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flaps and their efficiency

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

Re: flaps and their efficiency

Postby Alant » Fri Mar 10, 2017 3:14 pm

Simon

Yes, at the same airspeed the trimmed incidence will be less with the flaps down. However with flaps down you would normally be flying much slower, and things tend to cancel out.

In an extreme case an aircraft such as Concorde needs ridiculous incidence (i.e. nose up) at slow speeds on approach, even with the flaps down.
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Re: flaps and their efficiency

Postby Bomber » Fri Mar 10, 2017 8:55 pm

Alant wrote in Fri Mar 10, 2017 3:14 pm:Simon
In an extreme case an aircraft such as Concorde needs ridiculous incidence (i.e. nose up) at slow speeds on approach, even with the flaps down.


Alant I understand there's always exceptions to the rule, but we have to be careful that we're not clouding the issue of correct flight dynamic modeling by throwing these into the discussion.

Alant wrote in Fri Mar 10, 2017 3:14 pm:Simon

Yes, at the same airspeed the trimmed incidence will be less with the flaps down. However with flaps down you would normally be flying much slower, and things tend to cancel out.


I agree that things don't happen in isolation... However thanks to the French we have the word decalage and it's effects.... If you're flying slower the moment from the h-stab which ballances out the rotational moment of the wings is less.... meaning the nose drops and the angle of attack is reduced.

But the thing I'd like to draw to yours and others attention is that if in the flight model applying flaps simply raises the blue coefficient line upwards without moving the line to the left instead of the AoA being as you rightly said about -7 degs it ends up instead about 3 degs.. Is this the case, can it be demonstrated that the line moves to the left ?

Regards

Simon
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Re: flaps and their efficiency

Postby Alant » Fri Mar 10, 2017 11:26 pm

It is a matter of being able to interpret simple graphs.

If you want to find the lift for various flap angles at a given incidence you go upwards-downwards from one flap curve to the other.

If you want to find the incidence for various flap angles for a a given lift. you go left-right from one flap curve to the other.

Is it really that complicated?

Alan
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Re: flaps and their efficiency

Postby Bomber » Sat Mar 11, 2017 12:47 pm

Real time flight modelling is not the same as interpreting a simple graph.... if it was Flightgear would be full of perfect JSBsim flight models.
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Re: flaps and their efficiency

Postby Bomber » Sat Mar 11, 2017 1:28 pm

This plane is on final approach....

Image

Question....are it's flaps deployed ? they look it.
I can observe that It's elevators are hardly off centre (stick slightly back), it's descending so it's AoA is probably 2 to 3 degs
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Re: flaps and their efficiency

Postby Thorsten » Sat Mar 11, 2017 1:37 pm

After reading the thread three times, I still can't figure what the controversy is about...

that there has to be a reduction in AoA with the deployment of flaps... That means the nose goes down, it simply has to...


For the posted graph, that's what's going to happen. If the graph for a different plane's flaps looks different, that's not what might happen.

If anyones flight model isn't doing this.... then what conclusion can we only come too ?


That either there's a mistake somewhere, or that flaps for that plane act differently to the posted graph.

This plane is on final approach....


It's YaSim though.... but yes, flaps are deployed to nearly full, sinkrate might be too high though at this point, it's not on a 3-deg glideslope, it was my first landing after not flying the Twin Otter for quite a while, so this was not a textbook approach.
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Re: flaps and their efficiency

Postby Alant » Sat Mar 11, 2017 3:45 pm

With many aircraft their is negligible pitch trim (and hence elevator angle) difference when the flaps are lowered and airspeed reduced. The pilot adjusts speed with the elevator and descent rate with the throttle, adding more flap as he slows down during the various phases of the approach.
I too fail to understand what Simon´s problem is. I think that he is making things much too complicated for himself.

Alan
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Re: flaps and their efficiency

Postby Bomber » Sat Mar 11, 2017 4:04 pm

Gentlemen it's called a discussion on observations about flaps within flightgear...

