Drag Area Testing

[+saint aj 0]

While working on a new FM, I started to wonder how the sim calculates the drag forces incurred by weapon pylons and speedbrakes. So to determine this, I modified the stock Thirdwire F-104G so that its center weapon pylon had a drag area of 0.000 sq meters. Testing the aircraft at 600 kts true airspeed and 20,000 feet (the design cruise environment for my current project, so I had those calculations handy), I found the aircraft generated a little over 6500 lbs of drag in level flight, in a clean configuration save for one Mk81 on the test pylon. Noting this, I then altered the FM file so the pylon had the same equivalent flat plate area as the aircraft I'm working on, 0.09 sq m. Testing the aircraft at the same altitude and airspeed, with one Mk81 on the centerline as before, the Starfighter produced a bit over 6900 lbs of drag.

 

Now this last result I found somewhat confusing. The reason for this being that I had just calculated the drag for a flat plate of 0.09 sq m (0.97 sq ft) area at 20,000 feet and 600 kts TAS. According to NASA , the drag coefficient for a flat plate is 1.28. According to the standard drag equation, the drag should therefore be:

D = Cd * 0.5 * p * V^2 * S

1.28 * 0.5 * 0.0012765104 * (1008 fps) * (1008 fps) * 0.96 sqft = 796 lbf

 

So therefore, the pylon should have added just under 800 pounds of drag to the aircraft. Instead, it added about half this amount.

 

My first thought was of course that maybe the sim uses the body section's drag coefficient for weapon pylons. So I tried plugging in the fuselage's drag coefficient of 0.0254. This came out to 15 lbf. Obviously not the 800 pounds I was expecting.

 

In fact, the drag coefficient I'm coming up with is approximately 0.64 for weapon pylons, which is about half the NASA value for a flat plate. I thought this was pretty significant so I'm posting it here in the hopes that other designers will benefit when fine-tuning their projects.

 

Also I'd be interested in hearing an explanation of how I might have screwed this one up...

[Fubar512 1,350]

You're experiencing this because the game engine requires you to define a CD Mach table (expressed as "CD0machtable", CD at zero lift), where applicable. My suspicion is that it would otherwise impose a simplified (and as you've seen, incorrect) drag calculation on said object., as it does on missiles, bombs, bullets, (etc.).

 

You can try adding the declaration "HasAeroCoefficients=TRUE" under the pylon's entry, add the necessary coefficients, and then create a Mach table for it.

[+saint aj 0]

That works for weapon stations? I had no idea! I'll give it a go...

[Fubar512 1,350]

That works for weapon stations? I had no idea! I'll give it a go...

 

Now, I have not personally tried it, but as long as you declare the surface as having aerodynamic coefficiants, why wouldn't it work?

[+saint aj 0]

That works for weapon stations? I had no idea! I'll give it a go...

 

Hmm. Well I tried jacking the drag coefficient for the pylon up to the NASA spec for a flat plate, and it added a bit more drag, but not the expected amount. Well I'll have to keep tweaking, but I think I'm on the right track.

[Fubar512 1,350]

Hmm. Well I tried jacking the drag coefficient for the pylon up to the NASA spec for a flat plate, and it added a bit more drag, but not the expected amount. Well I'll have to keep tweaking, but I think I'm on the right track.

 

Try the tables, keeping in mind that doubling the velocity increases the force required, x4.

[Schorsch 0]

You're experiencing this because the game engine requires you to define a CD Mach table (expressed as "CD0machtable", CD at zero lift), where applicable. My suspicion is that it would otherwise impose a simplified (and as you've seen, incorrect) drag calculation on said object., as it does on missiles, bombs, bullets, (etc.).

 

You can try adding the declaration "HasAeroCoefficients=TRUE" under the pylon's entry, add the necessary coefficients, and then create a Mach table for it.

 

Hi.

I've found this topic while searching the forums. I have downloaded your Tornado model and I am trying to understand how the drag data is compiled. For fuselage it looks like this:

 

CD0MachTableNumData=7

CD0MachTableDeltaX=0.40

CD0MachTableStartX=0.00

CD0MachTableData=0.982,1.000,1.059,2.536,2.054,1.830,1.671

 

Maybe you can give me short answers on following questions:

1. The CD0MachTable gives drag coefficient over Mach number. I guess the data in CD0MachTableData needs to multiplied with CD0=0.0075. Right?
   
