### Help Support Homebuilt Aircraft & Kit Plane Forum:

#### skier

##### Well-Known Member
Only time will tell how far this thread will get before I get distracted by other ideas.

Rough Goals:
1) Asymmetric
2) Single Seat
3) VW Engine
4) LSA

Preliminary Sizing
Sw = 142 ft^2
b = 28.625 ft
W_gross = 1000 lbs
bhp = 80 hp

Screen shot from Solidworks (Thanks EAA)

#### Aerowerx

##### Well-Known Member
Twin engine?

Looks like Dan Raymer's "safety twin" (see his "Simplified" book).

#### Vigilant1

##### Well-Known Member
I'd think the empty weight (if typical aluminum construction) would probably be somewhere around 725 lbs, maybe a tad more (rationale: A Sonex typically weighs about 650 lbs, and you've got the same engine, 50% more wing, and an extra pod and the structure/fittings, etc to go with it. Yes, a Sonex is a two-place acft, but a small one and the extra seat doesn't require much additional material/weight).

#### skier

##### Well-Known Member
Twin engine?.
Nope, single engine. The number I gave it gives away a bit of more the basis. Stand on the ground and look up at it flying overhead and it should look something like the origianl BV-141 prototype (D-ORJE) at half-scale.
141/2=70.5

I'd think the empty weight (if typical aluminum construction) would probably be somewhere around 725 lbs, maybe a tad more (rationale: A Sonex typically weighs about 650 lbs, and you've got the same engine, 50% more wing, and an extra pod and the structure/fittings, etc to go with it. Yes, a Sonex is a two-place acft, but a small one and the extra seat doesn't require much additional material/weight).
You could be right, and using the weight estimation in Raymar's simplified book based on areas (with conservative lbs/ft^2), I appear to already be at gross without a pilot or fuel (ref: 946 lbs)

Wing ~ 213 lbs
Fuse ~ 265 lbs (including cockpit nacelle)
Vertical tail ~ 23 lbs
Horizontal Tail ~ 40 lbs
Prop ~ 20 lbs
Landing Gear ~ 100 lbs
Instruments & Avionics ~ 25 lbs (basic day VFR)
Furnishings ~ 20 lbs (could be on the low-side)
Electrical ~ 40 lbs
Engine ~ 200 lbs (installed weight)
Pilot ~ 200 lbs
Fuel ~ 144 lbs (24 gal)

The large amount of wing area was because I was not planning on using flaps and was hoping to get my stall-speed somewhere approaching that of a J3

#### TFF

##### Well-Known Member
Supposedly the Blohm Voss flew just fine. Getting eyes ahead of the engine in a cheap way was the intent. That is why they were willing to give up the extra weight and extra drag of a half fuselage. What they did was get rid of an engine as Germany had no gasoline to spare. You also will have to balance your pod weight distance with the primary fuselage as your lateral balance will need to be balanced. The percentage of weight of the pod will be higher than on the 141. It was a big plane like a twin bomber size actually a hair bigger than a bf 110. The weight penalty would not be as big as yours.

#### Aerowerx

##### Well-Known Member
Ok, I see.

What is the purpose of having a single engine offset like that? I would think you would have a lot of yawing moment.

#### skier

##### Well-Known Member
Ok, I see.

What is the purpose of having a single engine offset like that? I would think you would have a lot of yawing moment.
The initial RFP for an observation aircraft that resulted in the BV-141 design asked for a single-engine aircraft. The crew nacelle was offset to provide visibility like you would have in a twin-engine aircraft while meeting the single-engine requirement. The FW-189 eventually won the competition even though it utilized a twin engine configuration because the engine used in the BV-141 was needed for the FW-190.

As for the moment created with the offset engine, reports are that it actually flew more symmetrically than a conventional aircraft because the offset engine counteracted the left-turning tendency of conventional designs.

My design will benefit similarly (to some extent), but other design factors may mean that my design over-counteract these forces and end up causing a right-turning tendency at high power settings. The answer to that question will need a fairly accurate blade-element model of the propeller in a cruise attitude in flight and will come at a more detailed analysis phase further down the road. The design, as depicted in my first post has the engine centerline offset 1 foot to the left of the CG. That may need to grow as I try to balance the design laterally.

#### Pops

##### Well-Known Member
HBA Supporter
Log Member
To much airframe for a 80 HP , VW engine. Even with a belt drive you will be demanding to much HP for the cooling of the VW heads. Poor performance with high engine temps.
I do like the design, just think its not a design for the VW engine.

