0 votes
asked by (12 points)

I am attempting to develop a flight control scheme for a electric short take-off and landing vehicle for a class project. We need a flight control software that can accurately model the effects of the blown-wing propulsion system we've developed in order to pick a method of controlling our aircraft's flight path. What is the best way to model blown lift in X-Plane? Is it possible to model a change in blowing power for a fixed flap condition? 

High level, each wing has a large cruise motor at the wingtip and 4 blowing motors between the fuselage and the tip. 

I'm not worried about the motors and 3D model yet and am simply adjusting some of the Cessna 172 models to see what's possible. X-Plane 11's "Plane Maker" tool has two ways I *think* I could potentially model the effects of aggressive flap deflection and blowing:

1. Use the blown lift feature under Expert > VTOL or Helo Controls. Does the option on the right increment the CL by the listed amount per flap increment? Or is it just a 1-time increase in CL once blowing has been engaged?

2. Make the normal flaps "extra lift-y" by adjusting the values for maximum flap deflection. This option is under Standard > Control Geometry. I have some polar plots that tell me the theoretical impact of flap deflection and blowing on my wing's CL and CD but the effects are non-linear. Is the mapping of flap deflection to increases in CL and CD a linear one? Is there a way to map the flap deflection to more granular changes in CL and CD? 

Option 2 seems like it might be more along the lines of what I want but the putting in the max CL and CD for a given flap/blowing combo does not fully represent my airplanes ability to change blowing for a given flap deflection. 

2 Answers

0 votes
answered by (18 points)

Blown flaps are different than what this airplane does.

Blown flaps put air in (or take it out out) of the boundary layer to reduce airflow separation and get higher coefficients of lift from wings. The F-4 Phantom has them, and bonus points: they make a screeching noise when activated so flybys are better with them activated. (Fun-fact: Flyby score is directly proportional to the number of pants wet by spectators as a result).

But this is not what you want here at all.

I assume.

Instead you want to have the wings get plenty of propwash from your props to work at lo speed, and luckily X-Plane handles this quite well, but un-luckily I don't really get GOOD at it until 11.30, which is not in public beta yet but will be soon.

So in X-Plane before 11.30, it was just momentum theory to find propwash and put that over anything in line with the prop.

Now, in 11.30, it is much better: Air speeds up ahead of the prop, hits 60% of it's wash speed at the prop, and goes to terminal wash speed well behind the prop, all with assymptotic curves just as in reality. So, if the wing is behind the props, it will get washed by the props, and in 11.30 it will even be washed according to HOW FAR behind the props the wing is!

But here is the thing: X-Plane does NOT understand props that are just ABOVE or just BELOW the wing, trying to wash only ONE SIDE of it: The wings are either in propwash, or they aren't, and the wings speed is changed accordingly, same change to both the lower and upper surfaces!!!!

So your props below the wing will MISS the ing and have NO impact in X-Plane: They need to be in front of the wing to do anything! Just look at the Spruce Goose.

austin

commented by (3.6k points)
Hi knoedlaa,

Congratulations.  You have received an answer/comment from Austin, the developer of X-Plane.  Never been seen before.

All the best with your project.

Glenn
commented by (12 points)
edited by

Thanks for the response! To clarify my question a little bit more, I'm trying to figure out:

1. How I could input data from a polar that represents our aerodynamic model of blown lift?

2. What do those two parameters do in X-Plane's blown lift model? 

We considered using X-Plane to model the blowing by putting the blowers in the right spot and seeing what happened. Unfortunately for attempts to use X-Plane 11.30 for that approach, our professors want us sticking to the aerodynamic relationships we're getting from our Prof in JVL (http://web.mit.edu/drela/Public/web/avl/). Here's the lift/drag polar I'm working with. CX can be thought of as CD that can become a thrust force (negative CX) because the blowing gets too high. Cis a constant that represents the strength of our blown motors.  

For the first question, I was curious if I can translate the data I have into those CL and Cfactors for max flap deflection (the first red box in the the original post). I was hoping for some clarification on what those two coefficients did to the model of wing lift. Is it a factor that is multiplied by the wing's lift and drag coeffs at a max flap deflection? Is it a number that is added to the wing's coeffs at max flap deflection? How do the numbers change for non-max deflections? For example, if the CL factor was 1.5 and CD factor 1.1 at 40° of flaps would 20° of flaps would have factors of 1.2 and 1.05 respectively?

For the second question, I went and looked at how the F-4 used the blown lift originally as well as how the X-Plane F-4 is setup. Does the Cjust increase once by .15 past the flap threshold for activation of .05? Or is .15 an increment multiplied by the degree of flap deflection? Is Caffected at all?

I've included the side view of what our blowing motors look like as well as the theory behind the data we're using


Thanks again for the help!

0 votes
answered by (1.8k points)

Hey! Here is the input of X-Plane creator Austin Meyer. Thanks!

Blown flaps are different than what this airplane does.

Austin: "Blown flaps put air in (or take it out out) of the boundary layer to reduce airflow separation and get higher coefficients of lift from wings. The F-4 Phantom has them, and bonus points: they make a screeching noise when activated so flybys are better with them activated. (Fun-fact: Flyby score is directly proportional to the number of pants wet by spectators as a result).

But this is not what you want here at all.

I assume.

Instead you want to have the wings get plenty of propwash from your props to work at lo speed, and luckily X-Plane handles this quite well, but un-luckily I don't really get GOOD at it until 11.30, which is not in public beta yet but will be soon.

So in X-Plane before 11.30, it was just momentum theory to find propwash and put that over anything in line with the prop.

Now, in 11.30, it is much better: Air speeds up ahead of the prop, hits 60% of it's wash speed at the prop, and goes to terminal wash speed well behind the prop, all with assymptotic curves just as in reality. So, if the wing is behind the props, it will get washed by the props, and in 11.30 it will even be washed according to HOW FAR behind the props the wing is!

But here is the thing: X-Plane does NOT understand props that are just ABOVE or just BELOW the wing, trying to wash only ONE SIDE of it: The wings are either in propwash, or they aren't, and the wings speed is changed accordingly, same change to both the lower and upper surfaces!!!!

So your props below the wing will MISS the ing and have NO impact in X-Plane: They need to be in front of the wing to do anything! Just look at the Spruce Goose."

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