SSW D.IV Collector Aircraft & Fokker D.VIII compared with the Fokker D.VII

[Avimimus]

 

Hello,

 

Some people were curious about the Fokker D.VIII (coming out next week) and I think we are all a bit curious about the SSW D.IV (just announced).

Some observations:

 

 

Both of the new aircraft are small fighters compared to the Fokker D.VII (and the Albatross). Their loaded weight is smaller, which means that the retain a low wing loading in spite of having smaller wings. This means they remain relatively manoeuvrable.

 

The SSW D.IV has small wings. This makes it intermediate in wing area between a biplane and a monoplane (closer to some sesquiplanes, particularly the Nieuport N.17 which has almost the same wing area). The Fokker D.VIII has about half the wing area, although not half the lift, since monoplanes are more efficient (due to lack of wing interference) and since it has a new air-foil.

 

The SSW D.IV has a larger engine than D.VIII which gives it a good power to weight ratio. However, the fact that it is a biplane will probably hurt its acceleration and climb relative to the D.VIII. While its engine is smaller than the D.VII, the D.IV is also much lighter weight leading to an even greater power-to-weight ratio. This could also help it to sustain turns better than the D.VII.

 

Honestly, the airfoils are an unknown factor and airfoils impact lift, drag, handling and stability (e.g. a lot of the D.VIII performance comes from the airfoil itself). As a result these are crude comparisons. The same goes for different shapes of the control surfaces (the SSW D.IV has a rudder which is 'interesting' to say the least). If anyone has more information - I'd love to hear it.

 

In any case, it looks like it will be intermediate in many ways between the Fokker D.VII and Fokker D.VIII... it shouldn't increase speed (in fact it is much slower than the D.VIII) but it should have pretty good turning capabilities.

 

P.S. The Fokker D.VII has data given for the Mercedes D.IIIa not the BMW engine. With the BMW engine the Fokker D.VII is almost as fast as the D.VIII and has a power to weight ratio which makes it comparable to the SSW D.IV while retaining a lower wing-loading... so it may well be the superior plane when given the latest engine.

 

P.P.S. Let me know if anyone would be interested in me expanding the analysis to the Roland D.VIb and the Aviatik D.I ...two late war fighters that were more produced than the SSW D.IV, and in some ways higher performing than any of those listed here.

[Dutch2]

Interesting both the Roland and Aviatik. 

[US103_Baer]

8 hours ago, Avimimus said:

Honestly, the airfoils are an unknown factor and airfoils impact lift, drag, handling and stability

 

 

Maybe they'll give it the spad airfoil shape too. ?

Edited May 29, 2021 by US28_Baer

[Cybermat47]

9 hours ago, Avimimus said:

This makes it intermediate in wing area between a biplane and a monoplane (closer to some sesquiplanes, particularly the Nieuport N.17 which has almost the same wing area).

 

IIRC, the Siemens-Shuckhert fighters started off as a copy of the Nieuport 17. Here's the D.I:

 

[ZachariasX]

Actually, the SS D.IV is one of the better documented aircraft regarding flight performance. It is drasitically superior than most other rotary powered planes, for several reasons.

 

First, the engine. The engine is rated at 160 hp, limited by structural strengt. At full rpm at sea level, it would do 240 hp. This means, FTH is about 4000 m for the SS D.IV.

 

The SS D.IV not only has slightly smaller wings, it has also very narrow wings, meaning it has a very desireable span loading for a biplane and this makes it outclass the similar powered Pfalz D.VIII cosiderably, the more the higher they fly. While airspeed is about similar, max. sustained turnrate and climb are much better in the SS D.IV.

 

Biplanes are very poor designs regarding span loading, triplanes of course being even worse. It is the main power limiter of the Dr.I and making the Fokker D.VI a much better aircraft. (Or giving the Camel a slight edge in absolute performance.)

 

Span loading besically makes bi- and triplanes hit the brakes as soon as you pull that control colum back.

