Need Flight Model help

[Pete_SMS]

Hi,

Today I am requesting some help with respect to the Flight Model calculation in X-Plane. If this is not the correct place to post this, please move my topic to a more suitable area.

OK, here comes my problem.

After trying to achieve the best glide speed of my aircraft by a changing-testing-changing approach, I stopped working that way and try to tackle the problem with a mathematical and engineering approach, because the previous didn't work out.

OK, just a few basic assumptions I made. The best glide speed occurs when the lift to drag ratio is at its maximum. Also it is a known fact that the parasite drag (cd0) and the induced drag coefficient (cdi) are the same at this point.

Therefore I can say cd0 = cdi

==> cd = cd0 + cdi = 2 cd0

I also know, that the L/D ratio at best glide speed is, let's say, 15.0.

==> L/D = cl/cd = 15.0

We also know that induced drag is calculated by

cdi = cl^2 / (AR * pi * e)

where

AR = Aspect Ratio

e = Oswald efficiency factor

Having these equations, I would be able to set my planemaker and airfoilmaker settings to end up with the required cd0 and cl at the required airspeed.

However, classic engineering is using the wing area as a reference for lift and drag calculations

L = 0.5 * rho * Vtas^2 * cl * Swing

D = 0.5 * rho * Vtas^2 * cd * Swing

and it looks like, as if X-Plane is not using this approach. The reason why I make this assumption is that I made a test. I stabilized the aircraft at its current (L/D)max. I displayed all the parameters I am interested in inside X-Plane. I wrote down my total cl and cd, true airspeed, density, total lift, total drag etc.

Then I calculated the area X-Plane has to use, in order to come up with the total lift using its total cl. I noticed, that it is not only the wing area. It was the total area of all lifting surfaces. This means wing area plus horizontal tail area. OK. Fair enough.

Then I did the same for the drag, but this time the required area, was not equal to the one for the lift. In fact it was lower as the reference area for the lift. So I have the feeling that X-Plane is not using the same reference for lift and drag.

Based on this observation, I would like to make a few questions:

(1) Is my assumption correct, that X-Plane is using the total area of all lifting surfaces a reference in order to calculate the total lift, based on the total cl?

(2) Is my assumption correct, that X-Plane is not using the same reference area as for the calculation of lift ?

(3) I can set the drag coefficient for the fuselage and the nacelle in planemaker. Let's assume I have a prop aircraft and the prop and nacelle is directly sitting on the nose. The total frontal area of the fuselage has a certain area and so does the prop spinner / nacelle. Is X-Plane calculating the drag of of the fuselage and spinner by adding them up? I think this would be incorrect, because the spinner would already by covered by the fuselage frontal area calculation and would even help to reduce drag, due to its sreamlined shape. So the total drag of these components should be less then the fuselage alone.

(4) What is X-Plane using for calculating the induced drag. Only the wing AR and wing lift or the whole aircraft lift and AR?

It would be great if you could help with these questions, because I am trying to solve this glide speed problem for several days now and it looks like as if the more detailed I look into this, the more confusing it gets for me, because the classical approach is not really working.

Thanks in advance!

Best regards,

Pete

[Pete_SMS]

OK, I tested this thing a little further. This time I used one simple rectangular wing shape and the cycle dump file gave me exactly the wing area and aspect ratio I calculated by hand for this wing. Also the oswald efficiency factor was given to me. Having this info I went flying again and stabilized the aircraft at a certain speed.

Using X-Plane's parameter display, I wrote down the lift force and the drag force of the wing alone, the true airspeed, the density and so on. Then I went back caculating, using the following equations:

L = 0.5 * rho * Vtas^2 * cl * Swing

D = 0.5 * rho * Vtas^2 * cd * Swing

Since X-Plane was showing me the lift force for the wing alone, I calculated the CL of the wing for that condition.

cl = L / (0.5 * rho * Vtas^2 * Swing)

OK, the cl was a little lower than my airfoilmaker value, but that is OK, because I am using an finite wing instead of a 2D airfoil.