We CAN turn it into a flamefest, that's really easy.

OK then if it's so simple... either of you two guys able to produce a graph showing flaps plain or other, deployed on a standard configuration plane... ie no concorde or 5th generation fighter.

Simon.
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Re: flaps and their efficiency

Postby Alant » Sat Mar 11, 2017 4:11 pm

No flame war from here.

Datcom does a good job at conventional subsonic aircraft with standard flap configurations and is probably the easiest way to generate representative data - without needing know anything about fluid dynamics.

If you have a specific aircraft in mind please say.
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Re: flaps and their efficiency

Postby Richard » Sat Mar 11, 2017 4:24 pm

Simon is modelling things based on just the aerofoil graphs; so effectively he has to understand the complexities of the airflow past each surface.

Modelling flaps is one of those places where this gets harder; even with simple flaps[1].

When wing flaps are down, say 20 degrees, this is going to affect

- the lift / drag of the wing
- the pitch moment from the wing
- the downwash and therefore the incidence change at the tail.


I suspect to model flaps using the multiple aerofoil approach is going to require another set of tables for downwash due to alpha and flaps; otherwise you're never going to get anything even close to correct.

I'm just making a set of plots for the mrca wing section; to illustrate how I'm attempting to model this in VSPAero - which doesn't have support for flaps, so it can only do either simple flaps or you have to have multiple geometries to model more complex flaps. I've got a wing for 0,20,50 flaps and am generating tables from this.

--------------------
[1] and remember that I have no formal aerodynamic or aeronautic training
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Re: flaps and their efficiency

Postby Richard » Sat Mar 11, 2017 4:37 pm

This my current WIP using VSPAero for the increment in lift due to flaps.

Image

(sorry, the curve for 20 degrees is incomplete because I was too impatient to let it finish)
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Re: flaps and their efficiency

Postby Thorsten » Sat Mar 11, 2017 4:47 pm

With many aircraft their is negligible pitch trim (and hence elevator angle) difference when the flaps are lowered and airspeed reduced.


Yeah, the graph arithmetics done on the lift curve assume the airspeed doesn't change - assuming that deploying flaps generally tends to add drag, that means throttle also needs to be adjusted to keep airspeed for the argument to be valid.

Once that's not true (which is usually not during an approach as I'm trying to slow down) I can't see how the argument can be made without the drag table.

I readily agree that slowing down tends to counter-act the reduction in AoA for constant airspeed - whether the net effect is indeed negligible or not I do not know, I assume it would depend on how much you slow down as well.
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Re: flaps and their efficiency

Postby Bomber » Sat Mar 11, 2017 6:04 pm

Richard I don't know what percentage flap to wing ratio you're using but be aware...

on the Corby Starlet the inner part of the flap is 32.5% going to 35% at the outer part of the flap..

Ailerons go from 34% inner part to 24% outer part
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Re: flaps and their efficiency

Postby wlbragg » Sat Mar 11, 2017 7:37 pm

So how does that match up to the table I am using for the SuperCub that goes to 50deg?
It looks like the original set I hacked are closer to what it should be? What would you need to run your graph for the SuperCub?
Does this mean that the table might need to go up close to 2. at 50deg?
Code: Select all
  product
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            <property>metrics/Sw-sqft</property>
            <property>aero/function/kCLge</property>
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                    10.0000   0.3000
                    20.0000   0.5800
                    30.0000   0.7600
                    40.0000   0.8700
                    50.0000   0.9500
                </tableData-->
                <tableData>
                    0.0000   0.0000
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                    50.0000   0.3875
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Re: flaps and their efficiency

Postby Bomber » Sat Mar 11, 2017 8:10 pm

Well well an interesting question....

The question to ask you is this....

At 17 degs AoA.... if you deploy flaps, with the tables/method youre using does the lift increase or decrease ?
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