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Author Topic: Weight and Ballance  (Read 8959 times)

bomber

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Weight and Ballance
« on: March 02, 2015, 07:30:56 am »

http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/

Please read chapter 9

It's an easy read, yet very informative.. This is my bible on any subject with regards flight and I'd like to see it as the No.1 reference arbiter for discussion on a variety of subjects.

Once we're all clued up on CoG and Weights, I'll explain why I've split the vanilla JSBsim fight model file (see JSBsim manual http://jsbsim.sourceforge.net/JSBSimReferenceManual.pdf )  into separate files and what that means to you as a plane builder. We'll then talk about what assumptions have to be made and what simple tools are needed and any templates that can be provided to get a plane into the air with a very reasonable representation of it's performance.

Simon
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Uriah

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Re: Weight and Ballance
« Reply #1 on: March 02, 2015, 08:13:31 am »

Hi Simon.

That is a great resource I haven't seen before, great read. The next chapter, Chapter 10: Aircraft Performance, is also very informative. Thanks!

I see what you meant about calculating moments about the 'datum' point in the FDM.

I agree that splitting the FDM into multiple files for each section is best for many reasons. For one, a complex flight model could easily be tens of thousands of lines. Have you developed a different scheme than the one used in the original JSBSim FDMs? I'm really interested to hear what you have developed and how you suggest we go about developing more advanced flight models.

Regards,
Uriah
« Last Edit: March 02, 2015, 08:15:45 am by Uriah509 »
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bomber

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Re: Weight and Ballance
« Reply #2 on: March 02, 2015, 08:40:31 am »

A point of confussion with CoG's is that it's a moving point on the plane defined from another 'given' point... a  datum.

But if you find published data for a planes weight ballance (good luck with that) the datum isn't alway in the same place across different planes. sometimes the datum is taken from the engine mounting, prop spinner, pilot position...

I advocate using the pilots position as this allows the modeller to imagine that every object in front of him has a -ve X co-ordinate and every object behind him has a +ve X co-ordinate... this simplifies things and makes spotting errors a lot easier...

As to the FDM being different from the published JSBSim manual.... it's as different as chalk and cheese.

Let's get all the cards on the table with regards this, so as there's no confussion..

The JSBsim manual offers up one approach to flight modelling using it's very versatile programe. JSBsim can be used differently, and I've chosen to do so....

My flight models have no resemblence to the established accedemic norms and as such I've faced critism... but I'm quite happy to have people challenge the principle behind it.

I'm either an idiot, too simple to comprehend the mathmatical complexities or a genius.... the jurys out.
« Last Edit: March 02, 2015, 08:43:09 am by bomber »
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Uriah

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Re: Weight and Ballance
« Reply #3 on: March 02, 2015, 08:47:16 am »

Hi Simon,

What is the principle behind it? Can you post an example and explain the basics of how it works?

Regards,
Uriah
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bomber

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Re: Weight and Ballance
« Reply #4 on: March 02, 2015, 09:54:05 am »

Crickey ask a hard one why don't ya :)

In reality I only get one shot at this, it's either a hit or a miss, once people have made their minds up it's very difficult to change... once pronounced a crack-pot it's a tag very hard to shift.

The objective I set out to create was a system for producing flight models that an idiot could follow... a flight model at that point (idiot involvement only) that produced a flight model better than the best that could be produced at present but not as good as once a flight modeler got involved.

It's a 2 teir flight modelling design....

The principles...

Weight and Ballance are estimated using a spread sheet of build density.
2d force co-efficients are determined using java foil/prop programe for the airfoils and control surface positions.

Each wing is split into 8 sections, using 2d co-efficents the force generated using Angle of Attack (AoA), dynamic pressure and area is calculated and using trigonometry lift and drag vectors are resolved.
Each section of wing can have it's vectors adjusted acording to damage or surface condition
The resulting lift vectors for each wing are then summed to determine the overall lift of the wing and it's moments.
From the overall wing lift the co-efficient of lift is calculated and used to calculate the induced drag.
Form and Skin drag are calculated separately and applied to the airframe as armature distances to produce dampening moments.

So it's basically break the plane down to individual small pieces, calculate the forces, add them together, calculate the moments..... and apply forces and moments to the model.
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Uriah

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Re: Weight and Ballance
« Reply #5 on: March 02, 2015, 10:06:21 am »

I think that sounds like an excellent system! For one it would allow definition of different wing profiles and more complex aerodynamic geometry. I do not think that is a crazy idea whatsoever. Can it be more or less than 8 sections?

Regards,
Uriah
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bomber

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Re: Weight and Ballance
« Reply #6 on: March 02, 2015, 10:45:35 am »

Yes.... less or greater than, however the front end of the flight model (tier 1) must match the back end (tier 2)

If you have say real stubby wings (like on a cruise)  you might not wish to have 8  but instead 2 sections in which case you'd simply create a front end with 2 sections and ask a back end modeler to adjust these files whilst he's updating the co-efficients tables.

