TFF

Funny about scale plans and full scale plans is even someone building a homebuilt airplane rarely builds close so they could interchange with another like plane. One out of a high end kit, sure. Whe worst thing I have done is attach the top wing of my Top Flite SE5a with its designed hardware. Too scared to take the top wing off and rip the fittings out.

otrcman

Abufletcher is certainly right; a DR1 is going to be difficult by nature. But by examining WHY it is so difficult, we might be able to mitigate some of the problem.


1. High center of gravity: Not much to be done here. If the plane is shaped like a DR1, the CG is bound to be way above ground level.

2. Short coupled: Can't fix that without changing scale fidelity either.

3. Tiny vertical stabilizer: Ditto #2

4. Narrow main gear spacing: Ditto #2

5. Very nose-high stance in 3-point attitude: That means the airplane has to rotate through a large pitch angle as the tail comes up. Bad news, as the gyroscopic precession effect is somewhat increased by the amount of pitch rotation that occurs as the tail comes up. Thus more nose swing as soon as the tail begins to come up.

But this problem can be mitigated somewhat without changing the outward appearance of the model. How ? Install a yaw rate gyro. The gyro senses even the tiniest beginning of the yaw divergence as the tail begins to come up. The divergence with a model happens so quickly a human pilot has no hope of seeing the motion visually. So a gyro makes all the difference.

The second mitigation for the nose swing when the tail comes up is to hold just a little bit of aft-stick while advancing the throttle for takeoff. What you are doing is preventing the tail from coming off the ground too soon due to prop blast. And you are preventing the tail from rising so rapidly and through such a large pitch angle. Just don't hold so much aft-stick that the model is forced into the air prematurely.

6. Tailskid vs Tailwheel. TFF suggested some sort of snap on tailwheel. I agree. To maintain directional control, you need the ability to create side force at the back of the plane. The side force can be either something that rubs on the ground or something that reacts to the airstream. At the beginning of the takeoff, the rudder has little or no airflow with which to generate a stabilizing side force. So you are relying completely on the skid. The problem with a tailskid is that it develops very little friction against a hard surface runway. And once the model begins to move, the friction is reduced further as it becomes a sliding coefficient of friction rather than a static coefficient. So with the Fokker, you have almost no stabilizing force whatsoever at the back end of the plane.

The contribution from a tailwheel (as opposed to a skid) is that rubber generates a great deal more friction when in contact with the ground. And, once the model begins to move, the wheel rolls in the forward direction, but is essentially stationary in the sideways direction. So the friction coefficent in the sideways direction remains at the higher static value. Of course this is only true so long as the taiwheel is in contact with the ground. Once the tail begins to come up, all you have is the rudder to keep you straight.

7. Co-rotating main wheels. For an airplane to turn, the two main wheels have to rotate at different speeds. If you mount the wheels on a solid axle such that they are forced to always rotate at the same speed, the plane will be much more resistant to turning. A few modelers have employed this trick over the years with good success. The downside is that the model is more difficult to turn when taxiing. But then, how much luck are you having right now taxiing with just tailskid control ?

8. Pilot control. We've talked so far almost exclusively about takeoffs. Landing has its own set of problems. One thing that can help to go straight on landing is to keep the skid (or tailwheel) is solid contact with the ground as you slow down. You can do that by either doing three point landings and holding the stick fully aft until you come to a stop or by doing a wheel landing and bringing the stick back (and holding it) as you see the tail begin to settle.

9. One last thing about using a gyro. There are many people who feel that using a gyro in a model is somehow cheating, or "not experiencing the actual stability of the real airplane". In reality, the very act of making a subscale model of a real airplane destroys the scale physics of the design even though the outside shape of the model is identical. The problem is that you have drastically altered the inertia of the design in relationship to it's shape. And you can't have scale inertia without making the model incredibly heavy.

Reducing the inertia of the model increases the rate of divergence in all axes. Further, trying to see small divergences (on a small object like a model) is far more difficult than seeing - and feeling - divergences while sitting in the airplane. Put it all together, and you find yourself trying to detect (visually only) very small and very rapid motions. By standing alongside the runway you have surrdendered any kinesthetic sense. It's hard enough to do in the full scale airplane, let alone the scale model.