 

This brings us to the Fokker D.VIII, being head and shoulders above any competition regarding span loading. This means even with a modest engine, it really can have an edge on the competition regarding energy retention in maneuvers.

 

Level airspeed on the other hand is only a factor of the prop chosen. And there a compromise between climb and power at evisioned operating altitude is made.

[Voidhunger]

Fokker D.VIII was very good at Adleshorf when tested with more powerful engines like Oberrusel UR.III, Goebel Goe and Siemens Halske.

With Oberursel UR II it was no way better than SS DIV.

 

But Fokker D.VIII (EV) was not combat tested with those powerful engines and its possible that even the correctly assembled wing of the V.III could not withstand forces in real combat.

 

I really like both of these planes and Im very happy that we have opportunity to fly them.

[Avimimus]

Thanks! Very interesting and cogent observations.

 

Some more research:

 

The D.IV had only 80% of the wing area of the D.III, most of which came from reducing the size of the upper wing. However, the wing's airfoil was also changed and refined.

 

The D.III could climb to 8100 m (26,568 ft.) in 36 min. There is some debate over the climb performance of the D.IV but it was still good an didn't deteriorate as much as might be expected (improved upper wing airfoil, engine cooling, and better lift to drag ratio might all be factors leading to a similar sustained climb performance compared to the larger winged D.III)? 

 

The four bladed propeller and the somewhat unusual engine design should be interesting. The additional blades (and blade area) are to allow the prop to function at 900 RPM (compared to the 1400-1600 rpm typical of other fighters). The rudder has an usual design partly to deal with torque (in spite of the fact that the rotary engine design should reduce it as  well).

 

Some more information from surviving pilots' accounts:
- Sensitive on the controls. The four ailerons might give a good initial roll response (seen as an advantage over the Pfalz competitor). But it sounds like a bit of a handful overall.
- Spins with little warning (especially at altitude), but spin recovery is easy.
- Difficult to land compared to earlier fighters (higher wing loading led to a number of experienced pilots crash landing).
- Very good power at altitude.
- Superior climb.

 

It also sounds like it should be a bit more durable than expected (both to enemy fire and quite possibly significantly so in a dive).

[Avimimus]

On 5/28/2021 at 4:42 PM, Dutch2 said:

Interesting both the Roland and Aviatik. 

 

Okay! I'll give it a shot.

[ZachariasX]

On 5/29/2021 at 10:31 PM, Avimimus said:

There is some debate over the climb performance of the D.IV

It is actually about the best documented aircraft in this regard, and it performs far better than you just get by looking at wing loading. It is the span loading that sets it above the competition. Funny enough, it is because the upper wing is smaller (it has a more narrow chord, it is not just smaller) it is more efficient than for instance the Pfalz D.VIII with the same engine.

 

Climb speed is about ~9.3 m/s at sea level, ~6.6 m/s at 3000 m and ~3.3 m/s at 6000 m, from where things get sketchy.

 

Edit: In you post about the plane specs above in this thread, you should compare span load as well. It will tell you something. Also your speed estimates are semi useful, as the engine has actually a "critical" altitude of 4000 m, meaning it has far more margin up there and this shows in speed at "working altitude". Speed on the deck where you have to throttle back don't tell you the full story.

Edited June 16, 2021 by ZachariasX

[Avimimus]

8 hours ago, ZachariasX said:

It is actually about the best documented aircraft in this regard, and it performs far better than you just get by looking at wing loading. It is the span loading that sets it above the competition. Funny enough, it is because the upper wing is smaller (it has a more narrow chord, it is not just smaller) it is more efficient than for instance the Pfalz D.VIII with the same engine.

 

Climb speed is about ~9.3 m/s at sea level, ~6.6 m/s at 3000 m and ~3.3 m/s at 6000 m, from where things get sketchy.

 

Edit: In you post about the plane specs above in this thread, you should compare span load as well. It will tell you something. Also your speed estimates are semi useful, as the engine has actually a "critical" altitude of 4000 m, meaning it has far more margin up there and this shows in speed at "working altitude". Speed on the deck where you have to throttle back don't tell you the full story.