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Uriah

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Re: Weight and Ballance
« Reply #7 on: March 02, 2015, 11:30:41 am »

I see so Tier 1 is where define each section and Tier 2 integrates the forces?

With this type of configuration would it be possible to define all types of aircraft geometry? Fuselage, winglets, canards, and different configurations of stabilizers, also defining parameters such as dihedral, or wing that are above or below the fuselage. As we know, high set wings brine the center of lift above the center of gravity and such configurations are inherently stable. On the other hand low set wings, such as the F117 Nighthawk's make that configuration inherently unstable in pitch and roll, but this static instability leads to high maneuverability.

Also, how can we improve aerodynamics of swept wings as opposed to straight ones? Not to mention the Aerodynamic Center is further aft on swept or delta wings, but the drag is also reduced at higher Reynolds numbers. I would be interested in A B testing an aircraft with two different wing types having the same effective surface area and seeing how the flight characteristics compare. I am not sure if the JSB model currently can handle sweep, do you know?

You are aware that there was a big fix so we can now output flight data to CSV format?

Best regards,
Uriah
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bomber

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Re: Weight and Ballance
« Reply #8 on: March 02, 2015, 11:36:14 am »

All geometry is configurable... so you want to put canards on or have a bi-plane with 2 wings one about above the other... then it's there for ya.

Incidence, sweep and dyhedral for wings, h-stabs plus wing flex are all included

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Uriah

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Re: Weight and Ballance
« Reply #9 on: March 02, 2015, 11:55:18 am »

It sounds incredible! Excellent work Simon.

So we start with the simplest possible aircraft, Wright Brothers for example, and once we have that where we want add complexity over time.

What needs to be done? What areas do you need help in?

Regards,
Uriah
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bomber

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Re: Weight and Ballance
« Reply #10 on: March 02, 2015, 01:25:54 pm »

The Wright Flyer was a suggestion because it taps into a market, ie those that go visit the museum.

We need a model importing, I'd suggest it to could be used as a step by step tutorial.

I'd like to take this forum through the process of the front end,  step by step.

Simon.
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bomber

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Re: Weight and Ballance
« Reply #11 on: March 02, 2015, 03:09:47 pm »

I'd not however suggest the Wright Flyer as a tutorial for the few reason that it's not typical of the majority of single wing planes... it's a bi-plane, has two rudders and uses wing warping.....

It's not really going to help lead people into flight modelling their plane... I'll do it, on the side  because i think is should be done, but frankly I'd like an easier life.

I'd suggest another glider...
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HiFlyer

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Re: Weight and Ballance
« Reply #12 on: March 02, 2015, 03:34:57 pm »

The Wright Flyer already available for Flightgear is not suitable for adaptation?
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bomber

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Re: Weight and Ballance
« Reply #13 on: March 02, 2015, 03:42:26 pm »

I don't actually subscribe to using work created for another sim, the politics can be mind blowing.
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Uriah

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Re: Weight and Ballance
« Reply #14 on: March 02, 2015, 10:01:52 pm »

You're right, albeit one of the first, the Wright Flyer is an unconventional configuration.

I really feel that we at least need to do something powered. Single engine, traditional airplane configuration, but at least powered. This is because in a glider it really limits our flight testing envelope, considering the angle of attack cannot exceed a small margin without stalling and the glide slope has a maximum far below what we need for level flight, which I consider the minimum benchmark for comparing flight characteristics and performance. Also, consuming fuel during level flight will lead to improved flight characteristics due to less wing loading, so we can run a test with the autopilot set to hold roll and pitch, and output the CSV data to analyze the changes over time as the mass decreases. The ASK-13 is an excellent glider, and really your proof of concept for the system. I don't see a reason to do another glider as an example for people to work from, as not many people are going to build gliders honestly, there is only the ASK-13 that I know of for Outerra and about 40 jet aircraft. It would be far more helpful if we could dial in a powered aircraft FDM, however simple it is.

I spent about two hours flying the ASK-13, and have to say I dismissed it far too fast before. It is a powerful proof of concept. I would defend it against any criticism on the basis that it is already superior to any FDM JSBSim has to offer in their library. The post stall handling characteristics are exceptionally poor, and by that I mean excellent! If you go into a dive and pull back up, the velocity actually decreases due to increased drag like it is supposed to. Also the stability near stall speed feels far more realistic than any of the other aircraft.

I looked through your FDM and I can write a program to generate those .xml files automatically from an input configuration file. We could include parameters to define as complex of geometry as necessary, but initially it could be simple and expanded in scope over time.

There is also another option, we develop a modular back-end to adapt for changes in the front end? This way, T2 would have the maximum number of sections for a wing, and you would define 'n' number of sections in T1, and T2 would handle 0 - n sections as necessary.

Best regards,
Uriah
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