Fokker D.XXI Retractable Under Carriage

[Marcel]

I've read a lot of complains from Dutch fighter pilots about the fixed gear on this plane. They said a retractable version should have been much better. I know the Finns tried this on their version of the D.XXI. Does anyone know how it performed compared to the standard version?

 

[Mangrove]

It wasn't much better. The maximum speed only increased about 30 km/h but total weight was c. 200-300 kg larger than the fixed version. The explation may lie at the fact the Fokker was designed quite well at the beginning and the aerodynamics couldn't be improved as much as they would like to.

Mechanic Erkki Eve tells how he added 200 rpms to the Mercury engine by allowing the throttle to be used freely. It seems the Valtion Lentokonetehdas had inhibit using the full manifold pressure as it would damage the engine at long run. The extra 200 rpms gave the Fokker 30 km/h more speed but could only be used at short time. However this is only a story which I can't verifye from the sources right now.

For more info learn to read Finnish and buy Jukka Raunio's book "Lentäjän Näkökulma II". 8)

Martti

 

[Marcel]

Thanks, Martti, so it seems the retractable gear wouldn't have made a difference. Are there any Finnish test reports available?

 

[Snautzer01]

the retractable d21

ace publication -->> fokker d21

Fins adapted 2 planes

one was the FR 117 Test began jan 1941. when the gear collapsed in july 41 they build it back to fixed

 

[Snautzer01]

Denmark tried 20 mm cannons They bended the wing to much when fired

ace publication -->> fokker d21

 

[Snautzer01]

another reason is that the cost of installing the retractable gear was quite high.

 

[Marcel]

another reason is that the cost of installing the retractable gear was quite high.

Hoi Snautzer,

Yes that was why the D.XXI wasn't build like that in the first place. I read Jan Linzel's book and he was very angry about this decision. I was therefore wondering what difference it would have made, in sense of performance.

 

[Snautzer01]

i havent seen test with it but i guess if the fins didnt bother...

on the other hand if mangrove is right about 30 km/u increase with same power source... that would be a quite considerable improvement.

maybe the winterwar threw a spanner in the works.

 

[Marcel]

on the other hand if mangrove is right about 30 km/u increase with same power source... that would be a quite considerable improvement.

maybe the winterwar threw a spanner in the works.

Yes that would give the aircraft a level speed of, what, 490-500 km/h? Still slower than the 109, but the gap would definitely have been smaller.I think the Finns had prospects to get better, more modern planes by 1941.

 

[Snautzer01]

Yes that would give the aircraft a level speed of, what, 490-500 km/h? Still slower than the 109, but the gap would definitely have been smaller.I think the Finns had prospects to get better, more modern planes by 1941.

me 109 wasnt the opponent for the fins,

speed ~460 (fixed gear mercury engine)

better put it up to IL16 type 18

 

[Graeme]

The explation may lie at the fact the Fokker was designed quite well at the beginning and the aerodynamics couldn't be improved as much as they would like to.

Hi Martti.
Found an old article on the D.XXI which spoke of one example being fitted with "a completely redesigned wing, known as the E-I wing, which was tapered and had a considerable wash-out towards the tip".

Mention is also made of a Finnish reconnaissance version that had "transparent panels extending almost to the tail plane."

Any photos or drawings of these two versions?

Fokker was also considering various alternative engines...

 

[Graeme]

Hi Marcel.

What does your avatar say in English?

 

[Marcel]

It says:
Royal airlines

And below:
The flying Dutchman, once a legend, now reality.

It's a KLM comercial from the '20-'30ies.

 

[Mangrove]

On the other hand if Mangrove is right about 30 km/h increase with same power source... that would be a quite considerable improvement.
Maybe the Winter war threw a spanner in the works.

30 km/h more is not a good trade when you get 300 kg more weight on the wheels and the wing. The Air Force HQ knew 30 km/h wasn't enough, the change would take too much time, was too expensive and the 415-420 km/h was good enough for the Recco Squadrons (LeR 1) in 1941-1942.

