Pietenpol-List: VW engines in Pietenpols
Posted: Mon Feb 24, 2014 2:21 pm
Original Posted By: "Jack Phillips"
Recently there have been a couple of questions raised regarding the suitabilityof VW engines in Pietenpol aircraft. Following is an article written by Pietenpolbuilder Brian Kenney, and published in the BPA newsletter in 1996. Foodfor thought.Why a VW "Beetle" engine is not a good choice in a Pietenpol!A Volkswagen "Bug" engine and its derivatives can produce enough horsepowerto fly a Pietenpol Air Camper. This alone is not enough of a reason to selectit as a Pietenpol engine. This is why:The efficiency of a propeller (irrespective of how well it is designed)is a functionof the difference between the speed of the airplane and the speed of thepropeller wash. As the speed of the prop wash approaches the speed of the airplane,the propeller approaches 100% of theoretical efficiency. At the same time,thrust is caused by the propeller throwing air backwards. The faster the air,the more the thrust. As the air speed approaches the speed of the airplanethe thrust goes to zero! At zero forward speed the thrust is maximum.Put these two together and you see you can't have both at once. If the airplaneis not moving it has a lot of thrust, its efficiency is zero--it is doing nowork. As the airplane accelerates, the work (force times distance) and the efficiencyincreases but the thrust decreases. In the case of a zero drag airplanethe airplane can accelerate until it reaches the thrust speed. There is zerothrust and the propeller is approaching 100% efficiency.Since most aircraft don't have zero drag (especially Pietenpols!), the airplanecomes to equilibrium somewhere between the two extremes. In the case of a Pietwith a large propeller and a slow turning engine, it is when the propeller isat about 75% efficiency. You can not get better than that unless you clean upthe drag.One variable you can adjust in a propeller, that has an effect on efficiency, isthe propeller diameter. The bigger the propeller the more air it can push backwards.Therefore for a given propeller wash speed there is more thrust. Oranother way to look at it; for a required thrust, a bigger propeller needs lesswash speed. Therefore, if you remember about efficiency, there is more efficiencybecause for a given thrust the velocity of the propeller wash is less.Diameter also effects pitch. The larger the diameter, the less pitch you need (theair can move slower). This effects the speed range of the aircraft. A largepropeller is like having a car with one low gear. A small propeller is likehaving one high gear. If you have a slow airplane, a low gear can work fine, ina fast airplane, it won't work because the engine will not produce any powerto get going (fast airplanes always have surplus horsepower).The relationship for best propeller efficiency has been determined to be that thepropeller tip speed should be approximately 2.3 times the aircraft speed atcruise. This you can't achieve. The diameter is too large or the rpm is too slow.The bigger the diameter and the slower the engine the better. This is whyWorld War One aircraft perform so well (best propeller efficiency) on low power- 1400 rpm engines turning 80" propellers.. Because our engine choices requiremore revs to produce power, we have to compromise and lose efficiency. A Pietwith a 72" propeller, 2300 max. rpm and 65 horsepower is the typical compromisewith a Continental engine, but a Model "A" with 55 horsepower will do aboutthe same.Unfortunately, to retain reliability, Model "A's" are usually built to produce50 horsepower - or a bit less.The Corvair engine is another compromise. They have a loss of efficiency due tothe small diameter propeller and accelerate poorly (due to the tall gear effect)but produces good power.So how does this relate to a VW engine?In order to use a Volkswagen engine, it has to really rev (over 3300 rpm) to producesufficient power. This requires a small propeller to keep the tip speedsdown. It therefore has poor efficiency, or a too "tall" gear. If you pitch itso you can take off, you won't fly faster than about 50 mph. If you pitch itfor cruise you will need a 6000 ft strip for take-off! The Corvair works becauseit has surplus horsepower and can afford to waste some. The VW can't affordthe wasted horsepower.The only approach with a VW that will work is the one that Pazmany used on hisPL-4. It uses a reduction belt. You then need a starter, and alternator, wiringetc. You also need to think about cooling. At 60 mph there is not much ramair. The Corvair requires a blower to get proper cooling. If you go this routeyou have two projects instead of one! In Pazmany's configuration, the