Board index FlightGear Development Aircraft Flight dynamics model

FDM developer searched

Good sims require good FDMs (the "thing" that makes an aircraft behave like an aircraft).

FDM developer searched

Postby D-ECHO » Sun May 10, 2015 3:15 pm

Hi!
Is anyone out there who would like to test his skills by modelling the FDM for this aircraft:
Image
Contact me for more info :)
Regards
User avatar
D-ECHO
 
Posts: 2109
Joined: Sat May 09, 2015 12:31 pm

Re: FDM developer searched

Postby IAHM-COL » Sun May 10, 2015 4:54 pm

what kind of aerodynamics is that aircraft suppose to have?
Ive no idea how to make an FDM. And there are at least 2 excluyent versions in FG: Yasim and JSBsim.
Maybe a yasim structure for your model makes more sense.
If we gave everybody in the World free software today, but we failed to teach them about the four freedoms, five years from now, would they still have it? Probably not, because if they don’t recognise their freedoms, they’ll let their freedoms fall
User avatar
IAHM-COL
Retired
 
Posts: 4064
Joined: Wed Aug 08, 2012 5:40 pm
Location: Homey, NV (KXTA) - U.S.A
Callsign: HK-424D or ICAO4243
Version: 3.7-git
OS: Linux

Re: FDM developer searched

Postby D-ECHO » Sun May 10, 2015 5:16 pm

Sorry, but what do you mean with "what kind of aerodynamis?" in this case? YASIM or JSBSIM?
Well, since today I've only used YASim because that was easier...
User avatar
D-ECHO
 
Posts: 2109
Joined: Sat May 09, 2015 12:31 pm

Re: FDM developer searched

Postby IAHM-COL » Sun May 10, 2015 5:18 pm

If we gave everybody in the World free software today, but we failed to teach them about the four freedoms, five years from now, would they still have it? Probably not, because if they don’t recognise their freedoms, they’ll let their freedoms fall
User avatar
IAHM-COL
Retired
 
Posts: 4064
Joined: Wed Aug 08, 2012 5:40 pm
Location: Homey, NV (KXTA) - U.S.A
Callsign: HK-424D or ICAO4243
Version: 3.7-git
OS: Linux

Re: FDM developer searched

Postby wkitty42 » Sun May 10, 2015 6:32 pm

i'm still trying to figure out what i'm seeing that is the actual craft... is it just the egg shaped vehicle with tricycle gear or is it that with some sort of open triangular wing on the back with the point of the triangle being the furthest from the body of the craft... in other words, i can't make out the wings, if there are any, and i don't see any sort of propulsion appendages...
"You get more air close to the ground," said Angalo. "I read that in a book. You get lots of air low down, and not much when you go up."
"Why not?" said Gurder.
"Dunno. It's frightened of heights, I guess."
User avatar
wkitty42
 
Posts: 6549
Joined: Fri Feb 20, 2015 3:46 pm
Location: central NC, USA
Callsign: wk42
Version: git next
OS: Kubuntu 14.04.5

Re: FDM developer searched

Postby IAHM-COL » Sun May 10, 2015 6:55 pm

yup Wkitty
Another way of saying just what I am asking myself.
What is the aerodynamics of that model? if any, at all?
If we gave everybody in the World free software today, but we failed to teach them about the four freedoms, five years from now, would they still have it? Probably not, because if they don’t recognise their freedoms, they’ll let their freedoms fall
User avatar
IAHM-COL
Retired
 
Posts: 4064
Joined: Wed Aug 08, 2012 5:40 pm
Location: Homey, NV (KXTA) - U.S.A
Callsign: HK-424D or ICAO4243
Version: 3.7-git
OS: Linux

Re: FDM developer searched

Postby D-ECHO » Mon May 11, 2015 1:26 pm

Maybe this may help understanding the aircraft :)
Image
User avatar
D-ECHO
 
Posts: 2109
Joined: Sat May 09, 2015 12:31 pm

Re: FDM developer searched

Postby IAHM-COL » Mon May 11, 2015 1:37 pm

No. I still dont know how is that suppose to fly
If we gave everybody in the World free software today, but we failed to teach them about the four freedoms, five years from now, would they still have it? Probably not, because if they don’t recognise their freedoms, they’ll let their freedoms fall
User avatar
IAHM-COL
Retired
 