Found an old article on the D.XXI which spoke of one example being fitted with "a completely redesigned wing, known as the E-I wing, which was tapered and had a considerable wash-out towards the tip".

Hello Graeme!

Indeed there was a Fokker with elliptical wing design, the original design was for the Finnish VL Pyry if my memory serves me correctly. As the Finnish Aviation Literature isn't very high quality (no sources marked down) I can't be sure but what I remember from the visits at the Finnish National Archive the elliptical wing wasn't very good for D.XXI as the main strength of the Fokker in 1940- was agility, not speed. The modified Fokker couldn't dogfight as well as the original.

On the other hand all Finnish Fokkers were modified to have wing slots as in 109. The speed loss was like 0-5 km/h but the new wing was more agile and had better landing characteristics.

Mention is also made of a Finnish reconnaissance version that had "transparent panels extending almost to the tail plane."

Never heard of it. Fokker "FR-92" was fitted with American Fairchild F-24 camera in 1942 and "FR-110" a Soviet one but that was it.

Martti

 

[Cosmic Sea]

On the other hand all Finnish Fokkers were modified to have wing slots as in 109.

I haven't heard of 109's having slots in their wings. You might have mixed them with wings slats, which they did have. These were in the leading edge of the wing and popped up to increase lift in while manouvering or flying slower.
The idea of the slots was the same however, but simpler. Holes going through the wing.

Mention is also made of a Finnish reconnaissance version that had "transparent panels extending almost to the tail plane."

Could that refer to the twin wasp-engined fokkers with the extended canopy?
The second picture posted earlier is of a wasp-fokker, but the canopy doesn't show very well. Here's a photo from the side: Wasp-fokker


Oh btw, it's my first post here, hello everyone!
I registered a while ago but didn't participate until now.
And sorry if this is considered necroposting.

 

[Snautzer01]

30 km/h more is not a good trade when you get 300 kg more weight on the wheels and the wing. The Air Force HQ knew 30 km/h wasn't enough, the change would take too much time, was too expensive and the 415-420 km/h was good enough for the Recco Squadrons (LeR 1) in 1941-1942.
Martti

well plse see increase in weight and speed for lets say the 109.

30 k in speed better is very good. Pilots sold their mother to be faster. The reasons the fins didnt are other then mere lack of better results

 

[Marcel]

I haven't heard of 109's having slots in their wings. You might have mixed them with wings slats, which they did have. These were in the leading edge of the wing and popped up to increase lift in while manouvering or flying slower.
The idea of the slots was the same however, but simpler. Holes going through the wing.


Could that refer to the twin wasp-engined fokkers with the extended canopy?
The second picture posted earlier is of a wasp-fokker, but the canopy doesn't show very well. Here's a photo from the side: Wasp-fokker


Oh btw, it's my first post here, hello everyone!
I registered a while ago but didn't participate until now.
And sorry if this is considered necroposting.

Don't worry about necroposting, the thread hasn't been that old
Thanks for the pictures. Never saw thes extended glass cockpits on the D.XXI

 

[Cosmic Sea]

The extended canopy was a feature of the Finnish-manufactured twin wasp-engined Fokkers. As Bristol Mercuries were reserved for the Blenheim bombers so another engine had to be chosen to power the license-built Fokkers.
These series (so called IV and V) had some changes when compared to the Mercury-engined ones, the most prominent being the extended canopy offering better visibility. Also, as a result of the installation of a two-row engine, the two fuselage-mounted machine guns were moved to the wings.

Here's some more photos of Wasp-Fokkers: Photos of Finland's Continuation War, P.2

In case you're interested, there's two books about Finnish Fokkers, one for each type.
Suomen Ilmavoimien historia, osa 3a - Finnish Air Force History, part 2
Suomen Ilmavoimien historia, osa 3b - Finnish Air Force History, part 2b
They're bilingual, with photo captions in both in Finnish and English and there's an English summary at the end of the book.

 

[Snautzer01]

another view