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rotax618

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Everything Matthew proposes is sound, for a single place aircraft, what I thought we were proposing, an 8' centre section with 4' outer panels would make it both trailerable and storable in a domestic garage and with rectangular planform provide sufficient area and span.
 

Vigilant1

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Matt, regarding your chart in post 3,429: all looks fine to me with two minor quibbles:
- I'd probably look at the numbers a bit and see how they change with slightly reduced wing area. 140 SQ ft with an 1100 lb airplane may be more wing than you need. The span gives you the climb you want using modest power and unless you need an exceptionally low flaps-up stall speed, the large area makes things heavier and the flying less fun in choppy conditions.

- I don't think you'll see 85% propeller efficiency in this plane in cruise flight (wood prop, cruise is not very fast, not a very big prop disk). I'll check later and chime in, but I think that 85% is likely optimistic.

Observation: The 50 lbs allowed for fuel in the chart would be very limiting. It is fine for the weight chart, but, at very least, it would be good to have a 16 gal tank so flown solo or with two light folks the plane could be useful for going places. My wife and I flew a 300+ mile leg in my Sonex, it holds 17 gallons. Lots of folks go farther than that.
 
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Riggerrob

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I disagree. A 65p aircraft engine has a lot more cooling fin and mass than a VW. The VW is heat dissipation limited. While a good pilot can manage the VW with two people the average ham handed 25hr a year at most pilot is likely to have problems. Very likely. It is better to outfit the plane with an engine that can dissipate the heat more efficiently for two people especially if the two people care about each other and the pilot is low on time.
What if you install an extra fan in the cowling, to suck more air - faster - past the cooling fins?
 

cluttonfred

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Blane, I am not at all married to the VW engine when it comes to the VP-21, it’s just that I don’t know of any other options that can compete when it comes to a 65+ hp four-stroke engine complete and ready to fly. The VW is long proven and well understood and, even setting no aside Aeroconversions and Great Plains and Hummel, you can go out and buy a complete 2180cc long block for about $4,000 and do the conversion yourself for (spitballing here) maybe $2,000 more. I have my eye on the Goakin ATV engines but until there is a proven conversion and a reliable supply VWs are still my go-to solution. The Air Trikes and Aeromomentum Suzuki conversions would be my next choice.
 

cluttonfred

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Quick answers...my logic here is that climb rate is more important than speed in a fun flyer. 1100 lb and 141.5 sq ft give 7.77 lb/sq ft, 1040 lb and 130 sq ft (VP-2) give 8 lb per sq ft so we are not talking about a big change. I would prefer to err on the side of better climb than top speed, rarely does that latter become a matter of life or death for a sport aircraft, but climb matters a lot with every short field take off or climb over an obstacle.

I would absolutely try to get as much span as possible to maximize efficiency, but part of that comes down to structural weight. I have long been a fan of the bending beam approach to wing structures used in sailplanes in which a wing half is essentially hinged at the longeron and tied to the opposite wing by the spar so the fuselage sees mostly vertical loads. Taking that approach with a generous 4' fuselage width you could have two overlapping 18' span wing panels that would produce a 32' span wing but still fit in a 20' ISO container (or small trailer or workshop) when removed.

I don't know enough about propeller efficiency to tweak those numbers, I just went with the standard values from the LAA performance estimation spreadsheet knowing full well that it gave only very rough approximations.

Absolutely agree on the fuel limitation, though my goal would be to get to a point where you can carry full fuel (whatever that number turns out to me) plus 2 x 200 lb adults (or ideally 220 lb adults given my current weight), if for no other reason than people tend to want to fill the tank and go flying. My goal would be 225 kg/496 lb useful load, which is enough for two hefty people (100 kg/220 lb each) plus the fuel allowance. Two hours at 4 gph means 48 lb, so 25 kg (55 lb) is enough for two hours with 30 minutes reserve, which I can live with in a fun flyer. More would, of course, be better.

Matt, regarding your chart in post 3,429: all looks fine to me with two minor quibbles:
- I'd probably look at the numbers a bit and see how they change with slightly reduced wing area. 140 SQ ft with an 1100 lb airplane may be more wing than you need. The span gives you the climb you want using modest power and unless you need an exceptionally low flaps-up stall speed, the large area makes things heavier and the flying less fun in choppy conditions.

- I don't think you'll see 85% propeller efficiency in this plane in cruise flight (wood prop, cruise is not very fast, not a very big prop disk). I'll check later and chime in, but I think that 85% is likely optimistic.

Observation: The 50 lbs allowed for fuel in the chart would be very limiting. It is fine for the weight chart, but, at very least, it would be good to have a 16 gal tank so flown solo or with two light folks the plane could be useful for going places. My wife and I flew a 300+ mile leg in my Sonex, it holds 17 gallons. Lots of folks go farther than that.
 

Topaz

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...I don't know enough about propeller efficiency to tweak those numbers, I just went with the standard values from the LAA performance estimation spreadsheet knowing full well that it gave only very rough approximations.
For a wood fixed-pitch prop (all VW conversions of which I know ask for a wood prop), you're looking at more like np=0.70 best-case. A really optimized wood and composite prop might be able to get to 0.75, but I've never seen such a combination on a real-world VW-powered homebuilt.

Absolutely agree on the fuel limitation, though my goal would be to get to a point where you can carry full fuel (whatever that number turns out to me) plus 2 x 200 lb adults (or ideally 220 lb adults given my current weight), if for no other reason than people tend to want to fill the tank and go flying. My goal would be 225 kg/496 lb useful load, which is enough for two hefty people (100 kg/220 lb each) plus the fuel allowance. Two hours at 4 gph means 48 lb, so 25 kg (55 lb) is enough for two hours with 30 minutes reserve, which I can live with in a fun flyer. More would, of course, be better.
It's all going to be a trade around a useful load that'll be a bit under 500 lbs. How you slice it is up to you. Like Vigilant1, I tend to see 300 miles as a more-useful range than the 150-200 you're describing, and I personally think 50 gallons is going to be on the lower end of that range once you factor in a reasonable climb, etc. Every time I've done the numbers on a small VW-powered two-seater, I always end up with about 480 lbs of useful load, of which almost exactly 100 lbs is fuel, very similar to what Vigilant1 describes. I split payload between pilot, passenger, and baggage as follows: A split with a 180lb pilot and a 200lb passenger is the same total payload as a 180lb pilot, a 150 passenger, and 50 lbs of baggage, the latter of which is "tight but doable" for a couple on a "light" weekend trip. The 180/200 and zero baggage is the pilot and a buddy to an airshow for the day, a $100 breakfast, etc. Two "large" 250 lb passengers and 100 lbs of fuel ends up at 600 lbs of useful load, which I think is just too much to ask for an engine in the 65hp class for anything but a motorglider-like span. Even 550 lbs is probably asking too much, IMHO. Two 220 pounders with 50 pounds of gas fits, but you won't be carrying any baggage.

However you slice it, 450-480 lbs is probably the about most useful load one can expect in an airplane in this class without needing a span that makes it hard to build in the usual garage build-space.
 

cluttonfred

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Thanks, Topaz, for all the feedback. I am deliberately staying lighter than LSA limits to keep things affordable and get the most out of modest power: 1320 lb on a VW is very different than 1100 lb on the same engine. Target would be 600 lb empty for 500 lb useful load knowing that keeping an enclosed two-seater that light would be a challenge.

However you slice it, 450-480 lbs is probably the about most useful load one can expect in an airplane in this class without needing a span that makes it hard to build in the usual garage build-space.
 

Topaz

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Thanks, Topaz, for all the feedback. I am deliberately staying lighter than LSA limits to keep things affordable and get the most out of modest power: 1320 lb on a VW is very different than 1100 lb on the same engine. Target would be 600 lb empty for 500 lb useful load knowing that keeping an enclosed two-seater that light would be a challenge.
LSA is a good fit for an airplane like this, because the performance limitations help you work towards a lighter, less-expensive, structure, rather than try and get another 20 knots of cruise. 😉
 

blane.c

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Blane, I am not at all married to the VW engine when it comes to the VP-21, it’s just that I don’t know of any other options that can compete when it comes to a 65+ hp four-stroke engine complete and ready to fly. The VW is long proven and well understood and, even setting no aside Aeroconversions and Great Plains and Hummel, you can go out and buy a complete 2180cc long block for about $4,000 and do the conversion yourself for (spitballing here) maybe $2,000 more. I have my eye on the Goakin ATV engines but until there is a proven conversion and a reliable supply VWs are still my go-to solution. The Air Trikes and Aeromomentum Suzuki conversions would be my next choice.
I think that 65hp is a problem to be stuck on when it comes to two place. Well honestly it isn't the hp it is the cubic displacement. Two people are carried fine with 170ci/2800cc despite hp rating. Were as 115ci more or less isn't enough whatever the hp rating is for two up. This is direct drive of course, but there are options when you add a pto that are lighter. Of course if you want a two up that takes off at 40mph, fly's at 40mph and lands at 40mph well bigger wing it is, but I still question cooling at lower speed.
 

blane.c

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Thanks, Topaz, for all the feedback. I am deliberately staying lighter than LSA limits to keep things affordable and get the most out of modest power: 1320 lb on a VW is very different than 1100 lb on the same engine. Target would be 600 lb empty for 500 lb useful load knowing that keeping an enclosed two-seater that light would be a challenge.
Just to be clear I didn't suggest 1320lb and a VW engine, it was 1320lb and a Suzuki liquid cooled variant.
 

Tiger Tim

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What if you install an extra fan in the cowling, to suck more air - faster - past the cooling fins?
That’s kind of a good point. IIRC when Pietenpol designed his Corvair conversion he left all the factory cooling shrouds (and fan?) on because “GM spent millions designing this, I won’t do any better.”

A Beetle had quite a large fan shroud but later air cooled VWs had very compact cooling to fit the whole unit under the rear floor. I wonder if any gains could be found in climb cooling by keeping VW’s low-profile shrouding.

^this would be easy enough to cowl and keep a high thrust line
 

Vigilant1

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If a fan is desired, just mount it on the driveshaft. Lighter and it eliminates a possible belt failure issue, and the approach works just fine on industrial engines turning at about the same speed, the Cessna 337 rear engine, etc.
But if we just want 70 HP, a fan isn't needed if we have good ducting and reasonable airspeeds (not for air volume, but for air pressure).
 

Topaz

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...But if we just want 70 HP, a fan isn't needed if we have good ducting and reasonable airspeeds (not for air volume, but for air pressure).
People keep looking at the VW and saying, "inadequate cooling," which is utter nonsense. Used within its power, rev, and overhaul limits, a VW will last longer than any airframe you put it in. What happens, however, is that people put one in the wrong airframe (too heavy, too overloaded, etc.) and then blame the engine when it fails after being pushed WOT endlessly without any regard for the max. continuous horsepower limit.

Any VW-powered two-seater is going to be light, have more span than a similar C-65 powered airplane, and isn't going to exceed about 175 mph in level flight in any event because of the other characteristics of its airframe (area, span). It's not going to be limited to "40 mph" because there's no correlation between top speed and engine cooling for a well-designed system. VW's don't need "extra" cooling, they need to have good cooling and some respect for their limitations given by the designer and the pilot - just like any other engine. Don't overload the engine with too much airframe or payload, and when the POH says "After 5 minutes, reduce power to the maximum continuous horsepower setting", you do so or face the consequences of abusing your engine.

A properly built KR-2 (meaning, at the original empty weight and at no more than the design useful load, two conditions that are rarely met) will cruise 150-165mph until the tanks run dry on a 65hp VW and do it reliably for years. That people built them over-weight, overloaded them with too much cheeseburger-enhanced payload, and then ran the engine WOT for the entire climb for flight after flight isn't the fault of the engine. Abuse ANY engine and it's going to fail. Period.

"What everyone knows" on this subject is so far divorced from reality as to beggar belief. And it just gets worse as fewer VW's end up in airplanes. People in the '70's knew better, and VW's were plentiful and reliable powerplants. People today "know better" because they really know nothing.
 

Bigshu

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Blane, I am not at all married to the VW engine when it comes to the VP-21, it’s just that I don’t know of any other options that can compete when it comes to a 65+ hp four-stroke engine complete and ready to fly. The VW is long proven and well understood and, even setting no aside Aeroconversions and Great Plains and Hummel, you can go out and buy a complete 2180cc long block for about $4,000 and do the conversion yourself for (spitballing here) maybe $2,000 more. I have my eye on the Goakin ATV engines but until there is a proven conversion and a reliable supply VWs are still my go-to solution. The Air Trikes and Aeromomentum Suzuki conversions would be my next choice.
Why are we limited to 65 hp for a VW conversion? Cassler's site quotes prices for 85hp, and 20 extra hp solves a lot of performance issues. If the weight and rpm's are the same, I'll take the extra hp, please! Cooling probably would need extra attention, but that doesn't have to mean more structural weight. Personally, I'm a Corvair conversion guy. Around 100hp without too much trouble, and the weight would still be doable at the LSA limits, much less whatever the new LPA rules will be. You see them on lots of airframes, so why not a simple to build one?
 

Topaz

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Why are we limited to 65 hp for a VW conversion? Cassler's site quotes prices for 85hp, and 20 extra hp solves a lot of performance issues. If the weight and rpm's are the same, I'll take the extra hp, please! Cooling probably would need extra attention, but that doesn't have to mean more structural weight. Personally, I'm a Corvair conversion guy. Around 100hp without too much trouble, and the weight would still be doable at the LSA limits, much less whatever the new LPA rules will be. You see them on lots of airframes, so why not a simple to build one?
Because without extensive modifications to the heads, cylinder cooling fins, etc., you end up with an engine that has a 85hp+ WOT output that can be used for five minutes and then must be throttled back to about 60hp for the rest of the climb/flight. The architecture of the VW engine wasn't ever intended for such "high" power outputs continuously. Yes, you can force that much fuel and air into the cylinders and extract that much power from one - for a brief time. But the engine simply doesn't extract heat fast enough to sustain those power levels. You could force an O-360 to put out 300hp if you work at it hard enough, but you're going to have to throttle back to 180hp pretty quickly or that engine will burn up, too. It's the same thing. Scott Casler will tell you flat-out that his engines - all of them - have both maximum and continuous power ratings, and you respect those or you burn up your engine.

The 85hp VW offerings you see out there have had significant modifications done, to the point where "still a VW" might not even apply. And most of them have a pretty severe drop to the maximum continuous power limit, whatever it is. Might be 65hp, might be 70hp, but it sure as heck isn't 85hp all day long.

The ultimate fact of the matter is that 65hp maximum, 60hp continuous, is about what you can get out of an 1835cc "simple and inexpensive" VW conversion. If you want/need more than that, either opt for one of the larger (2100cc), more heavily-modified, and much more expensive 70-80hp conversions, or opt for a different family of engines, such as the Corvair. You're not going to find "simple", "cheap", and "80hp+" out of a VW conversion, any more than you'll find "simple," "cheap," and "300hp+" out of an O-360 "conversion."

You have to respect the engine for what it is, and not try to make it do more than what it can. "Add a fan" isn't going to change this.
 
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