Tool for AoA on H-stab

[bomber]

So here again I'll explain how I knife and fork this using separate apps.

We all know that if the wing airfoil experiences an AoA then due to the air stream coming off the back of the wing airfoil the h-stab experiences a different AoA.

Now for my flight modelling I model ALL the airfoils, lift, drag and side as accurately as they can be and have my errors in other areas. If you've 'whole plane' coefficients that come from wind tunnel testing then the standard JSBsim approach is possible but surely after all these years we can all agree that it's not even 1 in 10 planes that have this. The facts of the general flight model quality and the use of yasim speaks for itself in this regard.

So having used the airfoil tool to establish the polars along the wing I import this data into Javafoil, Xfoil could I believe be a viable alternative and from this I get the ccoefficients at various angles of attack.... I use -32, -16, -8, -4, -2, -1 0 1, 2, 4, 8, 16, 32 degrees as this gives a pretty good looking series of tables.

But what we're after is the airflow off the back of the wing... for this I use the polar tab on javafoil, xfoil I'm sure does the same. To get a graphical representation of the airflow off the wing.

https://drive.google.com/file/d/1fIl83D ... sp=sharing

The above image has been added into a drafting package and scaled such that the tail airfoil (additional blue airfoil) position is correctly located. From here it's a simple task to measure the angle of the flow stream and I do this for all the AoA as detailed above and produce a table of the AoA experienced by the inner wing section against what the tail experiences.

Regards

Simon

[IAHM-COL]

We all know[?] that if the wing airfoil experiences an AoA then due to the air stream coming off the back of the wing airfoil the h-stab experiences a different AoA.

You miss me here already.
How so?

AoA = Angle of attack.
Airstreams should nt affect AoAs, unless they cause a rotation of the airfoil (via a momentums, per example)

Differences in the AoA in wing and hstab shouldnt be cause on differential momentums. They seem maybe cause structurally, as in the actual shape of the flyable.

Can you elaborate a lot more on your first statement here quoted?

Thanks @Bomber,

IH-COL

[bomber]

Although it says increase... it's an increase in negative angle of attack

[IAHM-COL]

@Bomber

I'm still wrapping my mind around this, but I am starting to agree with this.

In addition to AoA of main airfoil, would airspeed have an effect on this downwash effect? Or do you think speed will play no role here?

[bomber]

Speed plays no role if you're just after the angle of downwash on the h-stab.

That said, this alone won't give you the actual AoA on the h-stab as roll rates p & q aren't taken into account.

What you actually need is the w-fps (down speed) that the h-stab is travelling at plus it's u-fps (forward speed) and with these and a bit of Pythagoras you can work out the actual AoA on the h-stab.. you'd end up with something like this

Code: Select all

   <fcs_function name="T4T/aero/h-stab_left/AoA-deg">
      <function>
         <todegrees>
            <sum name="T4T/aero/h-stab_left/AoA-rad">
               <atan2>
                  <property>T4T/velocities/h-stab_left/w-aero-fps</property>
                  <property>T4T/velocities/h-stab_left/uv-fps</property>
               </atan2>
               <product>
                  <v> -1 </v>
                  <sin><property>T4T/aero/h-stab_left/beta-rad</property></sin>
                  <value>0</value>         <!-- dihedral rad -->
               </product>
               <product>
                  <cos><property>T4T/aero/h-stab_left/beta-rad</property></cos>
                  <value>0</value>         <!-- rigging rad -->
               </product>
            </sum>
         </todegrees>
      </function>
   </fcs_function>

And if you've gone this far, you so far out of the box, that any textbooks you have on conventional flight modelling you can throw away as they aint going to help.

[IAHM-COL]

@Bomber

Code: Select all

               <product>
                  <v> -1 </v>
                  <sin><property>T4T/aero/h-stab_left/beta-rad</property></sin>
                  <value>0</value>         <!-- dihedral rad -->
               </product>


and

Code: Select all

 
               <product>
                  <cos><property>T4T/aero/h-stab_left/beta-rad</property></cos>
                  <value>0</value>         <!-- rigging rad -->
               </product>

should evaluate as zero on every condition, since

K*0 = 0 for every K that is a Real number.

I still don't grasp what we attempt here in terms of what is the input and what is the output seek for

[IAHM-COL]

accordingly that code should be validly simplified as

Code: Select all

   <fcs_function name="T4T/aero/h-stab_left/AoA-deg">
      <function>
         <todegrees>
            <sum name="T4T/aero/h-stab_left/AoA-rad">
               <atan2>
                  <property>T4T/velocities/h-stab_left/w-aero-fps</property>
                  <property>T4T/velocities/h-stab_left/uv-fps</property>
               </atan2>
            </sum>
         </todegrees>
      </function>
   </fcs_function>



In other words you would be saying that hstab-left AoA [radians] is simply

Code: Select all

 arctan(w/uv)

So,
1. if uv is forward speed, why you said above that speed not a factor?
2. why, then you want to use xfoil to calculate the flow of air around the main wing?

[bomber]

accordingly that code should be validly simplified as

In other words you would be saying that hstab-left AoA [radians] is simply

Code: Select all

 arctan(w/uv)

Yes it can, for this particular plane, another maybe not... this is a template.

atan2 is better to use as it never gives an infinity large result, which you sometimes get with arctan and this either crashes FG or gives a 'kick' to the flight.

So,
1. if uv is forward speed, why you said above that speed not a factor?

It's not a factor in determining the AoA on the H-stab... it is a factor in flight modeling.

It's my bad... you asked a question about the speed and I tried to answer it... this sometimes takes us down a rabbit hole of going off in a tangent.
It's just that I enjoy flight modelling and talking about it and I think it's rude to ignore a valid question from someone that's interested,

2. why, then you want to use xfoil to calculate the flow of air around the main wing?

[/quote]

The angle (downwash) that the air leaves the back of the main wing is caused by it's shape... this angle is not effected by the speed at which the air passes over the airfoil...

Simon

[IAHM-COL]

The angle (downwash) that the air leaves the back of the main wing is caused by it's shape... this angle is not effected by the speed at which the air passes over the airfoil...

Simon

Ok.
And the other factor affecting the angle of the downwash is the AoA of this wing?
And that's why you calculate a table of downwash vs AoA?

It's my bad... you asked a question about the speed and I tried to answer it... this sometimes takes us down a rabbit hole of going off in a tangent.

Thanks for answering questions. I can go down the rabbit holes if I can find a cookie that makes me larger

[mue]

And if you've gone this far, you so far out of the box, that any textbooks you have on conventional flight modelling you can throw away as they aint going to help.

Yup. Pitch damping due to tail surfaces isn't widely known in the aerodynamics community. E.g. only recently (1923!) this paper was published: https://ntrs.nasa.gov/search.jsp?R=19930091201