This past week PBS aired a 3 hour show called "PLANE CRAZY." Hope you can see it on public TV in your area!!

It was about a man who built (or attemptedto build) a plane in 30 days. Any-who, on his second 30 day attempt, he built a Fisher designed kit "YOUNGSTER", with the help of the Fisher family. (6 people it took)

A lesson can be learned here if you think about it; and the point about VW engines I want to demonstrate is this:

The Fisher plane weighed just 320 pounds (dry). The pilot added 180 lbs+. (It's a single place plane.) It took off of from a super nice flat blacktop runway of 2500+ feet. This "Youngster" ate up over half the runway, and barely flew -- with very little extra power -- at about 45-50 mph behind its' VW engine . . . "Struggle" may be a better word.

The thinking part comes in here: People are always asking: "CAN I USE A VW ENGINE IN A PIET?" The answer is a resounding "NO".

In the "Youngster" example you have an airplane weighing half that of an Air Camper, carrying only one person and operating in an almost perfect environment -- and it can barely fly.

Lets change a few elements in the equation: Using the same VW engine; double the weight to 650 lbs, double the people to 360 lbs, double the fuel to 80-100 lbs -- then take off from a short grass runway that has not been mowed in a month. Add a dew drop or two, and a few gopher holes. Hummmmmm. Do you want a VW engine? NO WAY!

VW proponents will come back and say: "Well, you know they made some larger displacement engines in the mid 70's." Increasing CC's does increase the torque. But the margin of improvement is not great enough, and most VW's are middle-of-the-road type.

How often have you ever seen a Sky Scout or Air Camper fly behind a VW engine? NEVER! It has been tried and it did not pan out.

Corvairs, A65's, A85's, Model A's, perform well, and I personally believe (not proven) that if a guy wanted to play around with that JABARU engine certified out of Austrailia, a lot of fun could be had at a moderate cost.

I see gear reduction kits and Rotax engines as places for things to go wrong, parts to break, and additional expense.

Follow the PLANS + Build it with Quality Materials = An Airplane that flies well, is safe and you can be proud of yourself when you fly/show it to others!!!

Andrew Pietenpol

A note from BPA Newsletter editor Grant MacLaren:
The author of this article is Brian Kenney (1483 Newlands Crescent, Burlington Ontario L7M 1N8 Canada) Brian is the builder of Pietenpol Air Camper C-FAUK, powered by a 65 hp. 'flat four' Continental aircraft engine. At my request, Brian supplied this article for use in our BPA Newsletter and for my use in answering the many Pietenpol newcomers who ask us "Can I power a Piet with a VW engine?''

March 6, 1996 - Grant; for your use as requested. Feel free to editorialize.

A Volkswagen "Bug" engine and it's derivatives can produce enough horsepower to fly a Pietenpol Air Camper. This alone is not enough of a reason to select it as a Pietenpol engine. This is why.

The efficiency of a propeller (irrespective of how well it is designed) is a function of the difference between the speed of the airplane and the speed of the propeller 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 airplane the 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 airplane is not moving it has a lot of thrust, its efficiency is zero--it is doing no work. As the airplane accelerates, the work (force times distance) and the efficiency increases but the thrust decreases. In the case of a zero drag airplane the airplane can accelerate until it reaches the thrust speed. There is zero thrust and the propeller is approaching 100% efficiency.

Since most aircraft don't have zero drag (especially Pietenpols!), the airplane comes to equilibrium somewhere between the two extremes. In the case of a Piet with a large propeller and a slow turning engine, it is when the propeller is at about 75% efficiency. You can not get better than that unless you clean up the drag.

One variable you can adjust in a propeller, that has an effect on efficiency, is the propeller diameter. The bigger the propeller the more air it can push backwards. Therefore for a given propeller wash speed there is more thrust. Or another way to look at it; for a required thrust, a bigger propeller needs less wash speed. Therefore, if you remember about efficiency, there is more efficiency because 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 (the air can move slower). This effects the speed range of the aircraft. A large propeller is like having a car with one low gear. A small propeller is like having one high gear. If you have a slow airplane, a low gear can work fine, in a fast airplane, it won't work because the engine will not produce any power to get going (fast airplanes always have surplus horsepower).

The relationship for best propeller efficiency has been determined to be that the propeller tip speed should be approximately 2.3 times the aircraft speed at cruise. 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 why World War One aircraft perform so well (best propeller efficiency) on low power - 1400 rpm engines turning 80" propellers.. Because our engine choices require more revs. to produce power, we have to compromise and lose efficiency. A Piet with a 72" propeller, 2300 max. rpm and 65 horsepower is the typical compromise with a Continental engine, but a Model "A" with 55 horsepower will do about the same. Unfortunately, to retain reliability, Model "A's" are usually built to produce 50 horsepower - or a bit less.

The Corvair engine is another compromise. They have a loss of efficiency due to the 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 produce sufficient power. This requires a small propeller to keep the tip speeds down. It therefore has poor efficiency, or a too "tall" gear. If you pitch it so you can take off, you won't fly faster than about 50 mph. If you pitch it for cruise you will need a 6000 ft strip for take-off! The Corvair works because it has surplus horsepower and can afford to waste some. The VW can't afford the wasted horsepower.

The only approach with a VW that will work is the one that Pazmany used on his PL-4. It uses a reduction belt. You then need a starter, and alternator, wiring etc. You also need to think about cooling. At 60 mph there is not much ram air. The Corvair requires a blower to get proper cooling. If you go this route you have two projects instead of one! In Pazmany's configuration, the installation probably weighs enough to allow an acceptable C.G. If not, this is your second major problem. If you solve these problems, get use to an engine revving like 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 to Oshkosh 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 limitation with diesel engines is their power-to-weight ratio. They tend to be heavy for the power they produce. The VW diesel engine is not that heavy, but I don't think that it will produce enough power. You need about 50 hp to fly an Air Camper - a bit less to fly the Sky Scout. You need this type of power at no more than about 2500 rpm. This is why a VW beetle engine is no good - because you have 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 you will 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. a gallon 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 hose nozzle in your Pietenpol filler tube?

I am not trying to discourage anyone from developing a new airplane design. Just think about it long and hard before you try it. (And understand that everthing you change on an airplane produces and/or requires changes elsewhere in the design. gem)

I think building an airplane it enough of a project. Building an engine is another complete project. I know someone who has been putting an auto engine in a flying airplane and is now in his fifth year in doing so!