(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