Then I did the same for the drag, and here something very strange became obvious. Knowing the cl at that condition from my previous calculation, I should have been able to calculate the induced drag of the wing by using

cdi = cl^2 / (AR * pi * e)

I did this and the calculated induced drag was already about 3 times higher then the drag X-Plane showed me for the wing. I then went to airfoilmaker and wrote down cd0, the drag coefficient of the airfoil at zero lift. Having cdo, I calculated the zero lift drag of the wing alone by using

D0 = 0.5 * rho * Vtas^2 * cd0 * Swing

Also this value was almost twice the drag force X-Plane was showing me for the wing. If I now add-up zero lift drag and induced drag for the wing, I am about factor 4 higher then the value X-Plane showed me.

Either I am doing something stupid, or X-Plane is not following the standard at all.

I am getting more and more the impression that tuning the performance of the flight model is a more or less "change-test-change-test" approach until you are close to the real vlaues, since a lot of X-Plane internals are unclear.

How do you guys and girls handle this issue? Would be great to have a discussion about this topic.

[Morten XPFW]

Reverse engineering is the only way, like you say, tune-fly-tune-fly.  Tailoring the airfoils.

With regard to drag, XP has issues so without a plugin you will not get it right.

[Pete_SMS]

Thank you for the answer Morten. Much appreciated.

At least I know now, that I can not solve the problem analytically by using an engineering approach.

When you talk about using a plugin for the drag, do you mean changing the fuselage drag sim/aircraft/bodies/acf_fuse_cd dynamically, or something else?

[Pete_SMS]

And yes, I did something stupid during my last test. I based all my calculations on SI units and converted kts to m/s, lbf to N etc. However, I forgot to convert my calculated drag force in unit [N] back to unit [lbf], in order to compare it with the values shown on my data display in X-Plane. Drag and lift is unit [lbf] there and that means I have to devide my calculated values by 4.44822162.

Does a "I was a little tired when I did this" count?

Well, now I can continue with my research of X-Plane internals...

[Pete_SMS]

Having a better understanding of X-Plane internals now, I can go to about 90% accuracy by using an engineering approach and the last 10% by trial and error. Well, I guess having a deviation of < 2% in the end should be satisfying, knowing that you could have this spread on a real aircraft as well.

Basically, I am very satisfied with my overall aircraft performance, but I still need to get that glide speed and the corresponding L/D correct. After getting a hint, I played with wing incidence and in fact this helped a lot.

Is here anybody who would share his strategy for moving the max. L/D to a lower speed and increase the ratio at the same time?

I figured that by increasing the wing incidence, the max. L/D is moving to a lower speed. Probably due to the fact, that the fuselage will be at a lower AOA and therefore closer to its minimum drag at 0 AOA.

Is the strategy to increase the wing incidence, lift curve slope and/or CL0 correct, or would you use something else.

I would assume that a lot of you have done this before and any help would be much appriciated, because this is basically the last nut I have to crack.

Thanks!

[Pokerface]

Having a better understanding of X-Plane internals now, I can go to about 90% accuracy by using an engineering approach and the last 10% by trial and error. Well, I guess having a deviation of < 2% in the end should be satisfying, knowing that you could have this spread on a real aircraft as well.

Basically, I am very satisfied with my overall aircraft performance, but I still need to get that glide speed and the corresponding L/D correct. After getting a hint, I played with wing incidence and in fact this helped a lot.

Is here anybody who would share his strategy for moving the max. L/D to a lower speed and increase the ratio at the same time?

I figured that by increasing the wing incidence, the max. L/D is moving to a lower speed. Probably due to the fact, that the fuselage will be at a lower AOA and therefore closer to its minimum drag at 0 AOA.

Is the strategy to increase the wing incidence, lift curve slope and/or CL0 correct, or would you use something else.

I would assume that a lot of you have done this before and any help would be much appriciated, because this is basically the last nut I have to crack.

Thanks!

Hey Pete,

as it looks like you are not getting any help, I talked to a colleague about your problem. He is a flight simulation engineer and is also working with X-Plane. He was the one who recommended X-Plane for my flight training preparation.

I told him about the problem you have and he said you can contact him any time with respect to questions.

