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

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

rotax618

Well-Known Member
Joined
Oct 31, 2005
Messages
1,090
Location
Evans Head Australia
To achieve centreline thrust/drag on a high wing aircraft the tractor engine needs downthrust as most of an aircraft’s drag is a result of lift. A high thrustline is when the engine is positioned some distance above above the wing. Having 2 small engines arranged as on the above amphibian is a pretty efficient arrangement with regards to trim drag.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,024
Location
US
In case nobody has pointed this out yet, the rate of climb information got overwritten by some engine info. Has some HP/rpm stuff instead.
Thanks for the note. The info is all there, it just got formated differently when I imported the table. The cell with the engine/prop info gives the thrust upon which the ROC figures are based. Below that, the rows give the conditions for the shown ROC (altitude, airspeed, one or both engines). I can't change the table formatting here, so I just went back and added some explanatory notes in red in the table to reduce any confusion.

The PDF version of the chart (attached to the post) may be clearer, it has the original formatting.
 
Last edited:

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,024
Location
US
To achieve centreline thrust/drag on a high wing aircraft the tractor engine needs downthrust as most of an aircraft’s drag is a result of lift. A high thrustline is when the engine is positioned some distance above above the wing. Having 2 small engines arranged as on the above amphibian is a pretty efficient arrangement with regards to trim drag.
I don't know about aircraft in general. The configuration I've been working with for this plane is similar to a small Cessna 337-- a "low" tractor engine in front (prop hub a few inches below the windscreen, about 20" above the cabin floor in this case, total cabin height of 43"), and a "high" pusher engine in back (prop hub centered about 10" below the top of the cabin/TE of the wing).
The location of the drag (and resulting trim required) varies a lot by airspeed. For example, for the Pops Beetlemaster:
-- At approx cruising speed (110 KTAS/about 130 MPH, 6000 MSL), there would be very little drag due to lift (induced drag is estimated to be about 25 lbs). Parasite drag predominates, and is expected to total about 145 lbs. The "low stuff" (cabin) has a bluff front and expected flow separation in the rear (esp if the rear engine quits), and a total wetted area of about 140 sq ft. And there are three wheels/struts even farther below. The "high stuff" (wing, booms, tail) does have most of the wetted area (about 370 sq feet) but very little frontal area and flow is expected to be very good over most of it (wings outboard of the prop blast area). So, I'd think at cruise with both engines producing thrust, things will be fairly well balanced.
- At lower speeds (e.g. best climb speed), induced drag is much more significant. At 70 knots at SL, induced drag is expected to be about 55 lbs, parasite drag will be about 57 lbs total. So, the majority of the drag will be "high," and we might need some nose-down trim. But, if we look at the magnitude of the total drag at cruise (170 lbs), it is much higher than at climb speed (112 lbs). And we expect to spend more time at cruise speed, so it may make sense to bias the thrust line(s) for the cruise drag condition.

It'll be worth taking a close look at trim drag in the critical single engine scenario (MTOW, rear prop stopped).
 
Last edited:

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
9,019
Location
USA.
All of my plans has been using the Revmaster 85 HP engine. Revmaster also has a turbo version of 90 HP but don't think its on the market.

Hotrod BeetleMaster. Two Continental 0-200 engines with C-85 pistons, ( 8.5 CR over the stock 7.5 CR ). The engine in BoB Barrows Bearhawk LSA is a C-85-8 with the O-200 crank, rods, 90 hp cam and special pistons for 9.5 CR. He says about 115 HP. Would lose some useful load with the extra weight of the engines. He says the engine weight is 188 lbs.
Two 0-200 engines with electrics with light weight starter and alt would be about 215 lbs each.
With a GW of 1600 lbs and a total of 230 hp = about 7 to 1 Power to weight ratio. I'm in love, hotrod for sure.
 
Last edited:

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,024
Location
US
Hotrod BeetleMaster. Two Continental 0-200 engines with C-85 pistons, ( 8.5 CR over the stock 7.5 CR ). The engine in BoB Barrows Bearhawk LSA is a C-85-8 with the O-200 crank, rods, 90 hp cam and special pistons for 9.5 CR. He says about 115 HP. Would lose some useful load with the extra weight of the engines. He says the engine weight is 188 lbs.
Two 0-200 engines with electrics with light weight starter and alt would be about 215 lbs each.
I know that the focus is on the VW engine, but If I were designing, I would try to have a design that would accommodate the Rotax 912 and the Corvair.
Okay, alternative engines. Jan Carlsson was pushing for D-motors, which would be fun, I guess they've sold a few of them.
I think the Suzuki engines are interesting, and if we could use an upright version they are especially economical. The Aeromomentum AM 10 weighs 139 lbs dry (add a few pounds for coolant and radiator), rated at 85 HP, and is priced at $8500. So, not far off from a VW Type 1 in those respects. It measures 18w x 22h x 23L, so we could make the front and rear cowlings tapered on the sides to a narrower front/rear. I don't have measured drawings, but I think the prop hub at the PSRU is lower than the VW (which is about 8" below the highest accessory on the engine). Anyway, the price and power is right, and they sip fuel.
I'm still focused on keeping the "Beetle' in Beetlemaster. But, I do think it would be wise to design the structure of the plane to allow higher weights than our target, especially if it can be done with little added empty weight (e.g. if using pultruded rod spar caps, beefing them up weighs very little, etc). If SE climb on 80HP proves better than expected, folks will want to carry more fuel, goodies, etc. And if it proves disappointing, it would be best to have the structure in place to mount larger engines. Anyway, if someone could build the major structure to accommodate 1800 pounds, with 225 on each firewall, and it can be done at small cost (in empty weight), I don't think anyone would regret it. People aren't getting any lighter...;)
 
Last edited:

Doggzilla

Well-Known Member
HBA Supporter
Joined
Jun 2, 2013
Messages
2,352
Location
Everywhere USA
Not sure if this has been mentioned, but this layout has a significant CG advantage over other designs.

If you look at the 337 the windshield sticks out significantly further than the other Cessna aircraft. This is because the wing is further back to accommodate the weight of the rear engine.

This means the cargo is closer to the wing and not so far behind. Much better for CG.

This also means changing the engine types won’t have a significant effect on CG.

And it means it could effectively be used to haul 1-2 50 gallon drums behind the pilot. This is not feasible with any other layout in this power range because other layouts would have huge CG swings between loaded and empty. The only other possibility is a twin pusher, but that has clear limitations. The only advantage that design would have is huge CG centered fuel tanks in the engine cowlings.
 

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
9,019
Location
USA.
Not sure if this has been mentioned, but this layout has a significant CG advantage over other designs.

If you look at the 337 the windshield sticks out significantly further than the other Cessna aircraft. This is because the wing is further back to accommodate the weight of the rear engine.

This means the cargo is closer to the wing and not so far behind. Much better for CG.

This also means changing the engine types won’t have a significant effect on CG.

And it means it could effectively be used to haul 1-2 50 gallon drums behind the pilot. This is not feasible with any other layout in this power range because other layouts would have huge CG swings between loaded and empty. The only other possibility is a twin pusher, but that has clear limitations. The only advantage that design would have is huge CG centered fuel tanks in the engine cowlings.

You are correct. With the pilots eyes on the leading edge of the wing, an load in the rear seat area will be under the CG. Also adding weight on the engines will almost balance out.
I also like the BeetleMaster with 2 RevMasters and not crazy about the finicky, highly tuned, R-912's. For alternate engines, it would be a VW or Corvair. 2 Corvairs would be great for a hot-rod Beetlemaster. Then we could call it the CorMaster :)
I always want to design where I'm not painting myself in a corner. Always want added strength built into the design for weight along with speed that I may want to add as in a higher HP engine. I hate an underpowered airplane. The JMR is designed that way. Rated at 6.7 G's I could have saved some weight and went to 4.5 G's. Bob Barrows and I had a discussion on this subject, amount others. Look at Bob's Bearhawk LSA. It can be built as an LSA but also with a 1500 lb GW if wanted.
Leave your options open.
 

Doggzilla

Well-Known Member
HBA Supporter
Joined
Jun 2, 2013
Messages
2,352
Location
Everywhere USA
That’s the great thing about this layout, you can use whatever engines you want without the weight difference throwing the CG all out of whack. They counter balance one another.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,024
Location
US
With the 75-80 Hp VWs, the drag estimates we've made, and the prop/thrust estimates Jan did, the Beetlemaster variants could all climb at MTOW on one engine and go 150 MPH at 6K ft on less than 60% power. Some of Jan's projections said a very slippery Beetlemaster could do 175 MPH with 75HP engines and fixed pitch props (I'm a little skeptical, at least if we still want props that will give enough low-speed thrust for SE climb.) So, more installed power would be useful mainly if we wanted:
1) More payload/fuel (through improved SE climb ability). That'll work. But, you could also get that with the VW engines by reducing drag due to lift (as AutoReply's variant proposed-- a long wing.)
2) Better climb in normal ops at present MTOW. That will work, but fuel capacity will be a limiting factor. Or, use a finer pitch prop on the present VW engines and make more thrust at climb speeds (but lower top speeds).

A higher top speed might best be accomplished with adjustable props on whatever engine is chosen. Bring money. Revmaster engines are (I think) still plumbed with the oil line needed to control a prop, but they don't make the 2 position props that (IIRC) Joe Horvath and Ray Maloof produced for use on the R2100D engine and mounted on motorgliders (with a turbo?). The combination was not trouble-free. Ivoprop makes an electric in-flight adjustable prop, but I don't know if the weight and vibration characteristics work on VWs. The Prince P-tip props do work well on VWs and they flex enough to give a pitch change of a few inches between climb and cruise, which could be particularly useful here.

As pointed out above, the layout provides a lot of flexibility because it can accomodate a range of engine weights (and there can be room for more fuel, too, right near the CG.) The low-budget options can use cheap engines and fixed pitch props to get good all around performance at modest GPH. Bigger engines and/or adjustable pitch props can offer more performance, if desired.
 
Last edited:

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
9,019
Location
USA.
Airfoil-- Still thinking of the 2414 but, Bob Barrows is always pushing the Riblett 30-413.5 What do you think?
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,024
Location
US
Dan,
I don't know enough to have an informed opinion on the Riblett airfoils. It seems to me that the key things for the Beetlemaster wing (at least as I'd use it) are:
1) No surprises. Docile stall behavior, no abrupt drag bucket, etc.
2) Low drag at our most critical flight condition--single engine climb. This works out to a cL of about 0.8 to 1.0 (depending on airspeed and weight).
3) Mild pitch moment
4) Low drag at high speed (i.e. low AoA/low cL).

As far as I'm concerned, a composite wing is still "in the running", so a laminar flow airfoil needn't >necessarily< be ruled out, but they can be finicky (bugs, rain, etc). It would have to be well behaved if it was dirty, wet, etc.

Another factor to consider is the possible need for a thicker airfoil (to make room for fuel, to make the spar stiffer at less weight, etc). There's not much of a penalty for this with the 24XX series, I don't know if the Riblett is more particular (i.e. how would a Riblett 30-415 behave?)

Jan offered some comments on airfoils, Riblett, etc and the Beetlemaster here (post 289 and post 281)

Bob Barrrows knows an awful lot about getting good performance from his planes. Every plane is a tradeoff, and my impression is that short field performance (i.e. high cL flight) is very important to the Bearhawk series, and they started from that criteria. I see the Beetlemaster operating mostly on pavement or nice turf strips, land at about 55-60 MPH, but no tundra tires or sandbars. :) Maybe the Riblett would be great, I just don't know much about it.

For the Bearhawk LSA (170 sq ft of wing, up to 1500 lbs, 30 MPH claimed landing speed (= stall speed?)), he'd have to be getting a cL of 3.8 (with all the tricks--flaps, etc).
 
Last edited:

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
9,019
Location
USA.
The Bearhawk LSA does not have flaps. It cruises fast and stalls very slow. We were going to have a contest on which one would fly the slowest, my SSSC and Bob Bearhawk LSA. We taxied out to the end of the runway and I had just changed the oil pump cover gasket on my engine and had to cancel the flight. So it never happen. The SSSC would fly off in ground effect at about 26/27K and stall at an indicated 30k. I believe he would have beat me with his LSA. Neither aircraft has flaps and my 120 sq ft and Bobs at 170 sq ft. My flying weight that day of about 780 lbs with a wing loading of 6.5 lbs sq ft and Bobs at about 1000 lbs with a wing loading of 5.8 lbs sq ft. The riblett airfoil does a good job between slow flight and cruise speed. When the Bearhawk B model went to the riblett airfoil it gain a few mph in cruise.
Talking to Bob about it a few weeks ago and he thinks the Riblett airfoil would have a couple mph higher speed than the 2414 airfoil. The Riblett seems to do a good job on both ends.
Decisions, decisions. :)
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,024
Location
US
Bob's certainly got more experience than I do, and some proven winners. I should get a copy of Riblett's book and learn more.
Sometimes it seems everyone with a laptop computer is running javafoil. They are instant aeronautical engineers that have discovered the perfect new airfoil.
I'm a lot more comfortable stealing an airfoil that has a good track record in a similar mission. If the Bearhawks fit that description, then that's valuable info.
 
Last edited:

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
9,019
Location
USA.
Bob's certainly got more experience than I do, and some proven winners. I should get a copy of Riblett's book and learn more.
Sometimes it seems everyone with a laptop computer is running javafoil. They are instant aeronautical engineers that have discovered the perfect new airfoil.
I'm a lot more comfortable stealing an airfoil that has a good track record in a similar mission. If the Bearhawks fit that description, then that's valuable info.
Tried and proven is why I like the 2412 or 2414. I know for sure what I'm getting. I played with airfoils in the wind tunnel for several winter months on NASA's web site back when I was thinking of building the JMR and I always came back to the 2412 for just a good all around performing airfoil not knowing what airplanes it was used on. Finally decided to go with the 2414 for the extra strength of the main spar.
 

Nicholette

Member
Joined
Oct 26, 2014
Messages
11
Location
UK
Two inline VW's.? In my ongoing quest to build a 2/3 or 1/2 Fairey Gannet in my garage, I am going to say.. Hey why not mount them side by side (Or even a pair of Rotax) then a couple of belt drives with some sort of clutch to a coaxial dual tube prop shaft. You run with it, perfect it, make it work. Then I can go ahead with my plan :)
 

BJC

Well-Known Member
HBA Supporter
Joined
Oct 7, 2013
Messages
12,876
Location
97FL, Florida, USA
I should get a copy of Riblett's book and learn more.
I predict that you will find it interesting.

Just ignore his bias against some widely, and successfully, used airfoils, and understand that none of his performance characteristics are based on wind tunnel testing. All are based on computer analysis. That said, I think that his modifications to NACA airfoils are worthwhile.


BJC
 
Last edited:

BJC

Well-Known Member
HBA Supporter
Joined
Oct 7, 2013
Messages
12,876
Location
97FL, Florida, USA
Two inline VW's.? In my ongoing quest to build a 2/3 or 1/2 Fairey Gannet in my garage, I am going to say.. Hey why not mount them side by side (Or even a pair of Rotax) then a couple of belt drives with some sort of clutch to a coaxial dual tube prop shaft. You run with it, perfect it, make it work. Then I can go ahead with my plan :)
A former bomb disposal technician once told me that the best way to learn is through mistakes - other people’s mistakes.


BJC
 

Nicholette

Member
Joined
Oct 26, 2014
Messages
11
Location
UK
But of course, and that has always applied to aviators hasn't it? I had my biggest lesson on the importance of weight distribution, loading and C of G from my good friend who was lucky to get away with his life and mobility with a walking stick from a tail spin. At least I did not say "Hey why not try... ". I was brazenly open about my intentions, forgot to add a lottery win will also be required though :)
 
Last edited:

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,024
Location
US
Two inline VW's.? In my ongoing quest to build a 2/3 or 1/2 Fairey Gannet in my garage, I am going to say.. Hey why not mount them side by side (Or even a pair of Rotax) then a couple of belt drives with some sort of clutch to a coaxial dual tube prop shaft.
Sounds . . .interesting. ;)
160 hp hauling all that hardware around and trying to make thrust with it, I dunno. It would probably be able to taxi around well enough, if the pavement was flat. And it had a tailwind. With the canopies open.
 
Top