Beagle Pup experiment

[KL-666]

I fly with a stick.

50 hz seems to make a difference. Yet it would be impractical because most other planes have adverse effects when not on 120 hz. Why not just use that freq?

I do not see how the stick has to do with inertia. If you roll, then you have a certain rolling speed. If you put the stick neutral, the rolling speed does not suddenly stop. In fact you may have to put in a slight counter roll to stop the movement fast.

Take off with flaps is in most planes a choice. Shorter run.

First goal of flaps is lift. Every other (sometimes desired) side effects are secondary. The behaviour of flap 3 is really abnormal. It is as if the plane is hung on a string. Most important: Trim authority can not make it descend with neutral controls at near stall speed. That is definitely very wrong. Did you ever fly small planes?

Into a spin with rudder, holding it for a while to not recover too early. Then just release the controls, and the plane recovers nicely by itself.

Kind regards, Vincent

[bomber]

50 hz seems to make a difference. Yet it would be impractical because most other planes have adverse effects when not on 120 hz. Why not just use that freq?

So the logic is all other poorer fdm's are unable to work at 50hz so you should make yours work at 120hz because people who haven't spend 5 years developing an fdm can't be bothered to spend 15 seconds changing the frequency setting ?

Also this fdm is designed to work in Outerra where we can have multiple planes flying at the same time and jump between them... so the lower the frequency the more planes we can run simultaneously.

I do not see how the stick has to do with inertia. If you roll, then you have a certain rolling speed. If you put the stick neutral, the rolling speed does not suddenly stop. In fact you may have to put in a slight counter roll to stop the movement fast.

The majority of a light planes weight is close to it's lateral CoG... If I was to put guns or fuel to the outer wings then we'd see a larger inertia.
Now I agree roll rate is fixed and if anyone knows the roll rate a beagle pup in rads/sec then we could compare.

Newtons first law states An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

So putting the stick in neutral is in fact the act of applying a counter force.

First goal of flaps is lift. Every other (sometimes desired) side effects are secondary. The behaviour of flap 3 is really abnormal. It is as if the plane is hung on a string. Most important: Trim authority can not make it descend with neutral controls at near stall speed. That is definitely very wrong. Did you ever fly small planes?

It always cracks me up when I see people think that the first goal is lift.... think about it lift doesn't change it's still equal to the weight of the plane, it doesn't really change the speed that much. What changes is the AoA that the lift is generated at... A much lower AoA, dropping the nose thus allowing the pilot to see the runway.... Which is nice considering that rushing towards the ground without a clear view can be considered dangerous.

what I'm hearing is that you're too heavy handed with the stick, take your hands off, drop the throttle to idle and let the plane fly itself to the runway.
Another techneque is to hold a pencil between your index and little finger whilst holding the joystick.

Did I ever fly a small plane.... no not really.

Into a spin with rudder, holding it for a while to not recover too early. Then just release the controls, and the plane recovers nicely by itself.

Odd I can neutral the stick and spin to the ground.... infact if you don't push the stick forward you go deeper and deeper into a flat spin...

Thanks for the feedback

regards

Simon

[KL-666]

what I'm hearing is that you're too heavy handed with the stick

Not sure what you mean by that. I tell you the plane goes up with flaps 3 and full trim down at 60 kts. What do you want me to do when i land like that?

1) Do nothing and climb?
2) Go even slower than 60 kts and risk stalling at low alt?
3) Or add to the full trim down with the stick to descend somewhat?

Anyway, i hope the info is of some help.

Kind regards, Vincent

[bomber]

A plane in decent is not the same as a plane in level flight...

The Beagle pup with full flaps will stall out at 46kts...

If you put full flaps on she'll comfortably glide at 54kts, with a glide slope of 8 degs and a 17deg attitude.... you don't have to touch the controls, she'll not stall out.

Yes all your comments help as it makes me re-look at the fdm... Don't for one second think, just because I'm countering your observations that I don't appreciate your feedback.

But equally if I feel your observations are because you're flying her incorrectly, then surely I must say so and allow you to counter ?

Simon

[bomber]

BTW I do think there's something wrong as she seems sometimes to rotate extremely fast in an uncontrolled manner... but I haven't quite worked out where the error is, probably in the AoA code or it might be a symptom of what you and Richard have observed with the lack of damping.

[KL-666]

Well, knowing it stalls with full flaps only at 46 kts helps. I'll try lower speeds next time. By nature i'm always a bit conservative with speeds avoiding stall in landing

Ow, i see you wrote again. In rotation i personally did not encounter a problem yet. Depends on rotation style i guess. And speed. I rotated at at least 60 kts.

Kind regards, Vincent

[bomber]

No I don't mean take off rotation, I mean when I'm doing aerobatics... something odd happens.

[Richard]

To be honest this is exactly what I'd expect, as initially the plane is flying in ground effect but if you climb out of it whilst still below stall speed you'll stall.

Overall all of the control surface movements in air seem to be far to effective and instantaneous. Possibly lacking damping (e.g. the affect of the rotation rate Q on CM); or this could be the moments of inertia. At this point all I can saw is that the control surface inputs are too directly coupled to the result. It warrants further investigation.

I'm kinda leaning to the control effectiveness being more down to the lack of pilot strength being taken into account within the control system than the actual ability of the plane to roll at the said rate... something I'll look at.

The ability to pitch up the aircraft at 20kts on the ground warrants further investigation. Seems to me that there probably isn't enough air going over the horizontal tail to make this possible.

The problem that is evident with the control inputs is not related to a lack of force simulation in the controls - I'm being gentle with the controls and it will stop rolling immediately with no need to correct; this just isn't right. There is either a lack of inertias, or it's the derivatives (moments due to rates). It is as though the controls are applying a fixed force with hydraulic ram attached with a moving sky hook, rather than the force building with airflow and surface deflection. It could also be that your control surface positions are directly connected to the stick position rather than simulating an appropriate rate of control surface movement.

I fly with mouse, joystick and keyboard depending on what I'm testing. Keyboard is great for controlled movements in a single axis; mouse for more precise control and avoiding the potentiometer glitches I get from pots (and joystick calibration drift).

There is definitely something that needs fixing in your model - and I don't think you can attribute it to the control system.

Running at 50hz makes your model fly better - less reactive than at the standard 120hz, I also tested 20hz and the model was unstable (in pitch and yaw) even with no control inputs. This indicates that you are using a fixed timestep rather than simulation/dt

[Richard]

I've just released a revised beta of my Beagle Pup model. http://chateau-logic.com/sites/default/ ... 6-11-11.7z

Fixed (or improved) flight handling.

* Elevator response, Pitch moment and stability; it may still be a bit twitchy
* sideslip due to roll
* subtle changes to the geometry to reach better stability.
* removed propeller aerodynamic effects and damage effects - I can put these back in later but they need more work to be useful.

To take off - throttle up whilst controlling the gyroscopic prop effects with the rudder. With no flaps smoothly rotate at 65kts keeping the yaw and roll under control. If you lose control of the yaw then you'll get excessive sideslip which in turn destroys the lift, generates roll and results in an undesired ground interaction. With 38% (one notch) of flaps you should be able to rotate slightly earlier, but the climb out will be slower and you'll need to be careful with the power and not to stall.

Landing I tend to come in at 50-60 kts with one notch of flaps, possibly two notches. Depending on weight you'll stall at around 45 kts.

With my model you do need to keep a watch on the yaw and rudder will be required to control this. I think this is like the aircraft based on comments that I've read - and this is coming out of the aerodynamics - it's not something that I've added.

At this stage let's keep the testing to take off and circuits - to provide comparative results use Fair weather.

[bomber]

The ability to pitch up the aircraft at 20kts on the ground warrants further investigation. Seems to me that there probably isn't enough air going over the horizontal tail to make this possible.

At 20kts (33fps) and the throttle firewalled the prop produces an induced velocity of 47fps which results in the h-stab seeing a velocity of 80fps equivalent to 47kts..

You would expect to have some elevator action at that speed.

Running at 50hz makes your model fly better - less reactive than at the standard 120hz, I also tested 20hz and the model was unstable (in pitch and yaw) even with no control inputs. This indicates that you are using a fixed timestep rather than simulation/dt

The flight model is all done using systems, so how can this be ?

The problem that is evident with the control inputs is not related to a lack of force simulation in the controls - I'm being gentle with the controls and it will stop rolling immediately with no need to correct; this just isn't right. There is either a lack of inertias, or it's the derivatives (moments due to rates). It is as though the controls are applying a fixed force with hydraulic ram attached with a moving sky hook, rather than the force building with airflow and surface deflection. It could also be that your control surface positions are directly connected to the stick position rather than simulating an appropriate rate of control surface movement.

There is either a lack of inertias

As I use point masses for the wings it can't be this...

or it's the derivatives (moments due to rates).

I don't use any derivatives,

I calculate the U speed the wing section see's

Code: Select all

   <summer name="T4T/velocities/wing_left/_1_u-aero-fps">
      <input>T4T/velocities/fuselage/u-aero-fps</input>
      <input>T4T/velocities/wing/_1_r_rotational-fps</input>
   </summer>

the W speed this wing sections see's

Code: Select all

   <summer name="T4T/velocities/wing_left/_1_w-aero-fps">
      <input>T4T/velocities/w-gf_left-aero-fps</input>
      <input>velocities/w-aero-fps</input>      
      <input>-T4T/velocities/wing/_1_p_rotational-fps</input>
      <input>T4T/velocities/wing/_1_q_rotational-fps</input>
   </summer>

I then calculate the true velocity that the wing see's

Code: Select all

   <fcs_function name="T4T/velocities/wing_left/_1_true-fps">
      <function>
         <pow>
            <sum>
               <pow>
                  <property>T4T/velocities/wing_left/_1_u-aero-fps</property>
                  <value> 2 </value>
               </pow>
               <pow>
                  <property>velocities/v-aero-fps</property>
                  <value> 2 </value>
               </pow>
               <pow>
                  <property>T4T/velocities/wing_left/_1_w-aero-fps</property>
                  <value> 2 </value>
               </pow>               
            </sum>
            <value> 0.5 </value>
         </pow>
      </function>
   </fcs_function>

I use this and the W velocity to determine the alpha AoA that the section see's

Code: Select all

   <fcs_function name="T4T/aero/wing_left/_1_alpha-rad">
      <function>
         <asin>
            <quotient>      
               <property>T4T/velocities/wing_left/_1_w-aero-fps</property>
               <property>T4T/velocities/wing_left/_1_true-fps</property>
            </quotient>
         </asin>
      </function>
   </fcs_function>   

and I combine this with the dihedral and incidence values to determine the true AoA that the section see's

Code: Select all

   <summer name="T4T/aero/wing_left/_1_AoA-rad">
      <input>-T4T/aero/wing_left/_1_dihedral-rad</input>
      <input>T4T/aero/wing_left/_1_incidence-rad</input>
      <input>T4T/aero/wing_left/_1_alpha-rad</input>      
   </summer>

Simon