Please explain the physics of the takeoff model

[TweetyBird]

I do manage to get a spit up manually, but in playing with the takeoff model,  something wierd is going on.

Spit IX
You give it a little throttle and and as starts moving down the runway, it starts to vear a little left. So you give it a little rudder, keep the throttle the same, and then it vears straight, then right. So you let off the rudder and it keeps going right???? What the heck happened to the torque effect that made it go left in the first place? It seems if you let off the rudder it would *eventually* start to go left again, but it doesn't.

I looked through the outside plane view and at that slow of speed, the back wheel is still straight. So what is making the plane continue to vear right when the rudder is released?

Edit: The only thing I can think of, is at slow speeds the rudder doesn't straighten out when released. But then, shouldn't the external view and cockpit instrument show that?

edit2:
Lets not just stop with the spit - what is going on with the Dora?
Again - very little throttle - doing less the 20 mph. Playing with the rudder keeping it straight on the runway. The wings are not banking at all at this slow speed. And then for no visual reason, the prop breaks. I'm thinking the takeoff model has a long way to go and if it hasn't departed from real to wierd.

[Kweassa]

Answer:

 Tail wheel + incorrect rudder adjustment.


 The tail wheel is modelled in the beta. At first, the tail wheel stays straight, and the torque will force the plane to veer right. However, obviously you either applied the wrong amount of rudder to correct this, or were unable to maintain the exact amount required for the plane to go straight.

 This turns the tail wheel, and your plane will start wobbling around as the tail wheel moves around.

 Try moving around in one of those office chairs with wheels attached in the manner how the tail wheels are attached to these tail draggers. I am sitting in such a chair right now, and moving the chair around by dragging my butt, is actually not so easy, because the angle of the wheels twist around its axis according to my butt input.

 Now, to that difficulty, add the torque factor which lifts one side of the wing before the other, during take off. Typically, during a take off run this makes you steer the plane down the runway with two wheels(one main gear, and the tail wheel, while the other main gear breaks contact with the ground). Follow the plane with a Chase view during take off, and you'll be able to see this happening. This unstable state, makes the plane even more wobbly.

 And, again to that, in the old betas, a bug caused a invisible wing tip to drag against the ground, which during take offs, caused a catastrophical ground looping.

 
 Luckily, the bug was squatted, and now it is much easier and comfortable to up Spits.

 I can up the Spits and Doras with no trouble at all - just a slight veering left and right due to rudder adjustments in the start of the take off, and then it goes straight down.

 I recommend either dampening the rudder input settings, or fining them out to smaller input variables.

[TweetyBird]

I'm going to do more test so they can be duplicated exactly. These tests I have been doing are at very slow speeds. There is NOTHING leaving the ground. If you were using the keyboard for throttle, I'm talking about 2 taps. The plane starts off swinging a little left - I don't know which plane you are using that starts right. Now I'm not talking about going so fast it swinging wildly left. Its going VERY slow. So I give it a *little rudder* then let off. But after straightening, the plane continues right as if all the forces that initially made it go left have departed.

As far as can think of, with all 3 wheels on the  ground, only two forces ( other than wind) can change the direction of the rear wheel - torque or rudder. Remove rudder, only torgue should be effecting it. And the direction it moves in because of torque chould be constant. With no rudder the torque moves the plane left.  Well why once rudder is applied removed, that is no longer constant? Perhaps the pivot on the rear wheel is very rusted and is effected more by rudder than torque?

I don't know how to explain it right, but it doesn't make sense if the rudder isn't sticking.

[TweetyBird]

Let me try again, as I may bve completely mistaken about how forces are acting during takeoff.

This is my understanding of it.

Once the engine starts, a torque effect created by the right spinning (pilot’s perspective) prop makes the weight on the right wheel less than the weight on the left wheel. With less weight on it, the right wheel travels faster than the left wheel creating a rotation  or yawing toward the left. The rear wheel simply follows this rotation.

When rudder is applied it thrusts the back of the plane opposite of the torque induced rotation.  It equalizes or perhaps even over compensates for the speed difference of the right and left wheels (i.e., it increases the speed of the left wheel by adding rotaion speed to its forward speed or whatever - BTW - wouldn't a gear mechanism that made the left wheel spin faster than right, or perhaps a little right break control the torque effect on the ground better than an unstable rudder?) ). If equal, there should be no rotation and the rear wheel should be following no rotation (i.e., it should be straight). Overcompensated and the left wheel should be traveling faster than the right wheel giving a right rotation and rear wheel following that rotation. Because at this point the only thing (other than wind) that can possibly make the left wheel go faster than the right wheel (of a plane with a right spinning prop) is the rudder, with a perfectly centered rudder, there is NO logical reason there should be any right rotation or yawing of the plane. The rear wheel always follows the rotation of the front wheels. It should NEVER govern the rotation or yawing of the front wheels. I mean isn't that the reason its on a pivot - so it DOESN'T hinder the rotation of the front wheels?

Is this correct to this point?

[Kweassa]

I've tested the Spit9.

 I don't see what you've explained happening.

 The plane veers left, I over compensate it with rudder, the plane veers right.

 And then, after a while, it veers left again.

 ...

 Maybe you should recalibrate your stick, or change stick settings.

[TweetyBird]

That might be it, thanks.

If it veers left again, then that makes sense - its probably a stick problem.

I'll test it using the keyboard for rudder.

Edit:

Ok i tested it and reproduced it 3 out of 3 times using the keyboard for rudder control to eliminate any stick problems.

Unmap rudder from joystick.
Disable auto takeoff.
Go to NE spawn point in a spit 9
Hit E (default) to start engine.
Give 2 increment's of throttle  (hit "=" twice)
As the plane moves it will begin to veer left.
Hit "d" twice and let the rudder overcompensate till the plane veers right.
Hit "s" key to center rudder.
The plane stays in a right rotation (or yaw)- just watch the compass spin.

It never goes left again.

[Ghosth]

Try useing the differential wheel brakes for some of that control, esp at mid range speeds where tailwheel is getting light but rudder hasn't much authority yet.

[Bullethead]

I think it would help me a lot if I understood how castering tailwheels worked on takeoff, and the method you apply to use them during takeoff.  I've never flown a plane with a castering tailwheel, just fixed tailskids and non-castering tailwheels.

Here's how it's worked in my experience....

With a tailskid, you want to get it off the ground ASAP because it slows you down, gets worn out, and keeps you from steering with the rudder very well.  So when you start rolling, you push forward on the stick to raise the tail as soon as you have speed for the elevators to work, then drive down the runway on the main wheels and steer with the rudder.

With a non-castering tailwheel, it's the opposite, at least to start with.  At the low initial speeds, the rudder itself doesn't have much effect, but the tailwheel moves with it and does steer well.  So you hold back on the stick to keep the tail on the ground as you accelerate down the runway.  Then, once you're fast enough for the control surfaces to get a good bite, you can either keep things that way and eventually make a 3-point takeoff, or you can relax the stick pressure, letting the tail come up, and finish steering your takeoff run with the rudder.

Now, if the tailwheel is free to caster, what do you do?  This would seem to provide the worst of both worlds.  The tailwheel would provide no resistance to the tail swinging, so you'd have to keep the plane straight with just the rudder.  But if you're not yet fast enough for the rudder to bite, how do you keep the tail from swinging?  Do you have to use individual wheel brakes?  Using brakes when you're trying to accelerate doesn't seem very efficient.

It's always been my understanding that castering tailwheels only came along when planes got too heavy for the groundcrew to pick up the rear fuselage.  With the castering wheel, they could just push on the rear fuselage sides to orient the plane on the ground without having to lift the tail.  But surely there was some locking mechanism to keep the tailwheel from castering during takeoff?  If so, then a so-called castering tailwheel should function on takeoff just like a non-castering tailwheel.  Is that true?

[TweetyBird]

>>Try useing the differential wheel brakes for some of that control, esp at mid range speeds where tailwheel is getting light but rudder hasn't much authority yet.
<<


Same wierd thing happens..


Spt iX...