2. The Mach Table is starting with 0, goes on 0.4 and has seven steps, hence the drag coefficients are given for Mach = [0 0.4 0.8 1.2 1.6 2.0 2.4]. Right?
   
3. Drag coefficients are interpolated between those points. Right?
   
4. Drag coefficients of several components are added. Right?
   
5. Is reference area per component, or is the reference area in the AircraftData table used for all coefficients?
   

I really appreciate some insight.

Thanks in advance.

Links to related topics always welcome.

 

Schorsch

[Fubar512 1,350]

Hi.

I've found this topic while searching the forums. I have downloaded your Tornado model and I am trying to understand how the drag data is compiled. For fuselage it looks like this:

 

CD0MachTableNumData=7

CD0MachTableDeltaX=0.40

CD0MachTableStartX=0.00

CD0MachTableData=0.982,1.000,1.059,2.536,2.054,1.830,1.671

 

Maybe you can give me short answers on following questions:

1. The CD0MachTable gives drag coefficient over Mach number. I guess the data in CD0MachTableData needs to multiplied with CD0=0.0075. Right?
   
2. The Mach Table is starting with 0, goes on 0.4 and has seven steps, hence the drag coefficients are given for Mach = [0 0.4 0.8 1.2 1.6 2.0 2.4]. Right?
   
3. Drag coefficients are interpolated between those points. Right?
   
4. Drag coefficients of several components are added. Right?
   
5. Is reference area per component, or is the reference area in the AircraftData table used for all coefficients?
   

I really appreciate some insight.

Thanks in advance.

Links to related topics always welcome.

 

Schorsch

 

The Tornado is not my model, nor did I have any association with it.

 

Regarding your questions:

 

1) Yes

2) Yes

3) Yes

4) Yes, but tables are defined per individual component as declared in the data.ini, and then "combined", by the flight engine

5) "Reference Area" in this case, is the total wing area in square meters.

[Schorsch 0]

The Tornado is not my model, nor did I have any association with it.

 

Regarding your questions:

 

1) Yes

2) Yes

3) Yes

4) Yes, but tables are defined per individual component as declared in the data.ini, and then "combined", by the flight engine

5) "Reference Area" in this case, is the total wing area in square meters.

 

Thanks for the reply.

You have - if I read correctly - built the MiG-17 aero model. I took a look in it, too. There you have given a more dense Mach-Drag table (M0.1-steps). The lift dependant drag is given as CDL (Drag due to lift) over Alpha.

 

You said that you got the drag information from the Have Drill presentation (I just went through it). Did you "guesstimate" the lateral derivatives?

 

I will try to check how the drag coefficients look when I use total aircraft coefficients. I may ask you as simulator insider: How well do simulation and reality (represented by some basic performance figures) match?

[Fubar512 1,350]

Thanks for the reply.

You have - if I read correctly - built the MiG-17 aero model. I took a look in it, too. There you have given a more dense Mach-Drag table (M0.1-steps). The lift dependant drag is given as CDL (Drag due to lift) over Alpha.

 

You said that you got the drag information from the Have Drill presentation (I just went through it). Did you "guesstimate" the lateral derivatives?

 

I will try to check how the drag coefficients look when I use total aircraft coefficients. I may ask you as simulator insider: How well do simulation and reality (represented by some basic performance figures) match?

 

In most cases, what I do is calculate a range, based on the premise that the calculated Cla at Mach 0.4 (which is the SF standard) is "100%".

 

For example, if the calculated CLa at Mach 0.4 was 3.286905, my Excel spreadsheet would look something like this:

 

Cla Mach Percentage +/-

3.200263 0.1 97.4

3.216983 0.2 97.6

3.245509 0.3 98.7

3.286905 0.4 100.0

3.342821 0.5 101.7

3.415707 0.6 103.9

3.509169 0.7 106.7

3.628587 0.8 110.3

3.782265 0.9 115.1

3.983700 1.0 121.2

 

 

The standard formula that we use does not take such variables as airfoil type, wingtwist, (etc.) into account, so if I were to use the figures I just calculated, I might find that the FM would perform well beyond the limits of the real aircraft, so I might have to "compensate" for that, somewhere down the line.

 

And that brings us to your next question. Strike Fighters simulates real world atmospheric and inertial values quite accurately. Generally, the end result is only as accurate as the data that you've used.