 

Definitely. The data in the charts is very primitive - just available numbers from publications. Comparisons like engine power to wing area provide only the crudest estimates of induced drag. We'd need corrections for the interference between upper and lower wings and airfoil data to do better. Even parasitic drag would be a bit of a nightmare to calculate - the external struts and bracing wires are a big part of the drag of an externally braced biplane - and the wires themselves can't be modelled simply as their vibration in flight produces a very large portion of the drag they create. Anyway, if you can do better - it'd be appreciated. I'm sure the dev team could do a fairly good job of modelling any of these aircraft... but it is beyond me. One thing that I do find interesting - cutting down of the upper wing to create the SSW D.IV (out of the D.III) is a less optimal choice than if one were to cut down the lower wing... yet they did it... presumably for structural reasons it was easier? I do wonder if an even more Nieuport like design would've had a somewhat better lift to drag ratio though.

[ZachariasX]

4 hours ago, Avimimus said:

One thing that I do find interesting - cutting down of the upper wing to create the SSW D.IV (out of the D.III) is a less optimal choice than if one were to cut down the lower wing...

In this case I think it was a good idea, as it increases the aspect ratio of upper wing that does most of the lifting. I‘d say it is generally a good idea to have high aspect airfoils on slow aircraft.

 

It is basically this wing configuration in which the SS D.IV differs from the Pfalz D.VIII and it yields a remarkabe improvement in performance.

 

[Avimimus]

I'd probably prefer it too... although I do suspect a clean sheet design with a higher aspect lower wing would've been more ideal than cutting back the upper wing for the same effect. In any case, you'd like the Feiro Dongó. I don't see a particular reason why such a design couldn't have a higher structural strength, but it might have had more trouble doing so while keeping the wing area high, and then there is that matter of roll-rate... I still agree with your general point though - higher aspect ratios might have worked in WWI. Anyway, it is still good to remember that a low wing loading was desirable - especially for landing on relatively rough fields...

 

 

Roland D.VI

 

A major competitor to the Fokker D.VII with comparable performance. It's performance looks better on paper, with the exception of unimpressive climbing ability. It had equal speed, a higher power to weight ratio and a lower wing loading.

 

Other than the rate of climb, the major downside of this design was its wooden clinker-built fuselauge, which was expensive and time consuming to build. However, it does add quite a bit of elegance.

 

It was built in relatively large numbers (more than the SSW D.III and SSW D.IV; similar to the Fokker D.VIII)! About 330 were built. They entered service in May 1918 and significant numbers were operational from June onwards.

 

Overview:

 

- Speed is roughly equivalent to the Fokker D.VII (190-200km/h depending on the engine).

 

- Better than average manoeuvrability (particularly at low altitudes - where it was said to be significantly superior to the Albatross). Lower than average climb rate.

 

- Pleasant flying characteristics, easy to land (although landing gear is narrow)

 

- Very good visibility in production examples (described as exceptional by an American test pilot)

 

- Wooden construction would respond to bullets differently (compared to fabric)

 

Note: The airfoil of the tail was inverted - which means that automatically it produced more downward force as speed increased (helping dive recovery). The design also had either balanced or unbalanced control surfaces (depending on the version). 

 

 

 

For reference, compared to the Fokker D.VII:

 

- Wing area is about 10% greater and wing loading is lower than other fighters (30.18-30.88 kg/m2 vs. 32.568-32.89 in the Fokker D.VII, and 32.41 in the Albatross D.Va)

 

- Power to weight ratio is higher than the Fokker D.VII (0.30-0.27 hp/kg compared to 0.27-0.26 in the Fokker D.VII and 0.27 in the Fokker D.VIII)

 

This largely explains its performance: It should have better instantaneous performance then the Fokker D.VII due to the higher power-to-weight ratio and the large wing. However, it shouldn't be as good at sustained climb or retaining energy (due to having a 10% larger wing). I suspect it would fly a bit like an Albatross D.III or D.Va if ti was upgraded to have performance close to that of the Fokker D.VII.

 

 

 

Note: 
- More examples reached service units than the SSW D.III and D.IV. It was built in similar numbers to the Fokker D.VIII, but less than the Pfalz D.XII
- An interesting note: One had a villar-perosa twin-barrelled overwing gun!
 

 

 

 

 

 

 

 

[Avimimus]

Given my excitement over the SSW D.IV - I thought it might be worth necroing this thread. \

I certainly want to talk about it more!

[BMA_Hellbender]

6 hours ago, Avimimus said:

Given my excitement over the SSW D.IV - I thought it might be worth necroing this thread. \

I certainly want to talk about it more!

 

If the Siemens turns out to be the turnfighter they are already advertising it to be, then there's going to be some 'splaining to do with regards to the Nieuport 28.

 

Up until now the only reason why the N28 apparently sucks in a sustained turn, is because it has low wing surface area, less than the SPAD 13 (20m²), officially 0.3m² less but I think the number used when building the FM was 3m² less (17m²). Now something that the devs have claimed from the very beginning of RoF, is that CLmax is highly dependent on wing thickness: thicker being better, with the ideal wing thickness for sustained turning being in the "medium thick" sweet spot where the Albatros D.Va is at and the Siemens also. Fokker wings are a bit too thick, British/Nieuport wings are too thin and SPAD wings far too thin. The N28 falls in between British/Nieuport and SPAD wings, so it should almost by definition be a better turnfighter than the SPAD 13, which it certainly was historically, but which is not the case in RoF/FC, because it has such low wing surface area. But more importantly: the N28 has higher aspect ratio than the SPAD, in part thanks to its rounded wing tips.

 

Now the Siemens has even lower wing surface area (15m²) than the Nieuport 28 (17m²), the lowest of any biplane in the sim. However it does have very high aspect ratio, even better than the Sopwith Dolphin, and certainly better than the Albatros D.Va.

 

So two scenarios are possible:

 

1. The Siemens (15m²) turns better than the Albatros D.Va (21m²), hence aspect ratio trumps surface area, hence the Nieuport 28 (17m²) should outturn the SPAD 13 (20m²)
   
   or
    
2. The Siemens (15m²) turns worse than the Albatros D.Va (21m²), hence surface area trumps aspect ratio, hence the Nieuport 28 (17m²) correctly turns worse than the SPAD 13 (20m²)

Edited July 16, 2022 by Hellbender

[Avimimus]

Well, all airfoils (wing cross sections) produce both lift and drag. As the airplane turns the oncoming air hits it at a different angle (i.e. angle of attack compared to level flight) and both the drag and the lift produced by the wing will increase. So the big question is:

 

For any given sharpness of turn, how much does the lift increase relative to the increase in drag?

 

I'm not sure about the SSW D.IV's airfoil. However, the greater aspect ratio (i.e. longer wings with a shorter front-to-back distance) should lead to less drag. There is also the fact that biplanes lose a lot of efficiency due to aerodynamic interference between the lower and upper wings, having a higher aspect ratio should reduce this. On the negative side reports indicate a rapid onset and propagation of stalls (possibly worsened by the high aspect ratio)? So I'd expect it to want to enter into spins if one isn't wary.

 

I'm expecting an aircraft that will lose less energy in a turn (i.e. lose less speed and altitude). It should have an advantage in sustained turn rates. However, it might not have an advantage in instantaneous turn rates.

 

Note: Different airfoils will also produce different results for a given airspeed/altitude/size of the wing, Hence why so many different wing cross-sections have been used throughout time. The relationship of lift to drag can change a lot depending on the specific angle of attack as well... those relationships can be quite surprising.

[ZachariasX]

8 hours ago, Hellbender said:

So two scenarios are possible:

When it comes to these things, you want to look at span loading as well. Power loading (at altitude!) is also important given the different design of the engines.

 

8 hours ago, Hellbender said:

If the Siemens turns out to be the turnfighter they are already advertising it to be

then the devs most likely did something that is most probably very right. We have comperatively much data on that crate to get an idea of what it can do.