Hit "e" to start engine.
Hit "=" twice to give a little throttle.
As plane veers left, tap "v" right wheel break.
The plane starts going straight then right and never left again as if everything that made it go left initially has disappeared.
Ok, tap left wheel bake "c". Plane goes left and never again goes right. Now obviously the problem isn't a stuck rudder

At a certain speed, it seems another model takes over and the plane makes more sense - need to test it more.
But at two taps of throttle, the model is definitely broken.

My guess is at slow speeds (or throttle settings) on the ground, the rudder and r/l wheel brakes are acting on the orientation of the prop and NOT the other parts of the plane, which is a completely weird way to handle it. Whatever it is, its sticking right and left at low throttle settings.

If you jam it to full throttle while hold down the wheel brake, the rudder movement make sense. You compensate for the the veering left with the rudder, and if you let off the rudder it veers left again, as it should. It doesn't stick. Its also an easy way to take off, but I doubt very realistic. Jam full throttle with wheel brakes on and let off the brakes. The rudder will behave as expected (i.e, not sticking to one side), and taking off is not much different than AH.

[Ecliptik]

I get the same thing with most planes when taking off.  
It's not hard to take off, but the natural movement of the plane seems odd.  In AH1, the aircraft would always veer in one direction, opposite to the rotation of the prop, all the way down the runway.

In AH2, I've found what happens with some planes (Spit IX included), is that with no rudder input at all (forget about keeping it straight, just let it go, hands off), the aircraft will veer one way for the initial 2-3 seconds, then come the other way for the rest of the takeoff.

The Spit goes left for a couple of seconds, then right the rest of the way.  Every time, rudder input or no.

[TweetyBird]

Yea I don't understand that initial jog to the right either. No controls other than throttle touched, what on earth could make it jog right for a couple of seconds?

[hitech]

As the tail lift gyroscopic presession forces nose to right.

Tail dragers are strange beast once they start turning 1 way that turn will tend to tighten unless halted with rudder input, i.e. unstable.

In real planes we call it dancing on the rudders. At mid range speeds where the rudder has very little effect other than the prop wash your feet are moving constantly .


HiTech

[TweetyBird]

So its sort of like a snowball effect? If the plane is turning right, the right wing is going to dip some takeing some weight off the left wheel which will cause the plane to turn more right which will cause the wing to dip even more?

Sorry, I'm not trying to ask dumb questions, but I've never been in a real tail dragger. Have only flow in commercial planes and I gota get drunk to do that

[hitech]

Correct tweetybird if not stop develops into a ground loop.

HiTech

[Booky]

Originally posted by TweetyBird

Once the engine starts, a torque effect created by the right spinning (pilot’s perspective) prop makes the weight on the right wheel less than the weight on the left wheel. With less weight on it, the right wheel travels faster than the left wheel creating a rotation  or yawing toward the left. The rear wheel simply follows this rotation.

Just wanted to clear something up. Not because of the game but just so you understand what happens in real life.

The left turning tendancy on aircraft with properllars that rotate to the right is caused by the decending blade on the right haveing a larger angle of attack than the acending blad on the left. This is especially noticed in taildragers during takeoff or any situation where you are high power settings low airspeed and high angle of attack.  When you pitch the nose up the decending blade creates more lift than the acending blade, therefor causing a yaw to the left.  The exact opposit happens in single engine aircraft that the propellar rotates counter clock (pilot perspective).

The torque created by the engine and the propellar have a very minimal effect on the aircraft.  

However the propwash created does have a noticable effect on the aircraft. As the wash spirals around the aircraft clockwise it ends up hitting the virtical stabilizer on the left.  This will be most noticed at high power settings low airspeed.  This type of left turning tendancy is seen in aircraft with a tailwheel as well as aircraft with a nose gear.

The prop wash and the lift effect from the blades angle of attack are the two most noticable types of left turning tendancy an aircraft has.

Im sure Voss or any other pilots will know what im talking about and maybe explain it better if I am not clear.  I just wanted you to understand what is happening when you are at a slow airspeed (or groundspeed) and have a high power setting.

Let me know if this helps out out some or if I came off totally unclear.