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

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#### Pops

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Dan,
Using your wing and your desired H-tail coefficient of .52, and a 14' lever arm (your 11' from TE of wing to front of H-stab + approx 2.5' from TE to CG + plus approx 1' from LE of H-stab to MAC of H-stab), I figured an H-stab+elevator size of 21 sq ft. Close to what you got?
Using the same method for the V-stab and rudders and your desired Vertical tail coefficient, I came up with 15.3 sq ft. In the ballpark?

I'll figure my numbers later, they'll be close to this. I was thinking theoretically the H-stab could probably be a bit smaller than the standard Raymer formula because of the endplates formed by the V-stabs and rudders. But it's always good to have a little bit extra pitch authority especially in a one-off with some unknowns.

25 feet long: Just a foot longer than a C-152 and your wings are just a foot longer. With wings folded back at the spars, they'll still be a bit shorter than than the tail. The whole thing would fit in a standard 40' long shipping container with room for parts and a workbench in the front.
I have the Hor-tail at 22.4 sq ft and the verticals at 14.88 sq ft total. I have a sub fin of 1.94 sq ft + the fin at 5.55 sq ft for a total of 7.44 sq ft each side. This is a vertical of .046. I worked on this last night before bedtime and was tired and sleepy. I found a mistake in my math from last night so the vertical changed from .045 that I quoted earlier to .046.

I'm ignoring the end plate effect of the verticals and like you I like to error on the high side of pitch authority.

The .046 will give it good yaw stability and I like large rudder authority so the percentage of rudder to fin area will be on the large side to keep the crosswind component as high as possible with the good yaw stability.

Room in a 40' container sounds good.

Added -- found another mistake so the numbers are correct now.

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#### Jan Carlsson

##### Well-Known Member
I wrote something very good and humble Before, but was not able to upload it, now i have some fever, prob some 30-40C me and my son in each corner of the sofa complaining on Sharp light, sound and Life in general.
maybe we got some data virus!

#### Vigilant1

##### Well-Known Member
I wrote something very good and humble Before, but was not able to upload it, now i have some fever, prob some 30-40C me and my son in each corner of the sofa complaining on Sharp light, sound and Life in general.
maybe we got some data virus!
Sorry to hear you are sick, but, well . . . if you think you might leave this world, you could first hammer out your thoughts, prop recommendation and thrust curves before you depart. We'll miss you, and will name the first Beetlemaster in your honor.

Better yet, get some sleep, drink plenty of fluids, and hang around for a few more decades.
Mark

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#### Pops

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Hope you are feeling better now. Life is good even when you are not feeling good.

What is next ? Area of rudder and elevator or maybe the landing gear?

#### Jan Carlsson

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Thanks, son is better, my turn to be better Think it was a bad giant baby syndrome with fever.

some years ago, like 30, i "invented" an automatic fethering propeller with aerodynamic / weight self adjusting propeller, a former friend, said that if that was good why isn't there anyone around? What did we know? not much.

#### Vigilant1

##### Well-Known Member
some years ago, like 30, i "invented" an automatic fethering propeller with aerodynamic / weight self adjusting propeller, a former friend, said that if that was good why isn't there anyone around? What did we know? not much.

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#### Pops

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I have the vertical fin with a bottom cord of 32" and 32" tall, TE straight, LE swept for a top cord of 18". I also have small bottom fin below the boom with a top cord of 32" and down 10" with a bottom cord of 24".
The rudder hinge line is also straight and not slanted for the same reason. ( One reason I like the straight tail C-172's over the swept tail). With the proper length main landing gear and wing at max AOA I don't think there would be a lower fin strike. I'll work that out latter when doing the main LG placement. I also want some of the rudder area as low as possible to avoid blanking of the rudders in a spin.

Considering the slightly difference in your wing and my wing, both Beetlemasters are coming out very similar.

#### Vigilant1

##### Well-Known Member
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

– 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.

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#### Pops

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For my mission, I will take what I have.
EW-- I would love 800 lbs but that weight would be very hard. Realistically maybe 850 lbs. I think 900 lbs would be max. I'll save every ounce and call it 850 lbs. Polished
aluminum wings with silver flap, if its not required for flight its not there, etc.
and ailerons for weight saving.
GW-- 1500 lbs max.
Fuel-- 200 lbs.
Payload with full fuel 450 lbs.
Wing area --- 140 sq ft.
Wing span --- 34 ft.
Airfoil --- 2414
Wing construction-- All aluminum, all flush riveted, Fabric covered alum flaps and ailerons.
Engines -- Revmaster 85 HP , 80 HP continuous.
Props ---- 57"x 48.5" each
Fuselage length -- 98" firewall to firewall . 24.55 ft less front spinner.
Fuselage width--- 30".
Crew -- 2 Tandem.
Fuselage construction -- 4130 steel tube with alum removable panels on side from firewall to door, Aluminum or CF from front of door to rear firewall. CF front and rear cowl.
Cabin floor to ceiling --43".
Tail booms to booms --- 8'
Horizontal tail --- 22.4 sq ft. HTC-- .053
Vertical tail --- 14.88 sq ft. VTC-- .044
Tail arm length -- 14 ft.

Single place with me (235 lbs) with full fuel ---Weight = 1285 lbs.

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#### Jan Carlsson

##### Well-Known Member
If we go with Vigilantett new number.
I set sea level 150 HP we get 182,5 mph just to set the drag in the soft to 3,0 sg ft, prop eff 86,9% 55"x61" (not just any prop)

6000´
122 HP WOT 3350 RPM 180 MPH 55"x60" 86,8% eff thrust is 100 kg

150 mph 79,3 HP is needed, thrust 73 kg eff 82,2% 3400 RPM to get 48.5" pitch 57"D and very low CL because of the low load.
the airfoil have little camber and airfoil semisymetric.
lost ~4.5% eff, the real loss will be larger due to difficult to make the propeller blade morph in camber with load.

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#### Vigilant1

##### Well-Known Member
For my mission, I will take what I have.
EW-- I would love 800 lbs but that weight would be very hard. Realistically maybe 850 lbs. I think 900 lbs would be max. I'll save every ounce and call it 850 lbs. . . .
GW-- 1500 lbs max.
Fuel-- 200 lbs.
Payload with full fuel 450 lbs.
. . . . Single place with me (235 lbs) with full fuel ---Weight = 1285 lbs.
She should climb really well. Even on one engine at 1500 lbs vs a Cessna 152 at gross, both at 70 knots:
You'll have less drag: (47 lb induced + 74 lbs profile drag with stopped prop = 121 lbs) (C-152: 61 lb + 102 = 163 lbs)
You'll have 26% less HP, but the HP you need to stay flying at 70 knots is also 26% less (per above).
You'll have a calculated climb rate of 390 FPM (assuming 205 lbs of thrust available). The C-152 POH says it gets 715 fpm at MTOW, but I have never seen 700+ FPM at that weight.

I'll make plans for a higher >>possible<< MTOW for my version of the Beetlemaster. There will be room in the wing (between cabin and booms) for about 36 gallons (215 lbs) total behind the spars, plus another 10 gal total (60 lbs) in aux tanks in front of the spars (lower 1/2 only, leaving room for control linkages above). I don't know how the total power available and fuel burn will work out but I might want to carry up to that 275 lbs of fuel: if I burn 8 GPH total (about 66% power) that gives 5 hours plus 45 min reserve. Add 2 people plus some bags. Anyway, if it doesn't cost me much weight or money to make a CF spar and other structure to carry a larger MTOW (say, up to 1700 lbs), I will do that just to keep my options open in case the performance (esp single engine) in real life looks like the calculations.

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#### Vigilant1

##### Well-Known Member
If we go with Vigilantett new number.
I set sea level 150 HP we get 182,5 mph just to set the drag in the soft to 3,0 sg ft, prop eff 86,9% 55"x61" (not just any prop)

6000´
122 HP WOT 3350 RPM 180 MPH 55"x60" 86,8% eff thrust is 100 kg
So, with those 55" D x 60"Pitch props: 100 KG total thrust (50 kg per engine/prop) at 180 MPH? That sounds pretty good. And you think the Revmaster engines will have no trouble turning them that fast, even with that pitch? Is it possible to know if just one of them will give us the 180-205 lbs (82-93 kg) of thrust we need at 70 knots (80 MPH)?

150 mph 79,3 HP is needed, thrust 73 kg eff 82,2% 3400 RPM to get 48.5" pitch 57"D and very low CL because of the low load.
the airfoil have little camber and airfoil semisymetric.
lost ~4.5% eff, the real loss will be larger due to difficult to make the propeller blade morph in camber with load.
At 150 MPH (130 knots), the planes need from 153lbs (70 kg, Autoreply) to 189 lbs (86 kg, Pops), so Autoreply could use the above prop at that airspeed and throttle setting, Pops might need something different. It sounds like a good economy cruise prop. If the prop in this post is the same one (I know it is 57"D but not sure if it is 48.5" pitch) then we also know it will give enough single-engine thrust at 70 knots for any of these planes.

I'm glad you are feeling better.

I know you have a better airplane performance spreadsheet than I do, but let me know if you want your own version of the Beetlemaster in my caveman spreadsheets. I just need the wing sq ft, span, approx cabin size, boom and tail size (I'll estimate the tail size if you know the length of the tail arms), gear type, struts or no struts, and (most important), what I should put in for skin friction coefficient (.006 for typical metal, .005 for typical smooth composite--or something else).

Mark

#### Pops

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A lot of work will have to be done in designing the wing for the Beetlemaster from the root out to the booms to get the largest fuel tank with running the controls past the tanks and also having flaps. For any useful range there might have to have auxiliary fuel tanks in the wings outboard of the booms with both tanks plumbed as one larger tank on each side. That means more weight. I'm allergic to weight.
I had the same problem with the JMR with the smaller 48" wing cord wing. I'm still not happy with the outcome because of the complexity and the size of the fuel tanks. Just having 17 gal total fuel with the C-85 fuel burn of 5 gph at cruise. I have drawing done for an auxiliary fuel tank under the baggage area in the fuselage but really would like to keep all the fuel in the wings.

#### Vigilant1

##### Well-Known Member
A lot of work will have to be done in designing the wing for the Beetlemaster from the root out to the booms to get the largest fuel tank with running the controls past the tanks and also having flaps. For any useful range there might have to have auxiliary fuel tanks in the wings outboard of the booms with both tanks plumbed as one larger tank on each side. That means more weight. I'm allergic to weight.
After reading your post, I re-read the write-up from one of the members of the Cessna 336 design team (My earlier post is here (post 184)with a link to a PDF of the piece. The PDF is from a paper scan and there are some optical-character-recognition errors in it, but not many). There's a lot of wisdom there for us. The rear wing spar is critical to the strength and rigidity of the tail booms--that's the first place they connect (obviously), plus their connection at the main spar. All the pitching moments and yawing moments from the tail get passed to those spars to move the airframe. So, that beefy rear spar is going to cost us some fuel in the inboard panel of the wing.

The booms themselves need to be stiff, apparently the C-336 had some problems (cracking?) where the tail surfaces mated with the spars, and reinforcements (which look like aerodynamic strakes, but are really there for structural purposes) were added.

The team had fits getting the control linkages to be relatively friction-free and without slop. No matter what we do, there's just a lot of turns and control-run distance required by this configuration.

And they talked about the better climb on the rear engine only compared to the front engine only, primarily because the reduced pressure in front of the turning rear prop helps keep the flow attached to the comparatively blunt rear of the aircraft. If the rear engine isn't making power, it gets pretty draggy behind that cowling.

The whole thing left me thinking that I hadn’t appreciated how convenient and efficient a "regular" tailcone is: Deep enough to be very stiff with minimal weight, straight control runs directly from the cockpit, load paths through tailcone longerons that are needed anyway and can pass loads directly to the shear panel/fuselage structure supporting the wing, and aft fuselage taper is gradual enough that it helps reduce turbulent flow.

Which, of course, leads to the sacrilegious consideration of the pros/cons of use of a single low tailboom in the Beetlemaster (below the rear prop). There would be more room in the wings for fuel, control linkages would be simplified (straight shot to the tail), weight might be reduced. It wouldn’t do anything to reduce separation drag at the rear of the fuselage, and the overall drag might go up (because the central low boom would need to go up as it went aft, to allow room for the plane to rotate/flare. That structure won’t be aligned with the ambient airflow (though it's airfoil profile could be), and it will also have scrubbing drag from the high-speed flow off the rear prop. But, just to put it out there—our rear prop tip will pass about 7.5” to 10.5” higher than the floor of the main fuselage, so there would be room for a low fusleage boom. Twin booms from the lower corners of the fuselage would also be a possibility and give more prop clearance.

Also, Pop's wing struts are starting to look more attractive, just as a means to run control wires to the booms with one less pulley. In fact, from a structural standpoint, having two struts per wing (front and rear spar), both possibly terminating at the main gear attachment point, would seem likely of doing a good job of stiffening up the tailbooms horizontally and laterally (pitch and yaw), obviously with a price to be paid in drag. Cessna used I-beam profiles in their C-336 and C-337 struts with a streamlined fairing over it.

Sorry to take a step back. At any rate, the paper from the C-336 guys may be worth a read for anyone interested who forgot as much as I did.

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#### Pops

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I have helped in doing 100 hr inspections on C-337's and it seems to almost be a flying inspection panel. They are not a low maintenance airplane. You do not retract gear until after take-off until in a cruise climb about at pattern altitude. When the gear doors swing open its like 2 air brakes. If I remember correctly there is about a 10 mph reduction in airspeed and you will go forward against the shoulder harness enough to feel them.
Like the C-210 there is a stc for removing the gear doors. I do like flying a 377, sort of like a heavy C-182. For some reason, driving a dump-truck always was on my mind. The Beetlemaster will handle better.

Even with running cables up the struts its still going to be hard in routing the cables/pulleys around the fuel tanks without paying the price of reduced fuel capacity. On the JMR, I used a flap torque tube forward of the rear spar for operating the flaps and also an aileron cable. The rear of the fuel tank had a clearance step to clear the tube and cable that cost some fuel capacity. As you can see in the pictures things are tight.
Have to remember the JMR is a small airframe of 600 lbs EW and a 1050 lb GW so things are small.

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##### Well-Known Member
Don't use spars for tail loads. Stressed skin is much better at that.

#### Vigilant1

##### Well-Known Member
Don't use spars for tail loads. Stressed skin is much better at that.
I see how the wing skins would work well for the "racking" loads (i.e. yaw, resulting from application of the rudder). Their utility in handling the pure pitching loads (i.e. from application of the elevator) isn't as obvious to me, though I'm still thinking through it. It may just be semantics: If the booms are attached to the skins (solidly, bonded) and the skins are attached to the spars/underlying ribs, then all should be well. And if the booms don't intersect/go through the rear spar web on the way to the main spar, that means it will be stronger and simpler to construct. Finally, if the boom is "underslung" or on top of the wing, it leaves more room inside the wing for the fuel tank while staying within the planned 8' max width. An extra 5" per side (my >guess< at the boom width, for back-of-the-envelope calculations) would give 3.5 gallons additional fuel per side behind the main spar for Pop's wing.

Even with running cables up the struts its still going to be hard in routing the cables/pulleys around the fuel tanks without paying the price of reduced fuel capacity. On the JMR, I used a flap torque tube forward of the rear spar for operating the flaps and also an aileron cable. The rear of the fuel tank had a clearance step to clear the tube and cable that cost some fuel capacity. As you can see in the pictures things are tight.
The welds on those tanks look nice. I'm sure the realities of getting the tank fit into the wing were harder than it appears in the final result.

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#### Pops

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For attaching the booms to the wings. I was thinking of the bottom of the boom on top of the rear spar with brackets to the spar. ( remember aluminum wing) The front of the boom would attach to the main spar with brackets. This is where rear wing struts would help with the load on the rear spar. Just thinking out loud.

Yes , there was a lot of work in fitting the tank in the wing. If I had it to do over I would make a few changes. #1-- the aileron cable that is forward of the tank would be run forward of the main spar. #2-- the the step in the rear of the tank would be a little larger for more clearance with the flap torque tube and tank straps. Also with the increase clearance I would use a next size torque tube dia. #3- increase the width of the tank a few inches, how much, I would have to do the math for that change. The tank has about 1/16" of clearance to remove. Drawing will be changed.

#### Pops

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Everyone quit working on the Beetlemaster ?

Dan