WHY NO FOKKER G1 REPLICA?

It has been a while when the last news was issued on this website but that had it's reasons one of them was that until now the redesign of the Fokker G1 consisted largely of interpreting existing data and incorporating this in a 3D design. Now the design has reached a maturity such that considerable design time has to be spend on the mechanical / aerodynamic side of flying a replica. Making new pictures from the 3D design is much easier than writing news about theoretical stuff like aerodynamics.

Although everyone has the dream to build as original as possible, the reality is quite different. Concessions for a flying replica are necessary for 4 main reasons :

The type of flying done with the replica differ substantially from the original. From originally a military function to a display aircraft. Operating in the vicinity of large crowds , considerable attention has to be paid to the safety of the aircraft. One of the major changes incorporated into the G1 replica is changing the engines from Bristol Mercury VIII to Pratt & Whitney R1340. The lack of spares for the Bristol engine influences the safety of operations. Annual flying time is limited, this is not so good for the currency of the pilot. Therefore it is better to reduce the workload of the pilot and provide him with an environment ( cockpit ) which is more like he is used to fly ( modern aircraft ). This means for example originally the G1 instruments for the engine were the single pointer type, differences between the left and right engine are then difficult to spot. With dual pointer type instruments , it’s much easier to detect functional differences between both engines. A glass cockpit is not in consideration, to me that's a bridge too far, it has to be a replica.

Many regulations since 1938 have changed. For any historic aircraft the major changes because of regulations are infrastructure related. This means instruments needed to fly in current airspace regulated environment. I don’t think there is one flying Spitfire in the world with an 100% original instrument layout. Also think about radio equipment and things which didn’t exist in 1938 such as : Transponder, GPS navigation.

Changing materials originally used can have several reasons. Better today available materials is the main cause. Think hereby for example about plastics, adhesives. One good example is the covering of the rudder which originally was done with silk but will be done using Dacron on the replica, the reason lifespan. Silk covering will last around 5 years, Dacron can last for 30 years with proper care. Also the application of Dacron is much easier.

Change of material can also be health related, think of asbestos in firewalls and radioactive translucent instrument dials.

The type of operations done with the replica differ substantially from the original. From a medium range reconnance , ground attack , fighter plane into a display aircraft. Medium range is no longer a necessity , a maximum endurance of 2 hours is enough. Carrying original weaponry and ammunition can be replaced by dummies which weigh much less. High top speed at 12.000 feet altitude is no longer important, flying low and ease of control are more important . For safety reasons the engines are changed but this is also because of financial reasons. For the price of 2 Bristol Mercury’s it is possible to buy 4 or 5 Pratt & Whitney engines. So it is easier to have 3 engines which one of them is a spare which can be exchanged quickly so as not to cause lose of revenue during the display season.

All of the above mentioned changes has it’s influence on the weight of the aircraft. This change of weight ( and sometimes location ) of a part influences the weight and balance of the aircraft which in it’s turn influences the flight characteristics of the aircraft.

Removing the weaponry makes the aircraft lighter, the only disadvantage with the G1 is that the front machine guns were placed far forward, removing these can only be done by adding weight in the nose the keep the weight and balance intact.

Changing the engines doesn’t need explanation in that this influences the flight characteristics. First feedback I got from a lot of people was : more horsepower , that solves your problem. This seams easy but it isn’t : Radial engines all have more or less the same horsepower to weight ratio. This means more horsepower is a more heavier engine and this influences the weight and balance. More horsepower means larger propeller is needed , but this has a negative effect on ground clearance and distance of the prop tip to the fuselage.

The bonus of a fighter replica over an airline replica is that fighters are generally overpowered and airliners are close of being underpowered. So we can loose some power. The Bristol Mercury engines were made for delivering maximum power at altitude not at sea level. Because display aircraft spend their career at sea level, power is there more important. The nice thing about the Pratt & Whitney R1340 is that this engine was designed to give maximum power at sea level.

But still there is a power gap between the original and the replica. But from the weight and balance sheet we have seen that the replica will be considerably lighter. So what do you have to do is to calculate the performance of the whole aircraft. You would think that finding information about 1938 technology would be a really difficult task. But actually if you know were to look it’s not so hard. The predecessor from the NASA was the NACA. Before spaceflight 100% of their research was aviation related. They did really good research for the American aviation industry and wrote nice readable reports on almost all subjects. Luckily someone in the USA must have thought it valuable to scan those documents and publish them on the internet

Using these reports I created an Excel file which automatically calculates the performance for the replica under different circumstances.

	Original Bristol Mercury powered with 2 speed propeller		Replica Pratt & Whitney R1340 powered with constant speed propeller
Max takeoff weight	4800 Kg		4400 Kg
Max takeoff power	1430 Hp		1200 Hp
Cruise speed @ 18000 feet	206 knots	383 km/u	168 knots	310 km/u
Max Speed @ sealevel	215 knots	399 km/h	188 knots	348 km/h
Max speed @ 6000 feet	235 knots	436 km/h	200 knots	370 km/h
Max speed @ 18000 feet	244 knots	452 km/h	225 knots	418 km/h
Max climb speed @ sealevel	2680 ft/min	13.6 m/s	1855 ft/min	9.4 m/s
Max climb speed @ 6000 feet	2680 ft/min	13.6 m/s	1860 ft/min	9.5 m/s
Max climb speed @ 18000 feet	1550 ft/min	8.1 m/s	1830 ft/min	9.3 m/s

These numbers correspond with the circumstances when everything works fine, unfortunately you have to take into consideration the possibility of engine failure. The worse moment to have an engine failure would be when you have to climb to avoid a collision. ( most of the time during the start ). You wouldn’t expect this but there are for a twin engine aircraft with constant speed props 4 conditions of engine failure , namely :

Stall Speed	80 knots	148 km/h
Max speed - situation 1	137 knots	254 km/h
Max speed - situation 4	114b knots	211 km/h
Max climb speed - situation 1	465 ft/min @ 97 knots	2.4 m/s @ 180 km/h
Max climb speed - situation 4	283 ft/min @ 87 knots	1.4 m/s @ 162 km/h
Cruise speed - situation 1	105 knots	194 km/u
Cruise speed - situation 4	86 knots	160 km/u

Most twin engined aircraft have what they call a critical engine. Aerodynamicly their can be a difference between a left engine failure or a right engine failure. To counter this they sometimes use counter rotating propellers so there isn't a difference. Worried about his effect Fokker used this also on the Fokker G1 prototype. But because of the twin boom twin rudder layout of the G1 they latter found out that there wasn't a difference between a left or right engine failure.