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

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nerobro

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Edited to add: From the notes published during the testing of the Gemini, Chris Heinz said the plane produced a single-engine ROC of 400 fpm at 3000' MSL. That's with full fuel (34 gals = 204 lbs) and one pilot, so about 240lbs below MTOW (depending on the weight of that pilot).
If I recall correctly, this is why the plane hasn't seen continued development. The twin engines provided almost no performance when one was out and the plane was heavy. I forgot where I read that though..
 

Pops

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Coming from a helicopter back ground, one factor that would be an important considerations for myself is noise. The push pull type have notoriously noisy cabins. Tolerable for a couple hours in a small helicopter but in a airplane on a cross country....... well, that would be very bothersome, tiring and totally not enjoyable. Noise alone would cause me to shy away from the push/pull to the more conventional wing installation. I understand twin center line thrust has a definite purpose in that sudden engine stoppage is much milder in yaw and torque reaction. Are there other benefits?

Ken
I have some C-337 time. If you do not have the engines in sync the engine noise produces a beat that is very annoying.
Also on take off to verify the rear engine is running, you lead with the rear engine throttle. ( some pilots has tried taking off the a dead rear engine). Do not retract the gear until approaching pattern altitude in cruise climb. When the gear doors open they are like a large drag brake and slows you down 10-13 mph and has put a lot of pilots back on the runway or in the trees. This has killed more than one pilot. I don't care much for the engines but I like the airframe. Just a big C-182. On single engine, it climbs better on the rear engine. Dan
 

autoreply

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So, if we had this VW mini-Skymaster with approx 35-40 foot thin wing, fairly slippery (tandem seats, well faired, etc): any opinions as to how fast it might cruise at 5000' and 10,000'?
Why tandem and not 4 seats (2+2)?
I think fixed pitch props with the climb (fine) pitch needed to produce enough thrust when single engine might mean, at higher speeds, we would hit redline RPM without fully loading the engine.
You pretty much need variable pitch, unless you're willing to lower MTOW a lot, or accept rather pathetic T/O and climb performance.
OTOH, the Sonex does an honest 140-150 MPH at altitude with a fixed-pitch prop (54" dia, 44" pitch), and I'd think the push/pull twin with twice the HP, similar props, and less fuselage frontal area might move along pretty well. Yes, the wing is long, but it's not twice as long as a Sonex (22 feet), and it would have significantly less chord. IIRC, the cruise speed "penalty" for a long wing goes down with altitude, and there's often a crossing point where the longer wing is faster.
Cruise drag (profile drag) goes down for longer wings (of the same area), not up. The Sonex is a pretty dirty design aerodynamically.

With a high wing (high AR), a faired LG and nicely blended rear cowling (see Deskpilot's designs for example), you'd easily do 200 mph @WOT (160 hp total). With an unfaired, low, short wing like on the Sonex you might easily be 20 mph slower ;)
 

StarJar

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If you are going to design a new airframe, and have twin booms, why not lay out a Boomerang type airframe? A straight wing (non-'boomerang'), non-symetrical airframe would be intersting to investigate on a two, or 2+2 seater. You could use an existing single seat (KR-2s or Sonex?) fuselage perhaps (and all the hardware), perhaps engineering a stretch. That way you could keep an eye on your mills, have substantial 'cool' factor, reduce the 'Defiant' vibration/noise factor, and have safety AND performance!
The short direct drive props, would help with ground clearance, or if it would work out, make it a mid-wing....?
Just throwing it out there before you fire-up the spreadsheets, or cut some material.:gig:
 

Pops

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I like the longer wings for high altitude performance. Also 2 place tandem for less drag and lighter weight. High power to weight and lower wing loading. All of this also make for better single engine performance.

Along with the high power to weight ratio and lower wing loading you will get the most all around performance when using the lighter and cheaper fixed pitch props

Dan



Dan
 

KAF

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You'd probably have to go with an in-line engine configuration for this type of project. A low-powered twin with wing-mounted engines and no constant-speed props would be a recipe for disaster, I think.

If you were slow and lost an engine about all you could do is kill the good engine, too; otherwise you'd find yourself upside down with a quickness.
 

Pops

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You'd probably have to go with an in-line engine configuration for this type of project. A low-powered twin with wing-mounted engines and no constant-speed props would be a recipe for disaster, I think.

If you were slow and lost an engine about all you could do is kill the good engine, too; otherwise you'd find yourself upside down with a quickness.

