So, now a look at cruise drag (i.e. thrust required for level flight at X airspeed).

Assumptions:

– Both engines are turning (i.e. no drag from a stopped prop)

– 6000’ MSL standard day

– Aircraft weight of 1600 lbs (a higher or lower weight won’t affect these figures much, induced drag changes little with weight at these airspeeds (Cl's))

The aircraft:

**Pops’ Configuration:** Strut-braced metal wing, two tandem seats. Welded tube fuselage (skins? TBD).

Wingspan: 34’ Wing area: 140 sq ft, Aspect Ratio: 8.24

Total wetted area: 508 sq ft

Skin Friction Drag Coefficient: .0006 (metal construction).

Total zero-lift flat plate drag area: 3.51 sq ft.

Drag at cruise: (Zero-lift drag + induced drag = total lbs) This is the combined thrust needed from 2 engines:

100 Kts: 100 + 35 = 135 lbs

110 Kts : 120 + 29 = 149 lbs

120 Kts: 143 + 24 = 168 lbs

130 Kts: 168 + 21 = 189 lbs

140 Kts: 195 + 18 = 213 lbs

150 Kts: 224 + 16 = 240 lbs

**Autoreply’s Configuration**: Composite construction, long cantilever wing, seating for 4.

Wingspan: 45’ Wing area: 126 sq ft, Aspect Ratio: 16.07

Total wetted area: 490 sq ft

Skin Friction Drag Coefficient: .0005 (smooth composite construction).

Total zero-lift flat plate drag area: 2.86 sq ft.

Drag at cruise: (zero-lift drag + induced drag = Total lbs) This is the combined thrust needed from 2 engines:

100 Kts: 81 + 26 = 107 lbs

110 Kts: 98 + 22 = 120 lbs

120 Kts: 117 + 18 = 135 lbs

130 Kts: 137 + 16 = 153 lbs

140 Kts: 159 + 13 = 172 lbs

150 Kts: 182 + 12 = 194 lbs

**Vigilant1’s Configuration**: Similar to Pop’s but with a slightly smaller, longer cantilever composite wing. Fuselage construction method undetermined.

Wingspan: 35’ Wing area: 126 sq ft. Aspect Ratio: 9.72

Total wetted area: 471 sq ft

Skin Friction Drag Coefficient: .0055 (composite wing).

Total zero-lift flat plate drag area: 3.00 sq ft.

Drag at cruise: (Zero-lift drag + induced drag = total lbs) This is the combined thrust needed from 2 engines:

100 Kts: 85 + 35 = 120 lbs

110 Kts: 103 + 29 = 132 lbs

120 Kts: 122 + 24 = 146 lbs

130 Kts: 144 + 21 = 164 lbs

140 Kts: 167 + 18 = 184 lbs

150 Kts: 191 + 15 = 207 lbs

Comments:

– Previously (Post 278) we had looked at the thrust required for safe single engine climb at 70 kts. In that situation, we found that approx 125 lbs (Autoreply) to 180 lbs of thrust would be needed for safe single-engine climb at 1600 lbs. Looking at the figures above for required cruise thrust, we see that the per-engine thrust output needed, even for airspeeds up to 150 kts (173 MPH) is just 97 to 120 lbs.

So, this is a (maybe obvious) mark on the wall for the total thrust that would be needed for cruise. The unanswered question remains: How much thrust is possible at 130-150 KTS from two fixed-pitch props driven by 80 HP VW-based engines

__IF__ each engine must also be able to produce 125-180 lbs of thrust at 70 Kts?

About that thrust: More to follow. But the pieces to date

**seem** to indicate (thanks, Jan!) that:

Two 55" x ??" props (post 293 & 294) driven by 75 HP engines will give us (

post 242 graph) a total of 275 lbs (125 kg) of thrust at 170 MPH (approx 150 kts). Per the above cruise calculations, that's enough to push any of these planes to 170 MPH. (FWIW, Autoreply's design would match the available thrust numbers from Jan's post 242 graph at about 205 MPH).

** Giant caveat**: Jan knows the assumptions behind the spreadsheet that produced that graph, and the limitations. It's very possible we hit limits on VW RPM, have blade flutter, locust invasion, or some issue that would modify the red thrust line on the chart. I shouldn't even be swimming in these waters. . .

NB: (Since I'm already in these waters) . . . The same chart also shows 340 lbs of thrust at 80 MPH (= 170 pounds from each engine at 70 Kts). As found in Post 278, 170 lbs of thrust is sufficient for safe SE climb at all/most weights envisioned.