Posts: 4064
Joined: Wed Aug 08, 2012 5:40 pm
Location: Homey, NV (KXTA) - U.S.A
Callsign: HK-424D or ICAO4243
Version: 3.7-git
OS: Linux

Re: FDM developer searched

Postby D-ECHO » Mon May 11, 2015 2:26 pm

well, the propeller pushes the aircrat forward :), then the wing lift the aircraft up.
User avatar
D-ECHO
 
Posts: 2109
Joined: Sat May 09, 2015 12:31 pm

Re: FDM developer searched

Postby IAHM-COL » Mon May 11, 2015 2:38 pm

The prop looks ok.
it has a rear configuration which some RL aircraft also have.
http://upload.wikimedia.org/wikipedia/c ... _O-2_3.jpg
Notice the Cessna 317.
But also, this one has a front prop (also).
I don't know of any monoprop with the helices only rear, but again, it could... be?

Abot the wing creating the lift, in that model...
I am not that convinced. I am still concern of thing like the center of gravity, and I do notice it is a tricycle.

The position of your ailerons, again, could they steer properly? and if so, what would be the behavior?.

Most rudders I've seen are alligned with the central axis of the fuselage. we can see the cessna317 above has a similar configuration as your rudders too

Do you have Horizontal stabilizers? and if not, what is the consequence of lacking them in the context of aerodynamics of an airplane?

All these questions need to be resolved to create an FDM for how such model would fly (it you build a RL object).
My guess is that it does not. So you are hired by Volkswagen, but your CV is rejected by Cessna.

;)
Last edited by IAHM-COL on Mon May 11, 2015 2:47 pm, edited 1 time in total.
If we gave everybody in the World free software today, but we failed to teach them about the four freedoms, five years from now, would they still have it? Probably not, because if they don’t recognise their freedoms, they’ll let their freedoms fall
User avatar
IAHM-COL
Retired
 
Posts: 4064
Joined: Wed Aug 08, 2012 5:40 pm
Location: Homey, NV (KXTA) - U.S.A
Callsign: HK-424D or ICAO4243
Version: 3.7-git
OS: Linux

Re: FDM developer searched

Postby IAHM-COL » Mon May 11, 2015 2:40 pm

Now. on the other part of the problem
Even if we were to make some assumptions on how it flies, I have zero knowledge to how to build/establish a functional FDM.
If we gave everybody in the World free software today, but we failed to teach them about the four freedoms, five years from now, would they still have it? Probably not, because if they don’t recognise their freedoms, they’ll let their freedoms fall
User avatar
IAHM-COL
Retired
 
Posts: 4064
Joined: Wed Aug 08, 2012 5:40 pm
Location: Homey, NV (KXTA) - U.S.A
Callsign: HK-424D or ICAO4243
Version: 3.7-git
OS: Linux

Re: FDM developer searched

Postby curt » Mon May 11, 2015 2:47 pm

Both JSBSim and YAsim require you to know in advance how the aircraft will fly. YAsim does more of the work of figuring out the specifics and nuances of flight, but also builds in many assumptions and simplifications. JSBSim allows you to capture just about any nuance of the flight dynamics and even "bad habits" of an airplane if you have the data and know how to represent it in the configuration file. Neither will predict how an airplane will fly from it's structure shape and mass characteristics.

X-Plane claims to do this prediction based on blade element theory (it divides up the aircraft into small pieces, computes the forces and moments on each piece, and then sums them up.) This is sort of like a virtual real time wind tunnel. It does a good job for what it does, but to be real time, it also has to make many simplifications and will never show you the 'bad habits' of an airplane. Also, if the prediction doesn't exactly match the reality, you have to start messing around with geometry and mass and other tuning factors to get the performance you expect.

There are also some basic equations you can find that will predict stall speed based on wing area and mass. There is a cubic wing loading formula that RC modelers like to use that will predict the feel of the aircraft (feather vs. brick) based on wing area cubed and mass. There are formulas to estimate needed tail surface area (and you could compare that against your actual surface area and perhaps estimate how effective your tail surfaces will be and how well your aircraft will track.)