Talking about a push-pull like a Cessan 337 Skymaster and NOT a low powered airframe like the factories manufactures.
Dan


Added--- I like the name "BeetleMaster"
 

Vigilant1

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If I recall correctly, this is why the plane hasn't seen continued development. The twin engines provided almost no performance when one was out and the plane was heavy. I forgot where I read that though..
Even at the weights Heinz was testing (well below MTOW, and probably with some of the fuel burned off getting the other performance numbers he apparently logged on the same test flight), I was still a bit surprised he was getting climb on one engine. The wing of the Gemini is not very long (shorter than a Thatcher CX5--and smaller, too), and I'm sure there was a lot of drag from that other dead engine and the rudder.

I have some C-337 time. If you do not have the engines in sync the engine noise produces a beat that is very annoying.
Okay, so that's something to think about. Any need to deal with this on the Beetlemaster? Can it be done simply/cheaply? I recall that the Powers-Bashforth Minimaster ended up with one three bladed prop (rear?) and one two-bladed prop. (Hey, I remembered correctly. See picture below)


I can't recall the reason, but it might have been vibration (to prevent occasional "beats" as the +/- pressure waves of the twin front prop occasionally matched the twin blades of the rear prop. having different blade numbers reduced this, and having 3 at the rear might have reduced vibrations on the elevator.)

Why tandem and not 4 seats (2+2)?
My head says a 2+2 might be possible, depending on what the equations/modeling say (we probably wouldn't know for sure until we tried it out). My "gut" says there's no way, regardless of what the magic equations yield, that four human beings, a couple hundred pounds of fuel, 200 lbs of dead engine with its drag-inducing prop clawing at the breeze, can be kept aloft, much less climb, using one VW powerplant with a small prop.:) I freely admit I have nothing objective to base that upon. From the other side: While a 2-abreast fuselage (2 or 2+2) is going to give up some top-end, it might not matter if we decide to go with fixed-pitch props (cost/weight/VW bearing concerns) since that 160+ MPH top-end might not be practical anyway (RPM limits + prop pitch).

Cruise drag (profile drag) goes down for longer wings (of the same area), not up.
Hmm. . . always? I understand that induced drag ( i.e. the drag associated with producing lift) goes down for the longer wing (given the same area), but that is not the case for the form drag (profile drag) of the wing. (I apologize for posting this without having the time to do the research.) But as speed goes up, form drag increases as a function of the speed squared, so at some speed the longer wing's form drag dominates that wing's lower induced drag. So, longer wings have lower total drag at low airspeeds, but a shorter wing (producing the same lift) has less drag at some higher airspeed. As we go to higher altitudes, the "total drag" curves for long wing vs short wing cross at a higher airspeed because the induced drag from producing the needed lift becomes a more significant factor (thinner air--> higher AoA for both wings to generate the needed lift. But the longer, lower AR wing will generate lower tip vortices drag). Again, apologies for the words in lieu of numbers.

You'd probably have to go with an in-line engine configuration for this type of project. A low-powered twin with wing-mounted engines and no constant-speed props would be a recipe for disaster, I think.
The inline configuration certainly does take a lot of the drama out of an engine failure. Still, the Gemini with its 80 HP engines and fixed props apparently handled okay. I did read that one of the engines was canted outboard 1.5 degrees to help with Vmc situation.
But, yes, get below the appropriate Vmc with a "conventional" twin and what happens next isn't pretty.
 
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Vigilant1

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The twin engine Zenith that I seen was white and I believe the builder/owner said that it was his mods. It was VW powered for sure. Dan
Thanks, I'll have to be on the lookout for that one. It might still be in my neck of the woods.

The Zenith Gemni, with two Jab 2200s, would not have been a low-price plane to build. A brand-new non-certified O-360 would be cheaper than two new Jabiru 2200s. Maybe the VW-powered Zenith you saw was trying to enter the "Zenith twin" club at a bargain-basement price.
 

saini flyer

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I did come across another very interesting configuration last year using 2XJabiru2200 on a J430. t has flown and is in the test phase. Maybe someone from SA can pitch in and talk about the numbers but the engines are so much closer to the centerline than being on the wings and also the CG is not altered as the engines sit back than the single Jabiru 3300.
IMHO this is the easiest way to go for minimal mods if choosing an existing airframe and achieve twin engine comfort/discomfort :gig:

Now the view forward and down.... well :emb:


Thanks, I'll have to be on the lookout for that one. It might still be in my neck of the woods.

