a JOB when you are having fun). Would take about as much time making and welding all the tabs on for attaching the CF skins and the fitting for wings, landing gear, etc, as cutting and fitting and welding the tubes. I made a little tool for the job from doing all the tabs on 4 Bearhawk fuselages. My drawing for the fuselage gives 96" between firewalls for the fuselage. I have the pilots eyes at the LE of the wing. Rear seat with the belly button ( Center of body weight) at the middle of the CG envelope, with a small baggage area between the rear seat back and the rear firewall. Front seat on rails so it can be slid forward for getting in the rear seat.

IF you have money to spend, buy the Revmasters. Like you, I like the Zenith carb so much, it would have to have a Zenith carbs on it. If that price hurts a little then go with Scott's engines. I think both are good engines. But for 85 HP and 80 continuous HP, for max performance I would go with the Revmaster, if possible.
Or you could always build your own engines for a little saving of dollars, but Scott's engines are a very good buy even against building your own engine. Have you checked the prices of VW mags lately. The Slick mag is gold plated :) Even the surplus Slick mag has increased in price.

Just having fun playing what if.
 

Vigilant1

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Do you know what airfoil is used on the CX5?
I couldn't find that, but it looks like the CX4 uses the USA35B (same as the Cub). The CX5 might be the same. Probably not the best choice.
 

Pops

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I couldn't find that, but it looks like the CX4 uses the USA35B (same as the Cub). The CX5 might be the same. Probably not the best choice.
I also looked but couldn't find it.
 

Pops

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No . I don't like any airplane with high thrust lines. Have no need to operate off water.
 

Vigilant1

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Building the steel tube fuselage would be a quick and easy job, (nothing is a JOB when you are having fun). Would take about as much time making and welding all the tabs on for attaching the CF skins and the fitting for wings, landing gear, etc, as cutting and fitting and welding the tubes. I made a little tool for the job from doing all the tabs on 4 Bearhawk fuselages. My drawing for the fuselage gives 96" between firewalls for the fuselage.
Welded clusters are a thing of beauty, but . . .
I wonder how much extra weight and "fuss" it would be to design and build the tube cabin frame as welded up flat sides and bolted-in crossmembers? It's not something you'd do for your own plane, but if somebody wanted to make parts for sale, shipping flat panels and crossmembers with tabs would be a lot cheaper. Even newby welders building at home would probably like to do all the welding (including any gussets) in 2D on a flat table , then bolt on the crossmembers.
Or, make the main crossmembers (firewalls, etc) as welded flat panels (with diagonals and gussets, as required), ready to bolt in.
The tube frame is a lot more compact (and shipable) without the empenage framing required for a conventional small airplane.
Any guess what the tube frame will weigh?
I'm well out of my depth here . . .
 
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Pops

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My drawing at 8' from firewall to firewall. The basic tube fuselage would be very easy to make and easy to mass produce with a fuselage jig like the jig Dallas and I made for the very complex Bearhawk fuselage. The tube fuselage would be a box with the tubing going up to the wing attach points and also the rear bottom slanted up to the bottom of the lower rear firewall. For the weight, would have to design it and work out the weight per foot of all the tubing. A box about 100" long and 52" tall and 32" wide should do for shipping.
Picture of the Bearhawk fuselage. Also had sub jigs for wing attach , stab attach, etc fittings that bolted on. Enough of the top and front cross pieces unbolted to remove the fuselage the jig out with the fin tail post on. 90 percent of the welding was done in the jig. Just a few places had to be finished on the bottom longeron tubes.fuselage jig.jpg
 

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