Even something like top speed is subject to engine rpm and prop pitch ... a propeller powered aircraft simply can't out run it's own prop, so you can often judge the upper limit on cruise speed just by knowing the max engine rpm and prop pitch.

There are also some offline tools you can use to help predict or understand your expected aircraft behavior in advance. Xfoil is one that pops into my head, but I've never used it myself and I really haven't explored this area of aerospace engineering myself. There are more high end ($$$) computational fluid dynamics packages that will predict airflow over the surface of your design in as much detail as you have memory and cpu and time to compute. Of course there are wind tunnels if you have access to them (and knowledge how to use them effectively.)

I think drawing up a new design and then coming up with a flight dynamics model that will accurately predict what happens in flight (before ever flying it) is a difficult proposition. There are many things you can do though to get in the ball park ... but it's hard to get down to the detail level where you can start predicting good or bad habits without doing a lot of extra work (like building a scale model and flying it, or wind tunnel tests, or very advance/expensive CFD analysis.)

That said, I bet you could get 80% of the way there in an evening of tinkering around with a YAsim configuration if you are willing to make some good guesses about expected performance.
curt
Administrator
 
Posts: 1174
Joined: Thu Jan 01, 1970 12:00 am
Location: Minneapolis, MN

Re: FDM developer searched

Postby IAHM-COL » Mon May 11, 2015 2:50 pm

curt wrote in Mon May 11, 2015 2:47 pm:
That said, I bet you could get 80% of the way there in an evening of tinkering around with a YAsim configuration if you are willing to make some good guesses about expected performance.


Yes. I am 100% with curt here. But you definitely will need
1) good guesses, or some guesses of the behavior
2) some idea how to implement that

Take a look again at the Cessna317 in my picture, because I see some similarities in these models.

[and the FG C317 flies wonderfully!]
If we gave everybody in the World free software today, but we failed to teach them about the four freedoms, five years from now, would they still have it? Probably not, because if they don’t recognise their freedoms, they’ll let their freedoms fall
User avatar
IAHM-COL
Retired
 
Posts: 4064
Joined: Wed Aug 08, 2012 5:40 pm
Location: Homey, NV (KXTA) - U.S.A
Callsign: HK-424D or ICAO4243
Version: 3.7-git
OS: Linux

Re: FDM developer searched

Postby D-ECHO » Mon May 11, 2015 2:53 pm

I will think about the other aspects later, just to the rear propeller and the both rudders:
Image
User avatar
D-ECHO
 
Posts: 2109
Joined: Sat May 09, 2015 12:31 pm

Re: FDM developer searched

Postby curt » Mon May 11, 2015 3:26 pm

Here is a link to an article I wrote some time ago. I used YASim to model a composite flying wing UAS as part of a project I'm involved with.

http://gallinazo.flightgear.org/uas/resolution/simulation-modelling/

This isn't really an howto article, but more along the lines of describing what I did with the tools. Maybe it inspires others to take a look into YASim themselves? To create a yasim model you input the physical layout of the airplane and the basic mass characteristics. Then you provide a cruise speed and an approach angle of attack and YAsim figures out the rest for you. From there you can tune power (rate of climb) control surface effectiveness, mass distrubution and balance and it's actually a lot of fun to see things start to come together.

Subsequently, I've used the YASim fdm to do quite a bit of further flight controller development and testing, including software in the loop and hardware in the loop testing of the system. I know the YAsim model isn't perfect, but it has about the right cruise speed, roll rates, power off rate of decent, climb rate, etc. so it's a great stand in for doing a whole range of things.

Hopefully the following link will work and show a bunch of pictures of our little aircraft (which we think is really cool.) :-)

https://plus.google.com/photos/+CurtisOlson/albums/5628118912247541201?banner=pwa
curt
Administrator
 
Posts: 1174
Joined: Thu Jan 01, 1970 12:00 am
Location: Minneapolis, MN

Next

Return to Flight dynamics model

Who is online

Users browsing this forum: No registered users and 0 guests