#### Riggerrob

##### Well-Known Member
Great idea!
AZ-705 parallels my concept studies of a half-scale BV-141 replica.
Half-scale is about the smallest you can scale down the cockpit nacelle and still fit a 95th percentile man.
The biggest limitation is the distance between the main wing spar and the propeller. Personally, I would prefer to install the prop disc 6 inches or a foot in front of the pilot's toes ..... to prevent a repeat of the lesson-learned on P-61 Black Widow night-fighters. Problem was with propellers directly inline with the second seat. A few P-61 aircrew got chopped by thrown propeller blades during wheels-up landings.
I need to study the reclined seating in modern sailplanes.

To ensure lateral balance, I would build cockpit nacelle, boom, tail feathers, wings, landing gear, wings, etc. And weigh them all before building the wing center section last.

Blohm und Voss' assymetrical airplanes take advantage of the assymetrical thrust created by climbing airplanes. Since the descending prop blade takes a deeper bite, it always produces more thrust than the ascending blade. The closer the descending blade is to the centreline, the straighter the airplane climbs.

Any engine installed in an AZ-705 would need to spin the propeller clockwise (as viewed from the pilot's seat).

As for engine output, Thatcher CX-4 flies great on a small VW enigine while CX-5 carries two guys, quickly on only 85 horsepower. My first sketches of a BV-141 replica started with a WAR FW-190 cowling concealing a Continental O-200 of 85 horsepower. They also included wings and tail copied from Falconar F-12 Jodel.
Later studies were based on Thatcher CX-5 wings and rudder.

As for structure: steel tube cockpit with sheet metal or Rutan mold-less composite airframe. Since the BV-141 airframe is mostly straight lines and conical curves, it should be easy to build a stand-off scale BV-141 with sheet aluminum and pulled rivets. But I would cheat by using the local
maker lab to CNC cut particle-board forming blocks for ribs and bulkheads.

A steel tube cockpit would help protect the pilot during crashes, but I still want a sacrificial layer of polystyrene foam under the pilot's seat to absorb the worst impacts.

#### skier

##### Well-Known Member
Too much airframe for a 80 HP , VW engine. Even with a belt drive you will be demanding to much HP for the cooling of the VW heads. Poor performance with high engine temps.
I do like the design, just think its not a design for the VW engine.
I do respect that you have more experience with VW engines that most on here (I believe you used them in a couple of your planes), however I don't believe this to be true.

Based on those numbers, it would seem to me that the AZ-705 would be the perfect candidate for an 80hp VW conversion. Namely the Aerovee. She definately won't be a speed deamon, but I'm planning for a cruise of 80-90 mph.

Great idea!
AZ-705 parallels my concept studies of a half-scale BV-141 replica.
Half-scale is about the smallest you can scale down the cockpit nacelle and still fit a 95th percentile man.
The biggest limitation is the distance between the main wing spar and the propeller. Personally, I would prefer to install the prop disc 6 inches or a foot in front of the pilot's toes ..... to prevent a repeat of the lesson-learned on P-61 Black Widow night-fighters.

I need to study the reclined seating in modern sailplanes.

As for structure: steel tube cockpit with sheet metal or Rutan mold-less composite airframe. Since the BV-141 airframe is mostly straight lines and conical curves, it should be easy to build a stand-off scale BV-141 with sheet aluminum and pulled rivets.

A steel tube cockpit would help protect the pilot during crashes, but I still want a sacrificial layer of polystyrene foam under the pilot's seat to absorb the worst impacts.
I was amazed at how well the BV-141 at half scale works as a single seater. It's almost like they planned for this when they did the initial design. The cockpit nacelle scales almost perfectly to a reasonable width (24"). As you can see with my sketch, I too was concerned about the prop chopping off my feet and have planned that the nose of the nacelle will be behind the prop-disk.

I plan to have the pilot in a semi-reclined seating position. I should have roughly 36-38 inches of height on the nacelle for the pilot. Note that the RV-8 has 41 inches for the pilot and 40 inches for the passenger. It will be tight, but I think I will be able to make it work.

As for materials, the AZ-705 will be typical sheet metal construction. I am not planning on trying to dutifully reproduce the shape of the original. In fact, I will probably have a rectangular cross section on the main fuselage similar to what you see on the PA-28 line of aircraft.