The Zenith Gemni, with two Jab 2200s, would not have been a low-price plane to build. A brand-new non-certified O-360 would be cheaper than two new Jabiru 2200s. Maybe the VW-powered Zenith you saw was trying to enter the "Zenith twin" club at a bargain-basement price.
 

Steve C

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I bet that 3 blade prop in the rear was for greater ground clearance. There may be a case for noise as well. Pushers have that extra buzz if the prop is near the wing, so 3 blades would at least change the frequency.
 

Vigilant1

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I bet that 3 blade prop in the rear was for greater ground clearance. There may be a case for noise as well.
Ground clearance might have been an issue, but I don't think it was the driving reason. The first versions had just 2 blade props (most pictures I found showed it like that) and I remember reading about some type of interference between the two being the reason that they switched to a 3 blade at back (I don't recall the results of that interference: noise, felt vibration, performance issues . . ). But, if they are indeed shorter then ground clearance could be another "plus".

I wish Bill Husa was still with us (for so many reasons . . . ) , He had some involvement with the Minimaster development and would probably know the details.

Mark
 
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Sockmonkey

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Hmm. . . always? I understand that induced drag ( i.e. the drag associated with producing lift) goes down for the longer wing (given the same area), but that is not the case for the form drag (profile drag) of the wing. (I apologize for posting this without having the time to do the research.) But as speed goes up, form drag increases as a function of the speed squared, so at some speed the longer wing's form drag dominates that wing's lower induced drag. So, longer wings have lower total drag at low airspeeds, but a shorter wing (producing the same lift) has less drag at some higher airspeed. As we go to higher altitudes, the "total drag" curves for long wing vs short wing cross at a higher airspeed because the induced drag from producing the needed lift becomes a more significant factor (thinner air--> higher AoA for both wings to generate the needed lift. But the longer, lower AR wing will generate lower tip vortices drag). Again, apologies for the words in lieu of numbers.
A longer span wing tends to have both a smaller chord and a thinner profile than a short one of the same area and more of it will be operating in "clean" air so form drag isn't going to spike like you seem to be suggesting.
The faster you go, the less wing you need, right up to mach 1 where you can get away with almost no wings at all. Stubby wings are going to need a high takeoff speed unless you make use of high AOA tricks deltas and pancakes use.
 

autoreply

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Indeed. A longer wing with the same wing area has slightly less drag.

2/3 blade props makes a lot of sense to avoid resonance between both props..


It surprises me a bit that save the Minimaster nobody has undertaken such a design, especially with two VW's being far less expensive than your typical 150-180 hp Lycosaurus.
 

Vigilant1

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A longer span wing tends to have both a smaller chord and a thinner profile than a short one of the same area and more of it will be operating in "clean" air so form drag isn't going to spike like you seem to be suggesting.
Well, not "spike", but go up smoothly. But, since I'm not able to do the research to actually produce an answer, I'll just shut up about it. And I'll look for examples of long-winged "go fast in a straight line" racers. :)

Stubby wings are going to need a high takeoff speed unless you make use of high AOA tricks deltas and pancakes use.
Yep, there's no way this design is going to have short wings--generous span will be driven by the need for acceptable (i.e. safe) single engine performance.

Another thing: I think many "T-Hangars" (at least in the US) have doors 40' wide, and some are smaller than that. So, if fitting in one and avoiding hangar rash is important, then 36'-38' wingspan should probably be the max. (Though I'd still like to incorporate an easily folded wing).

It surprises me a bit that save the Minimaster nobody has undertaken such a design, especially with two VW's being far less expensive than your typical 150-180 hp Lycosaurus.
Yep, it surprises me, too. Engines do fail--certified ones and non-certified. While I feel safe enough in a single engine airplane, an inline twin designed right would significantly reduce the likelihood of an off-airport landing. That's a very significant factor in some areas, less so in others. This plane would climb well, and that provides a safety factor of its own in normal operation (go-arounds, terrain avoidance, etc). But, aside from the safety issues, the "fun factor" would be pretty high. Finally, for normal cruising around, it would be very practical to get good cruise speeds (110 MPH or so) while operating each engine well short of WOT. That is going to increase engine life and reliability, too.
 