So in a very real sense, the gyro is restoring more scale like inertias and divergence rates and in a way increasing the scale fidelity.

Sorry for being so long winded,

Dick

abufletcher

I one of those Luddites who are disturbed by the idea of putting a gyro in a WWI model. But I'd be even more disturbed by putting a beautiful scale WWI model in the closet!

otrcman

One of those Luddites ? Heck, you are THE Luddite.

But no worries. It's that same mindset that drives you to hand paint your roundels, which I find exquisite.


Dick

carlgrover

As far as the button head screw thing: I ordered some 2-56 button head screws from micro fasteners and also the special allen key to install them (not a ball driver). They stripped out like crazy. I complained to micro fasteners and they said no one had ever told them about this before. He sent me some others and told me to trash the bad ones. The big problem with button heads is that for a given size, they use a smaller hex key than the same basic size in a cap screw. I really like the look of a button head but until someone comes up with a very high quality hex key and tightly tolerances the head, they will continue to strip out. Grinding the allen key will help as they do wear easily but that gets old in a hurry.

Biggest scale screw up? Not building a subject that i'm totally in love with!



carl

abufletcher

Sometimes it's lonely all alone in my cave. (But seriously, if the only choice is between using a gyro and putting the model in a closet, I'd definitely go with the gyro.)

FlyerInOKC

My Feedback: (6)

I was speaking with the owner of the LHS the other day about the DR.1 in his opinion adding a tail wheel to the skid, which I am doing on a VK Models DR.1, would increase the chance of a nose over. His theory is the skid drags and the resistance keeps it from nosing over on landing. I have a coupe of small metal wafer like wheels I plan on insert into the tail skid. The horizontal stab is removable so I can make a standard skids with assorted materials covering the dge and swap them out for comparison. Of course this all goes out the window if I suffer from y usual dumb thumbs. That loose nut holding the sticks is my biggest liability.

allanflowers

This is the tailskid solution on my VK Camel, a pair of clear plastic wheels - non-steering but still helpful. The other pix is from my Siemens Schuckurt D.III which has a piece of matte knife blade inserted into the skid. Its crisp edges bite into an asphalt runway fairly well and the hard steel doesn't wear down so fast. By the way, both of these planes has a gyro on rudder for takeoffs and landings.

TFF

My VK Dr1 flies great. Nosing over is all pilot knowing the plane. If you just want to land and have the tail settle and it stay straight, its not what happens. I land on the mains and after a second, full up elevator to pull the tail down. If it continues to roll on the mains, I feel the elevator actually runs out of authority and it noses over. Same with takeoff. The wings will takeoff before the tail. Takeoff roll is getting enough airspeed that the tail is flying, wings are already there.

mgnostic

I've also had a VK Triplane which I quite liked. Although I tend to like smaller airplanes small size tends to exacerbate problems. Scale loading is an issue and with with triplanes in particular the high center of gravity issue gets magnified. Although they aren't very practical for me I can see why the competition guys are rolling out 1/3rd scale and larger planes. With the larger models things like fokker nuts and such become practical. One other issue that I noted with my VK Fokker was that under some lighting conditions it became a Borg Cube flying through the sky. One could point to the wrong paint scheme as a scale problem. The standard streaked green Fokker paint scheme was dark enough that at any distance there were aircraft positions where there wasn't enough contrast to be able to see the model's orientation.

FlyerInOKC

My Feedback: (6)

I'm about half blind so I was going with a dark Vermilion with the post 1917 marking that look like a "+".

mgnostic

I should have said "any distance over a hundred feet". I forget that not everyone speaks Okie where it is understood that you are saying "any significant" distance. For a .60 size plane the VK is a pretty small aircraft. I still have good distance vision it it still got small pretty quick. A good color scheme can be a big help. I'm betting that one's choice of color scheme could open a whole new can of worms with regard to scale modeling mistakes.

FlyerInOKC

My Feedback: (6)

I have a bright yellow Cub that can be interesting to find in the right light and distance.

mgnostic

Now that I think on it we are kind of straying from scale errors into flyability errors. A lot of deviations from scale are driven by flyability.