Supposedly the Blohm Voss flew just fine. Getting eyes ahead of the engine in a cheap way was the intent. That is why they were willing to give up the extra weight and extra drag of a half fuselage. What they did was get rid of an engine as Germany had no gasoline to spare. You also will have to balance your pod weight distance with the primary fuselage as your lateral balance will need to be balanced. The percentage of weight of the pod will be higher than on the 141. It was a big plane like a twin bomber size actually a hair bigger than a bf 110. The weight penalty would not be as big as yours.
A preliminary lateral balance with the numbers I mentioned above indicate that moving the main fuselage 6 inches towards the left wingtip would almost perfectly balance the aircraft (assuming full fuel). This would place the engine roughly 1.5ft from the CG. There's also the possibility of keeping the distance between the main fuselage and the crew nacelle the same and shifting both to the left as lesser amount. This would be only about a 3 inch shift of the engine center-line.

Last edited:

#### Riggerrob

##### Well-Known Member
If we can agree that we need an engine in the 80 to 100 horsepower range, and we limit ourselves to engines that are already proven in light airplanes, let's review what is available.

Early WAR planes used 60 or 80 horsepower VW conversions. Nowadays a half dozen shops produce reliable VW conversions (Aero Vee, Great Plains, Hummel, Revmaster, etc.) with 2 litre displacement producing upwards of 80 horsepower. The disadvantage of most direct-drive VW conversions is counter-clockwise rotation.
Without clockwise propeller rotation we lose the BV-141's "secret sauce:" descending propeller blade close to the airplane's center-line.
Fortunately, Great Plains also offers VW conversions that drive the propeller off the flywheel end so that they rotate in the correct direction (clockwise).

Since many WAR planes were built too heavy, most builders installed 100 hp Continental 0-200 engines. O-200s are readily available (e.g. salvaged from Cessna 150 trainers) and rotate in the correct direction.
Jabiru 4-cylinder engines also fit our requirements.
Rotax 912 also fit our requirements.
We definitely need to install a 3-Bladed Whirlwind propeller to maintain the stand-off-scale look.

For crash safety and balance, I would like to install the fuel tank in the fuselage boom, above the wing center-section. That reduces the simplifies fuel line routing and reduces the risk of fuel spills in the cockpit to zero.
As for engine instruments, just bolt large, round "steam gauges" flush with the right side of the boom, almost level with he pilot's eyes. Since the throttle will be built into the left cockpit armrest, it will be instinctive for the pilot to glance outside at engine instruments while adjusting power settings. One of the German WW2 bombers, used similar engine instruments (Junkers 88?).

As for fuselage to nacelle spacing, I would start by mocking up the cockpit nacelle, followed by mocking up the fuselage boom. Thirdly, mockup a center wing section roughly 3 feet wide, wide enough that prop blast does not directly impinge on the cockpit gondola, not ridiculously far apart to aggravate lateral balance issues.

Last edited:

#### Riggerrob

##### Well-Known Member
A replica of a German warbird deserves a German engine.
I looked briefly a conversions of BMW R1200 engines but concluded that most propeller speed reduction units (popular on ultra-light airplanes and Rogallos) turn the propeller in the wrong direction (counter clockwise).

For the ultimate "look" install a Rotec R2800 radial engine. Rotecs pump out 110 hp and rotate in the correct direction, but they weight 225 pounds! Yikes!

All that extra weight might make a BV-141 replica too nose-heavy. It will already be nose heavy with both the pilot and engine ahead of the main wing spar. Further aggravating nose-heaviness is that the prop disc needs to be 4 or 5 feet ahead of the spar to reduce the risk of a thrown prop blade amputating the pilot's foot.
In the long run, we should design the boom firewall location for the heaviest engine (Rotec?) then install longer engine mounts to balance lighter engines.
This is where a 3D weight-and-balance spread sheet earns its keep.

#### Pops

##### Well-Known Member
HBA Supporter
Log Member
I do respect that you have more experience with VW engines that most on here (I believe you used them in a couple of your planes), however I don't believe this to be true.

Based on those numbers, it would seem to me that the AZ-705 would be the perfect candidate for an 80hp VW conversion. Namely the Aerovee. She definately won't be a speed deamon, but I'm planning for a cruise of 80-90 mph.

I was amazed at how well the BV-141 at half scale works as a single seater. It's almost like they planned for this when they did the initial design. The cockpit nacelle scales almost perfectly to a reasonable width (24"). As you can see with my sketch, I too was concerned about the prop chopping off my feet and have planned that the nose of the nacelle will be behind the prop-disk.

I plan to have the pilot in a semi-reclined seating position. I should have roughly 36-38 inches of height on the nacelle for the pilot. Note that the RV-8 has 41 inches for the pilot and 40 inches for the passenger. It will be tight, but I think I will be able to make it work.

As for materials, the AZ-705 will be typical sheet metal construction. I am not planning on trying to dutifully reproduce the shape of the original. In fact, I will probably have a rectangular cross section on the main fuselage similar to what you see on the PA-28 line of aircraft.

A preliminary lateral balance with the numbers I mentioned above indicate that moving the main fuselage 6 inches towards the left wingtip would almost perfectly balance the aircraft (assuming full fuel). This would place the engine roughly 1.5ft from the CG. There's also the possibility of keeping the distance between the main fuselage and the crew nacelle the same and shifting both to the left as lesser amount. This would be only about a 3 inch shift of the engine center-line.
No contest, If I was using a VW engine it would be the Revmaster over the Aero-V. Max torque at lower rpm's and a better cooling heads that they make, not VW auto heads as on the Aero-V. No one been in business longer making aero-VW engines than Revmaster.
Then just don't have the slick ads and red parts. Even the America Indians fell for the shiny things.

#### Riggerrob

##### Well-Known Member
Rev Masters may be better engines than AeroVee, but the problem is that they both rotate in the wrong direction (counter-clockwise) for a BV-141 replica.
The secret to BV-141's good handling is that the descending blade is closest to the airplane's center-line. This helps reduce P-factor and assymetrical thrust and all the other handling horrors that light twins suffer when an engine quits.
IOW try to think of a BV-141 as a light twin that just suffered engine failure on the right side. If that light twin has engines that turn in opposite directions (clockwise-turning left engine with a counter-clockwise turning right engine) then the pilot will not have to worry about "good engine" versus "bad engine" flying characteristics because the descending propeller will always be closest to the center-line minimizing yaw.

I have survived one forced landing in a King Air that lost an engine and did not much enjoy it. I will never walk straight again.

Another viable option is Werner Scarlett, 7-cylinder, radial engines made in the Czech Republic. They are about the same weight and width as other suitable engines and pump out just over 100 horsepower.

#### Jon Ferguson

##### Well-Known Member
You could just put the pod on the other side LOL... Or rotate the engine 180 degrees and drive off the other end.

#### Riggerrob

##### Well-Known Member
While BV-141 airframe looks complex at first glance, external loft lines can be simplified to a few cones.
For example, the fuselage boom only has four basic shapes.
We start by stealing a cowling off a passing WAR FW-190.
The center portion is a simple cylinder back to the trailing edge of the wing, then a simple cone aft of that. The most complex part is the aft pylon with the horizontal tail bolted on top and tail wheel bolted on bottom.
The vertical fin bolts/rivets onto the aft edge of the pylon. The most complex shape is the leading edge of the tail pylon.
The only major complications are the main wing penetrating the center section with a fuel tank on top.
So we could build a boom with only 5 bulkheads, with the first 4 being circular.

#### skier

##### Well-Known Member
The disadvantage of most direct-drive VW conversions is counter-clockwise rotation.
Without clockwise propeller rotation we lose the BV-141's "secret sauce:" descending propeller blade close to the airplane's center-line.
Thanks for pointing that out. I hadn't realized that. Since I'm not after a faithful BV-141 replica, my first thought after you said that was as Jon said below.

You could just put the pod on the other side LOL... Or rotate the engine 180 degrees and drive off the other end.
I'm pretty set on a VW. They're cheap, readily available, and have been around for a long time. I also am more leaning towards the Aerovee because it has a company behind it that I think will be around for a while. As a result, I'll probably make the AZ-705 a mirror image of the BV-141. That being said, it would be pretty easy for a builder to "flip" the engine and nacelle were they to want to go with a "normal" engine. It wouldn't change the design process at all, they would just have to be a bit clever about mirroring some of the components.

For the ultimate "look" install a Rotec R2800 radial engine. Rotecs pump out 110 hp and rotate in the correct direction, but they weight 225 pounds! Yikes!
If I remember right, those are pretty darn expensive. Another goal of my AZ-705 is to be relatively affordable. I'd be happy if the final cost was <$30,000 and overjoyed if it were <$20,000.

All that extra weight might make a BV-141 replica too nose-heavy. It will already be nose heavy with both the pilot and engine ahead of the main wing spar. Further aggravating nose-heaviness is that the prop disc needs to be 4 or 5 feet ahead of the spar to reduce the risk of a thrown prop blade amputating the pilot's foot.
In the long run, we should design the boom firewall location for the heaviest engine (Rotec?) then install longer engine mounts to balance lighter engines.
This is where a 3D weight-and-balance spread sheet earns its keep.
My quick numbers from earlier in the thread had the AZ-705 significantly tail-heavy. However, since I am going for the look of the D-ORJE my pilot will be pretty centered over the wing.

For crash safety and balance, I would like to install the fuel tank in the fuselage boom, above the wing center-section. That reduces the simplifies fuel line routing and reduces the risk of fuel spills in the cockpit to zero.
As for engine instruments, just bolt large, round "steam gauges" flush with the right side of the boom, almost level with he pilot's eyes. Since the throttle will be built into the left cockpit armrest, it will be instinctive for the pilot to glance outside at engine instruments while adjusting power settings. One of the German WW2 bombers, used similar engine instruments (Junkers 88?).

As for fuselage to nacelle spacing, I would start by mocking up the cockpit nacelle, followed by mocking up the fuselage boom. Thirdly, mockup a center wing section roughly 3 feet wide, wide enough that prop blast does not directly impinge on the cockpit gondola, not ridiculously far apart to aggravate lateral balance issues.

I agree with the fuel in the fuselage over the wing. I actually thought that would be a good "selling point", though I don't think I'd ever offer plans for my design (too much liability).

I think your suggestion for instrument location would be good, but I also like the idea of them overhead as in this photo (I don't know what plane it is):

Since my cockpit will be modeled off D-ORJE, I will likely have a pretty standard panel. I'm thinking the cockpit may end up pretty similar to the Long-EZ with the back seat cut off.

My minimum distance was 2 ft between the crew nacelle and the fuselage. I think 3 would definitely have some benefits (noise reduction) in getting the nacelle a little further away from the prop-tips, but I wasn't sure I wanted to deal with that much offset in the thrustline. It's starting to get "large". From what I can see, the Aerovee on a Sonex has approximately a 60" prop. With a 24" wide fuselage and 24" between the side of the fuse and the cockpit, that's 36 from the engine center-line to the crew nacelle. A bit tight, but not too bad. Since my preliminary weight and balance showed that moving the fuselage 6" to the left would put the plane in balance, That's now 42" from the centerline (a 12" buffer on the prop-tips) of the engine to the side of the crew nacelle and 30 from the fuselage to the nacelle (getting pretty close to that 3ft you suggest).

I'd think the empty weight (if typical aluminum construction) would probably be somewhere around 725 lbs, maybe a tad more (rationale: A Sonex typically weighs about 650 lbs, and you've got the same engine, 50% more wing, and an extra pod and the structure/fittings, etc to go with it. Yes, a Sonex is a two-place acft, but a small one and the extra seat doesn't require much additional material/weight).
I know this was one of the first posts in the thread, but I wanted to comment further on it. your suggestion of an extra 75lbs for the nacelle seems pretty close. Based on the weight estimation method in Raymar's "Simplified" book, I got around 88lbs for the crew nacelle (without any of the furnishings that would be a dupicate from the Sonex).

Last edited:

#### skier

##### Well-Known Member
I submit for your evaluation the second iteration of AZ-705. I mirrored the fuselage and the nacelle, moved the engine 6" forward, shifted the fuselage 6" outboard, and shortened the horizontal tail arm by 12". This should result in a fairly balanced plane (laterally) and get the longitudinal CG closer to the correct location. It will slightly decrease the tail-volume coefficient, but it should still be between .5 and .7 (typical tail volume coefficients for homebuilts and GA aircraft according to Raymar's big book).

The fuselage and wing remind me a lot of the CX-4, which I have to admit is a pretty good looking plane.

#### Vigilant1

##### Well-Known Member
From what I can see, the Aerovee on a Sonex has approximately a 60" prop.
FWIW, the Aerovee on a Sonex usually turns a 54" diameter x 44" pitch prop.

#### Riggerrob

##### Well-Known Member
The BV-141 cockpit gondola looks a lot like my first sketches - based on a Jodel F-11. Sitting the pilot on top of the wing simplifies a variety of balance issues.

However, I decided that I wanted a BV-141B replica because that huge, angular windshield is so EVIL. So my second sketches started with pushing the pilot's seat back until it presses against the wing's main spar and sinking the pilot's butt until it is almost at the bottom of the gondola. ...... calculating the distance to rudder pedals, then adding a few more inches to keep toes clear of shed prop blades. Once I fix the prop-to-main spar distance, review weight and balance and start sketching the second iteration.

The primary reason I started with adapting a Falconar F-11 Jodel is that Jodel wings resemble BV-141 wings and as long as I keep flying surface similar to Jodel's then it should handle like a Jodel. Models have a half-century of training student pilots in Europe.

Last edited: