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Aircraft Topics related to WWI aircraft, aircraft engines and armament

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Old 7 April 2007, 12:34 PM   #1
BDD
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Fokker DVIII stability

I have a drawing of a DVIII which seems to indicate that the horizontal stabilizer had a positive angle of attack. Is that accurate?

While the cambered airfoil should have a nose down pitching moment the positive lift on the tailplane would only add to this moment. The large, low landing gear and low "sub wing" would also only add to this nose down pitching moment. Did the DVIII have such an aft center of gravity that the horizontal stabilizer had to produce lift to balance the airplane in flight? I imagine that the large, delta tailplane could produce a lot of lift if necessary.

This seems, however, like a formula for pitch instability.

What was the actual situation for these planes?
 
Old 7 April 2007, 02:59 PM   #2
Taz
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BDD- Nearly all the Fokker aircraft were trimmed for tail heavy/low flight since they spent so much time climbing compared to descending. The airfoil wings were credited with producing enough lift to carry the weight of the landing gear, although I have not seen much evidence to back that up. Aircraft trim in WW-I was, to a large extent, trial and error or evolutionary, although some aircraft did have trimmable horizontal stabilizers (tailplanes) to allow the pilot to take some of the loads off the stick or wheel.

I read one account of a D.VIII replica where the pilot complained he had a hard time getting the aircraft to descend because of the tail heaviness. In general, though, the Fokkers were very light on the pitch and yaw controls, so overcoming the tail down trim was not too much of a hardship. Maybe we can get a report from someone on how Kermit Weeks E.V/D.VIII replica with the 160 hp Gnome 9N flies.

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Old 7 April 2007, 11:11 PM   #3
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In general, WWI airplanes were "tail-heavy". More exactly they had Center of Gravity behind Center of Lift; as a consequence, they required positive tailplane trimming, besides, they were longitudinally unstable.

Both german drawings and british rigging diagrams required positive angles for tailplane trimming, although there were weird instances like Pfalz D.III having negative cambered airfoil with positive angle of incidence (!?).

An interesting reading about this matter could be NACA Report No.120 "Practical stability and controllability of airplanes".

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Old 8 April 2007, 06:37 AM   #4
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I test flew Brian Coughlin's first D.VIII replica (with 160 Gnome engine, Kermit's D.VIII was Brian's second), and we were surprised at how far aft the CG was when we did the weight and balance before the first flight. If memory serves me correctly is was about 31 per cent MAC at empty weight. We found a McCook field report on a D.VIII which gave weights in level flight attitude for mainwheels and tailskid, and figured out that the CG on the original was very close to the CG on the replica. On the first flight a lot of forward stick pressure was required, but it wasn't unmanageable, as all of the Fokkers have plenty of elevator authority. We shimmed up the leading edge of the stabiliser, and lengthened the rear wing struts also I think. It still required some forward stick pressure, but was better. It was a nice flying airplane, I always say it would be a good sport plane with a Warner engine.

I always thought that Fokker airplanes were designed intentionally tail heavy because they realised that a tail heavy airplane is less stable and therefore more maneuverable. The pilots wouldn't have a problem with it, and in fact could use it to their advantage.

Also, the Triplanes I've flown both had Warner engines, but neither was as tail heavy as the D.VIII replica, although both were a lot more sensitive in pitch. I flew the old Blue Max D.VII that is now in New Zealand a few times when it was with Javier Arango in California, and it was a delightful flying airplane.
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Old 8 April 2007, 11:30 AM   #5
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DVII Wing Washout?

Comrades,
I think Achim mentioned in an earlier post that he was going to build washout into the upper wing on the DVII's and had some drawings indicating this was done in the originals. Since my questions would only apply to biplanes with ailerons on upper wing only, maybe it isn't a generic question.

If purpose of washout is to preserve roll-control into the last stages of a power-on stall (which seems a good thing on a WWI fighter), how are the other lifting surfaces rigged? Do both wings stall at the same time?

seems like this would be tricky to do. If the last surfaces to continue to fly in a power stall were the washed out upper wing outer panels, then for this to have any real value, the lower wings could not stall either before the upper wing or after but would have to stall simultaneously which seems unlikely to me. what happens if the landing gear airfoil stalls a little before the wings?

It seems to me that providing the washout is a waste of effort but that having really big ailerons would be more assuredly effective.

Any thoughts?

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