installationprobably weighs enough to allow an acceptable C.G. If not, this is yoursecond major problem. If you solve these problems, get used to an engine revvinglike crazy, making much noise, as you cruise along.Aircraft are a compromise. In a slow airplane you must use a large displacement,slow turning engine if you want to keep it simple!How about a diesel?Someone in the 1970's put a Mercedes diesel in a Pietenpol. They brought it toOshkosh and created a lot of interest. The problem was it didn't have enough power.The engine was replaced with something else and it flew OK. The limitationwith diesel engines is their power-to-weight ratio. They tend to be heavyfor the power they produce. The VW diesel engine is not that heavy, but I don'tthink that it will produce enough power. You need about 50 hp to fly an AirCamper - a bit less to fly the Sky Scout. You need this type of power at no morethan about 2500 rpm. This is why a VW beetle engine is no good - because youhave to rev it too high to obtain the needed power.The other factors to consider in the over-all weight of a VW Diesel, is that youwill probably need a gear reduction unit, and if you use a gear reduction unit,you will also need a starter. Water in the cooling system weighs 10 lbs. agallon etc. I am sure you will be over 250 lbs. when you are finished.Fuel is also an issue. You could use Jet A, but how will you get that big hosenozzle in your Pietenpol filler tube?I am not trying to discourage anyone from developing a new airplane design. Justthink about it long and hard before you try it.I think building an airplane it enough of a project. Building an engine is anothercomplete project. I know someone who has been putting an auto engine in aflying airplane and is now in his fifth year in doing so!Read this topic online here:http://forums.matronics.com/viewtopic.p ... __________
Recently there have been a couple of questions raised regarding the suitabilityof VW engines in Pietenpol aircraft. Following is an article written by Pietenpolbuilder Brian Kenney, and published in the BPA newsletter in 1996. Foodfor thought.Why a VW "Beetle" engine is not a good choice in a Pietenpol!A Volkswagen "Bug" engine and its derivatives can produce enough horsepowerto fly a Pietenpol Air Camper. This alone is not enough of a reason to selectit as a Pietenpol engine. This is why:The efficiency of a propeller (irrespective of how well it is designed)is a functionof the difference between the speed of the airplane and the speed of thepropeller wash. As the speed of the prop wash approaches the speed of the airplane,the propeller approaches 100% of theoretical efficiency. At the same time,thrust is caused by the propeller throwing air backwards. The faster the air,the more the thrust. As the air speed approaches the speed of the airplanethe thrust goes to zero! At zero forward speed the thrust is maximum.Put these two together and you see you can't have both at once. If the airplaneis not moving it has a lot of thrust, its efficiency is zero--it is doing nowork. As the airplane accelerates, the work (force times distance) and the efficiencyincreases but the thrust decreases. In the case of a zero drag airplanethe airplane can accelerate until it reaches the thrust speed. There is zerothrust and the propeller is approaching 100% efficiency.Since most aircraft don't have zero drag (especially Pietenpols!), the airplanecomes to equilibrium somewhere between the two extremes. In the case of a Pietwith a large propeller and a slow turning engine, it is when the propeller isat about 75% efficiency. You can not get better than that unless you clean upthe drag.One variable you can adjust in a propeller, that has an effect on efficiency, isthe propeller diameter. The bigger the propeller the more air it can push backwards.Therefore for a given propeller wash speed there is more thrust. Oranother way to look at it; for a required thrust, a bigger propeller needs lesswash speed. Therefore, if you remember about efficiency, there is more efficiencybecause for a given thrust the velocity of the propeller wash is less.Diameter also effects pitch. The larger the diameter, the less pitch you need (theair can move slower). This effects the speed range of the aircraft. A largepropeller is like having a car with one low gear. A small propeller is likehaving one high gear. If you have a slow airplane, a low gear can work fine, ina fast airplane, it won't work because the engine will not produce any powerto get going (fast airplanes always have surplus horsepower).The relationship for best propeller efficiency has been determined to be that thepropeller tip speed should be approximately 2.3 times the aircraft speed atcruise. This you can't achieve. The diameter is too large or the rpm is too slow.The bigger the diameter and the slower the engine the better. This is whyWorld War One aircraft perform so well (best propeller efficiency) on low power- 1400 rpm engines turning 80" propellers.. Because our engine choices requiremore revs to produce power, we have to compromise and lose efficiency. A Pietwith a 72" propeller, 2300 max. rpm and 65 horsepower is the typical compromisewith a Continental engine, but a Model "A" with 55 horsepower will do aboutthe same.Unfortunately, to retain reliability, Model "A's" are usually built to produce50 horsepower - or a bit less.The Corvair engine is another compromise. They have a loss of efficiency due tothe small diameter propeller and accelerate poorly (due to the tall gear effect)but produces good power.So how does this relate to a VW engine?In order to use a Volkswagen engine, it has to really rev (over 3300 rpm) to producesufficient power. This requires a small propeller to keep the tip speedsdown. It therefore has poor efficiency, or a too "tall" gear. If you pitch itso you can take off, you won't fly faster than about 50 mph. If you pitch itfor cruise you will need a 6000 ft strip for take-off! The Corvair works becauseit has surplus horsepower and can afford to waste some. The VW can't affordthe wasted horsepower.The only approach with a VW that will work is the one that Pazmany used on hisPL-4. It uses a reduction belt. You then need a starter, and alternator, wiringetc. You also need to think about cooling. At 60 mph there is not much ramair. The Corvair requires a blower to get proper cooling. If you go this routeyou have two projects instead of one! In Pazmany's configuration, the installationprobably weighs enough to allow an acceptable C.G. If not, this is yoursecond major problem. If you solve these problems, get used to an engine revvinglike crazy, making much noise, as you cruise along.Aircraft are a compromise. In a slow airplane you must use a large displacement,slow turning engine if you want to keep it simple!How about a diesel?Someone in the 1970's put a Mercedes diesel in a Pietenpol. They brought it toOshkosh and created a lot of interest. The problem was it didn't have enough power.The engine was replaced with something else and it flew OK. The limitationwith diesel engines is their power-to-weight ratio. They tend to be heavyfor the power they produce. The VW diesel engine is not that heavy, but I don'tthink that it will produce enough power. You need about 50 hp to fly an AirCamper - a bit less to fly the Sky Scout. You need this type of power at no morethan about 2500 rpm. This is why a VW beetle engine is no good - because youhave to rev it too high to obtain the needed power.The other factors to consider in the over-all weight of a VW Diesel, is that youwill probably need a gear reduction unit, and if you use a gear reduction unit,you will also need a starter. Water in the cooling system weighs 10 lbs. agallon etc. I am sure you will be over 250 lbs. when you are finished.Fuel is also an issue. You could use Jet A, but how will you get that big hosenozzle in your Pietenpol filler tube?I am not trying to discourage anyone from developing a new airplane design. Justthink about it long and hard before you try it.I think building an airplane it enough of a project. Building an engine is anothercomplete project. I know someone who has been putting an auto engine in aflying airplane and is now in his fifth year in doing so!Read this topic online here:http://forums.matronics.com/viewtopic.p ... __________
Let me ask you a question:There are several variants of VW engine. The smallest is obviously too small topower a Piet, but then we also have the bigger VWs, which have more power. Iam talking about VW with 2500ccm, which produces 85hp. What would happen if Iinstalled a larger propeller on it (170cm) - it would decrease the rpm, the powerwill be lower but torque will increase. Am I right?Would that be a sufficient solution?You know, the advantage of the VW which is connected directly with the propeller- so no reduction drive- is that you don't need to cool down the engine so much,you don't need an electric starter. So I can save let's say 15kg in weightand coolants, liquids.Because this VW is lighter, it would need to be moved slightly forward, but I thinkthat not more than 5cm.So if my ideas are not totaly wrong, it looks like this 2500ccm (2,5litre) VW wouldbe sufficient for a decent flight with a passenger in a Pietenpol.And now I stabbed the wasp's nest 
--------My production of WW1 propellers, trophies and constructions:Read this topic online here: