A Better LEGO GA8 Airvan Engine?

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Niels Abildgaard

Well-Known Member
Jun 13, 2010
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LEGO has donated a GA8 aircraft to MAF who fly out to help people in trouble all over the world.
I have bought a lot of LEGO toybricks for kids and grands so I am proud co -donor of this gift.
The MAF ratio collected money/given help ,depends first and foremost on engine mass and fuel consumption .
It is an acceptable,decent and worthwhile pastime to scheme a better engine for this aircraft.
The donated GA8 aircraft is pulled by a 2.134 m dia propeller turning 45 turns per second and absorbing 300 horsepower.
The same job can be done by two interweaving 1.6 m props placed 1.2 meter apart as shown on first picture .
Author did time on motor-torpedoboats with 72 diesel cylinders and 264 poppet valves .
Diesels and multicylindered four-strokes are misconceptions.

My mental limitations cause a self generating layout shown on picture 2.

It is a single 140 mm bore cylinder with two pistons stroking 210 mm each or 6.5 litre.
For having a sensible piston mean speed of 14 m per second we will turn 33.333 rounds per second .
150 horsepower on each shaft means that we must blow and fuel-inject for 10.5 bar mean effective pressure and this is very mild for a Junkers Otto-engine.
WW2 engine DB 601 is ca 20 kg per litre and works to 15 bar mean effective pressure.
Specific mass of our simpler headless engine can surely be 17 kg per litre or less.
Engine mass including blower and cooler will come to 110 kg.
Existing engine is heavier and uses more and leaded petrol to do the job
Noise from new arrangement is dramatically lower due to the much lower propeller-tipspeed.
With so many good points surely something must be wrong.
To find the weak point (if any) modelling is cheaper and less dangerous than full size.
If there is a problem it is most likely harmonic vibrations between the two propellers over the bevel gears and carbon shaft .
I am trying to make crankshafts and bevel gears for a small test engine shown on picture 3.

Anybody interested in doing something worthwhile for mankind before closing time?




How dose one chuck a cat :big: Im joking. That looks like a very interesting project in deed and you are doing a wonderful job of it.

Hello Dave and thank You for kind words.The cat owns me and I am to big to be chucked.

Somehow I got fed up with the layout for test engine and parts shown with cat.
The pictures show
Improved (Easier to make) layout
Close up of same
How I intend to make crankshaft

The problem for amateurs is of course that the hard crankpin is not easy to make.
On the other hand I have no difficulty cutting a hard commercial pin and drill a central hole
If the screwed up scheme fails I will either braze with copper (I know one who can) or go the hard way:turn ,casecoaling ,remowing unwanted carbon surfaces ,harden and grind.




buisness end.jpg
An intriguing idea for sure ,Niels,but I wonder if the losses in the gearing wouldn't be too much.Also,both propellors would be causing massive turbulence,their "wake" being intermingled.Would they also not need to be contra-rotating?Not wishing to rain on your parade,but there are some natural forces at work here that can't be changed at will.
I wish I could help you to figure it out,but "a little knowledge is a dangerous thing".I have enough trouble getting a simple engine to run :big:
Hello Hans

Purpose of gearing is to synchronize the two shaft/prop assemblies.The power absorption of the two props will be made identical in a stationary single shaft test rig .The only power going via the carbon shaft is difference in friction of the pistons and the blades going partly into the wake of each other.We did a test on two 20 kW electric motors with 1.6 meter props.Synchronization was done by a crossed bicycle chain and it ran for hours with lubrication from an oil can when stopped.If there is a problem it will be due to the three times motor rev where one blade absorbs a little less power due to going into wake of a neighbour and this is the whole point of the exercise to find out and how bad.The Boeing Vertol helicopters have the same problem.
Hi Niels

you could simplify the Synchronization a lot without bevel gears, by just using a crank connecting rod between the two shafts at the opposite end to the propellers. This would turn the props in the same rotation. Which is OK if the props are short enough relative to the engine stroke (not so good for intersecting arcs)

If the shafts must rotate in opposite directions, add another con rod a pair of spur gears that would do it a lot easier than bevel gears :idea:


less friction surfaces and much simpler to build

Hello Bez

To be vibrationfree opposite rotation is a must.
Then I would rather use a crossed chain as the Wright Brothers did.
They did a crossed chain to starboard and uncrossed to backboard and achieved that prop reaction torque on airframe was zero.My scheme is one better as engine constant torque is zero also.
The NSU Supermax motorbike and Prinz minicar used a connecting rod drive to camshaft and was silent but much more expensive than chains or timing belt.And a pest to repair.Thermal expansion of the rods relative to engine frame makes it difficult.
Bevel gears must be possible as every rear drive car and some front drive has a pair but it is certainly a borderline case for amateurs
Hej Niels

I thought that the only fast motor torpedoboats we had here in Denmark where gasturbine powered .... 55 knots.

Back to your questions. I've got a problem with your use of different size pistons - the one on the exhaust side being much bigger and heavier => a side to side rocking engine !

I can see a problem with crankcase breathing too - you have a very high crankcase compression .... so, unless you run the crankcases under vacuum, you're in trouble, since you aren't using them for charging the cylinder.

I believe that the shaft will perform perfectly, if the dimension are correct (strong enough) and as pictured you already got the props running opposite directions.

Last problem and not the least, will be the engine cowl. All props are first and foremost active in creating lift (suck) and much less as push, BUT you still have to move some airpast the blades and over the cowl.
Hello Admiral

The boats You are referring to (Sealion class) were planing and did more than 60 without torpedoes and in cold air.
I did time on Falcon class that were uprated German E-boats and the 20 cylinder Benz engines were made for airships before WW2.We never saw more than 34 Knots.
The pistons must have same mass but in two strokes piston mass is not quite so harmfull as in fourstrokes (mechanical misconceptions)
The cold inlet piston can be cast iron to make it as heavy as the exhaust one.
For the test engine I in tend to rely on crankcase scavenging but the drawing is not very clear.
I think the installation will be better aerodynamically than present engines.A smaller part of the slipstream glides along the fuselage than with a corresponding single prop.
I have started to make a crankshaft the difficult but correct way and picture shows a piece of 16MnCr5 carboracing steel and the mild steel disc to put it in my four jaw.


Hi Niels

So you did service on one of these : http://www.navalhistory.dk/danish/Skibene/Skibsklasser/Falken_klassen(1962).htm

And my former boss did on one of these (the source of my stories) : http://www.navalhistory.dk/danish/Skibene/Skibsklasser/Soeloeven_klassen(1965).htm

My www alias Admiral_dk has nothing to do with the sea, but all my favorites where used at the first place on the net where I tried to create an account.

Back to subject. Almost all my two-stroke experience has to do with racing engines and in those you try to get the reprocicating mass as low and balanced as possible + small surface area to reduce friction. So I do not get the idea behind your big "exhaust" piston .... It just appears to me that you got a very under squared design or are I mistaken ?
Best engine performance is obtained with a design between square and very over squared (modern high-power engines has a bore 2-2.5 x stroke). My geeling is a squared design will fit the bill for an aero-engine nicely.

Equal pistons will also help with getting same friction on both cranks and minimize powertransfer in the syncro-shaft.

To save weight, use the syncro-shaft as the generator and other "service" functions.

Best wishes Per
Hello Per

My experience is mostly two stroke shipdiesel .
Some have the worlds best thermal efficiency and have strokes up to four times their bore so I am not convinced of Your arguments.
The Junkers JuMo 205 diesels of WW2 was 105 mm bore and 160 mm stroke and had a power to mass ratio not much bettered today.
My test engine will have 40 mm bore and 60 mm strokes and a piston speed of 12 m sec IE 6000 rpm.This is more or less what the Vespa Ciao moped does day after day and piston rings and conrods come from mopeds.
Here is a picture of a solid crankdiscshaft being cut and next picture where it is admired (by me and cat) just before being posted to the hardening people who will put carbon into the surface.
When it gets back most of the surface will be removed and sent to hardening proper.The pin will be very hard and the rest tough but mashineable.
Grind the pin from its present 16.3 mm to 16.000 mm and turn the rest to wanted dimensions.
Very easy after all the crazy schemes I have thougth of in vain.
Is .3 mm overdiameter reasonable?


Hi Niels

To me a high power two-stroke is a multiply off 125cc cylinders, some 400-600HP/liter, engine + clutch + gearbox on a 4 cylinder 500cc with 260HP (at 11,000 Rpm), weighs some 45-55 Kg ...!!!

I'm guessing that you're talking more about fuel consumption vs. weight and power - a figure that's very relevant on planes and boats.

"My test engine will have 40 mm bore and 60 mm strokes and a piston speed of 12 m sec IE 6000 rpm.This is more or less what the Vespa Ciao moped does day after day and piston rings and conrods come from mopeds."

I thought that they looked familiar ;) I was going to suggest that you used the pistons too, but you got too much stroke for them to cover the exhaust and transfer ports near TDC.

I've just realized something else - an aero engine is supposed to continue to run, even with some of it's parts not working, but there's not much redundancy on a single cylinder engine .... and I'm guessing that you want it to be approved by the authorities ....

I will follow your progress, since it's an interesting project.

Best wishes Per
Hi Niels,

You have taken on quite a project. I have been involved with civil aircraft maintenance here in the US for over 43 years. One thing that is consistant is, it is up to you to prove what you make works everytime in every condition the airplane operates in no matter what it costs. Some GA8s are flying here in the US, but I have not seen one in my area. You are by fact going to change the airplanes Airworthy Certificate to an experimental. Have you looked at insurance costs? Its a lot. Deduct that from your fuel savings. Have you looked into the fuel you will burn for the test flights? Its a lot. Deduct it from your fuel savings. Remember, it isn't up to you to say its airworthy, its someone else and the cost to make sure its right does not matter to them. It costs a lot. Deduct that from the fuel savings. Have you designed in your required instrumentation for the engine?

Here is some of the required regulations for the airframe, engine, propeller and appliances. Parts 21, 23, 33, 35, 43 will apply to to what you are doing.


Last thing. What is wrong with the Lycoming IO-540K thats in it now? It's a 300 hp engine.


Hello Kenny
I think the 540 is lousy heavy noisy antisocial and I do not care a dam for the well being of American lawyers
!2 million are starving in east Africa because they must be supplied by road and the roads are controlled by gunnuts(Muslims in this case and not Americans.)


Hi Niels,

I happen to have an IO-540 on an airplane. It's a very strong and dependable engine. You do know that its the propeller that makes the most noise.

Best regards,
Hello Ken

A lot of our generation was braindamaged by the lead from petrol.
Only these stupid aircooled aircraft engines need it today exactly because of product liability lawyers and employees of FAA.
To reduce noise from Your 540 aircraft You can reduce prop tipspeed by lower rpm or reduce diameter of prop.
Less power or thrust and aircraft stays on ground.

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