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goldrush

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Just something which I am sure you have pondered.
1. the majority of "engine failures" relate to "auxiliaries"............. ignition.... fuel etc... not pure "engine mechanics", although the probability of 1 engine failure due to mechanical means when using 2 similar engine is higher

There is NOTHING more reliable than..................................... NOTHING.:)
 

Pops

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Just something which I am sure you have pondered.
1. the majority of "engine failures" relate to "auxiliaries"............. ignition.... fuel etc... not pure "engine mechanics", although the probability of 1 engine failure due to mechanical means when using 2 similar engine is higher

There is NOTHING more reliable than..................................... NOTHING.:)
Yes, like I always say," IF it's not there, it weights nothing, cost nothing, and is 100% reliable".
Dan
 

Pops

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Well, not "spike", but go up smoothly. But, since I'm not able to do the research to actually produce an answer, I'll just shut up about it. And I'll look for examples of long-winged "go fast in a straight line" racers. :)

Yep, there's no way this design is going to have short wings--generous span will be driven by the need for acceptable (i.e. safe) single engine performance.

Another thing: I think many "T-Hangars" (at least in the US) have doors 40' wide, and some are smaller than that. So, if fitting in one and avoiding hangar rash is important, then 36'-38' wingspan should probably be the max. (Though I'd still like to incorporate an easily folded wing).

Yep, it surprises me, too. Engines do fail--certified ones and non-certified. While I feel safe enough in a single engine airplane, an inline twin designed right would significantly reduce the likelihood of an off-airport landing. That's a very significant factor in some areas, less so in others. This plane would climb well, and that provides a safety factor of its own in normal operation (go-arounds, terrain avoidance, etc). But, aside from the safety issues, the "fun factor" would be pretty high. Finally, for normal cruising around, it would be very practical to get good cruise speeds (110 MPH or so) while operating each engine well short of WOat is going to increase engine life and reliability, too.

In operating my VW engine, I cruise at 2650 rpm, so at that rpm burning 3 gph it developing about 36 hp, for a 60 hp, 1835 cc engine. I have it in an airframe that performs very good on that HP.
For a successful Beetlemaster it should be designed so the engine can be run at a lower percentage of power and still have good performance. That's light weight, low drag,and wing area.
Dan
 

Vigilant1

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Just something which I am sure you have pondered.
1. the majority of "engine failures" relate to "auxiliaries"............. ignition.... fuel etc... not pure "engine mechanics", although the probability of 1 engine failure due to mechanical means when using 2 similar engine is higher
Yes, agreed. If a twin can't be safely flown on a single engine, then it is >less< safe than a single.

The USAF O-2 (a version of the Cessna 337 Skymaster) was used in the Vietnam "conflict" for forward air control duties. I have heard that after everything was added (2 people, lots of radios, fuel, pods for rockets, a little bit of armor, some personal weapons and ammo, survival gear, etc) that the single engine service ceiling was fairly low. Add in typical Vietnam hot, humid weather and (reportedly) the SE service ceiling was lower than much of the terrain. At that point, as the joke has it, "the purpose of the second engine is to get you to the crash site." In such circumstances, a twin was less reliable, overall, than a single would have been.
But, in actual use--if an available SE crash/bailout site is among "friendlies" and the zero-engine crash site would have been among "hostiles"--having the second engine was a very good thing.
 
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Vigilant1

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In operating my VW engine, I cruise at 2650 rpm, so at that rpm burning 3 gph it developing about 36 hp, for a 60 hp, 1835 cc engine. I have it in an airframe that performs very good on that HP.
For a successful Beetlemaster it should be designed so the engine can be run at a lower percentage of power and still have good performance. That's light weight, low drag,and wing area.
Dan
I think we can get there. For comparison: A C-172 has a 36' wingspan (same as we are talking about) but more wing chord. It has a MTOW of about 500-600 lbs more (= more induced drag to keep that aloft), and it has 160 HP (same as we'll have). At 2000' MSL and a throttle setting that gives a fuel burn of 6.1 GPH, the C-172 gets 100 KTAS (= 115 MPH). If we had the Beetlemaster's 2 engines at your cruise setting (3 GPH each), that's the same as this C-172 (approx). And I'm fairly sure the Beetlemaster can be made quite a bit "cleaner" than a C-172, so we should expect to do better than 115 MPH.

At altitude (10'K MSL), even at WOT the 80HP engines could only burn about 4GPH due to the thin air (= 56 HP each, about 70% of SL max power) = 8 GPH total. At that fuel burn and altitude the C-172 goes 122 KTAS (= 140 MPH). Again, the lighter, cleaner Beetlemaster would be expected to do even better on the same fuel burn.

(all Cessna performance numbers above came from a Cessna POH. So . . .)
 
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