He knows that it is tough going from classic simulation to planemaker, airfoilmaker and finally X-Plane, without knowing the details of X-Plane internals. Send me your e-mail address and I will get you in contact with him.

Cheers,

Steph

[Pete_SMS]

Hi Steph,

Thank you very much for the answer and talking to your colleague would be very nice.

I am an aerospace engineer myself and work on professional flight simulators and totally agree, that there is a learning curve in X-Plane, even you have the full academic background and lots of experience with professional simulators in general.

This is due to the fact, that you don't have full control of the underlying model X-Plane is using, and lots of things are kind of unclear or not known. You have to find out what effects all the settings in planemaker have and often they don't follow the standard or what you would expect in the first place. (planemaker is the aircraft design program that comes with X-Plane) Once you get it, it does make sense though.

For specific problems you end up in a trial and error cycle. Currently, I am trying to break that cycle by doing extensive research in order to understand how X-Plane is ticking internally. I put these results into mathematical models, based on classic engineering and aircraft design equations and refine them until I get close to the results of X-Plane. This way I get a pretty good picture of X-Plane internals and I can approach the whole thing better than by just using trial and error, because I can calculate things already offline. At least I am in the right ballpark before firing up X-Plane. There are still a few topics where I have to do more and deeper investigations, but I think my glide speed issue is on a good way now.

Flight Dynamics and handling qualities are not that big of an issue, if you know what you do and here you have the chance to influence most of the stuff yourself by coding plug-ins. Gizmo is great for that. Not sure if your colleague knows about it already, but if not, I let him know

I assume he uses X-Plane in his freetime or is he doing professional day job stuff with it?

I am looking forward to talk to him.

I will send you my email via PM and really appreciate that you came in to help. I could hear my own echo already pretty loud. *just kidding

Have fun on the forum, because this is a very cool place and do not hesitate to ask questions, if you have any.

Have a great day,

Pete

[D82]

Hello,

how did you end up? Succeeded?

I'm currently facing the same problem. Need to tune a glider speed polar. Did you somehow find how fuselage drag is calculated (added) to the overal plane's drag?

-Joe

[Pete_SMS]

Hi Joe,

Yes, but it was lots of trial and error until I got it right, playing with wing incidence, fuselage drag and airfoil lift and drag.

The project is on hold right now though, because I am working on the Beaver. When I will continue with it, the flight model will be done from scratch again.

As for the fuselage drag, I had a quick chat with Austin, but he didn't want to reveal how X-Plane is calculating fuselage drag. However, at zero AOA it is related to your frontal area and at AOA other than zero, I could imagine that X-Plane is using the projected area, plus some other "black magic".

Edited December 12, 2011 by Pete

[arno54]

in cycledump.txt you will find this :

Now for MI from FUSELAGE:

The frontal area is 2.132 square meters.

The side area is 9.222 square meters.

The top area is 8.318 square meters.

(for instance)

now for the fuselage drag, i came with :

math.sqr(fusecd) * frontalArea / sideArea

this adds to total wing drag and this total uses no polar but a linear projection that substracts from total lift for a given speed.

this explains 3 things :

why a very slight change in frontal area results in a major change in lift (actually, in vertical speed)

why a very slight change in fusecd has tremendous impact on rolling distance before take-off

why elongating the plane increases thickness to absurd proportions

another way to say it, is that fusecd actually acts straight onto thickness, but not really on drag.

I might be wrong, but countless hours on thickness troubles give me a 99% certainty.

In the end, as this could not be solved, I decided to use 0 fusecd everytime it's possible, and to set the overall drag through wing drag, using custom airfoils - I was never able to set correct thickness in an other manner.

[D82]

Thank for your replies.

One more thing. I'm not sure how to add induced drag to overal. I know how to calculate the coeficient but not sure how to use it to calculate the drag produced.

I guessing:

- calculate overal drag of the wing (sum of drags produced by every segment)

- calculate wings drag coeficient (Cd = 2*D/(rho*Vtas^2*Swing)

- then add Cdi to Cd and back calculate the overal drag?

[D82]

Ok i figured it myself

I got the flight model finaly where I wanted. Even if it was not reached using exact math, knowing what's behind the scene made it easier.