Free Flight, Free Speech

Posted: **Sun Jul 12, 2020 5:58 pm**

So you're flying in the area where ground effect takes place... usually defined through a table such as..

        <function name="aero/function/kCLge">
            <description>Change_in_lift_due_to_ground_effect</description>
            <table>
                <independentVar>aero/h_b-mac-ft</independentVar>
                <tableData>
                      0.0000    1.2030
                      0.1000    1.1270
                      0.1500    1.0900
                      0.2000    1.0730
                      0.3000    1.0460
                      0.4000    1.0280
                      0.5000    1.0190
                      0.6000    1.0130
                      0.7000    1.0080
                      0.8000    1.0060
                      0.9000    1.0030
                      1.0000    1.0020
                      1.1000    1.0000
                  </tableData>
            </table>
        </function>

So that's wingspan above ground level..

Here's something to think about, and we're only thinking about ground effect on LIFT.... but if you're flying at a positive AoA generating LIFT, ground effect has an effect... But if you're flying at a negative angle of AoA, sufficiently so as to not be generating lift but instead SINK (if that's the correct term) ground effect shouldn't have an effect.

Granted if you do do this within one wingspan of the ground, you're not going to notice this effect for very long.

Simon

Posted: **Mon Jul 13, 2020 12:47 pm**

So lets look at ground effect on drag...

Statement : Ground effect only influences the wings induced drag by interrupting with the wing vortex.

https://en.wikipedia.org/wiki/Ground_effect_(aerodynamics)

this is the ground effect drag influence table..

Code: Select all

        <function name="aero/function/kCDge">
            <description>Change_in_drag_due_to_ground_effect</description>
            <table>
                <independentVar>aero/h_b-mac-ft</independentVar>
                <tableData>
                      0.0000    0.4800
                      0.1000    0.5150
                      0.1500    0.6290
                      0.2000    0.7090
                      0.3000    0.8150
                      0.4000    0.8820
                      0.5000    0.9280
                      0.6000    0.9620
                      0.7000    0.9880
                      0.8000    1.0000
                      0.9000    1.0000
                      1.0000    1.0000
                      1.1000    1.0000
                  </tableData>
            </table>
        </function>

Got no problems with this... as you get closer to the ground the drag gets less...

But this is where it's applied...

Code: Select all

            <function name="aero/coefficient/CDwbh">
                <description>Drag_due_to_alpha</description>
                <product>
                    <property>aero/qbar-psf</property>
                    <property>metrics/Sw-sqft</property>
                    <property>aero/function/kCDge</property>
                    <table>
                        <independentVar lookup="row">aero/alpha-rad</independentVar>
                        <independentVar lookup="column">fcs/flap-pos-deg</independentVar>
                        <tableData>
                                         0        10       20       30
                             -0.0873   0.0041   0        0.0005    0.0014
                             -0.0698   0.0013   0.0004   0.0025    0.0041
                             -0.0524   0.0001   0.0023   0.0059    0.0084
                             -0.0349   0.0003   0.0057   0.0108    0.0141
                             -0.0175   0.002    0.0105   0.0172    0.0212
                             0.0000    0.0052   0.0168   0.0251    0.0299
                             0.0175    0.0099   0.0248   0.0346    0.0402
                             0.0349    0.0162   0.0342   0.0457    0.0521
                             0.0524    0.024    0.0452   0.0583    0.0655
                             0.0698    0.0334   0.0577   0.0724    0.0804
                             0.0873    0.0442   0.0718   0.0881    0.0968
                             0.1047    0.0566   0.0874   0.1053    0.1148
                             0.1222    0.0706   0.1045   0.124     0.1343
                             0.1396    0.086    0.1232   0.1442    0.1554
                             0.1571    0.0962   0.1353   0.1573    0.169
                             0.1745    0.1069   0.1479   0.1708    0.183
                             0.1920    0.118    0.161    0.1849    0.1975
                             0.2094    0.1298   0.1746   0.1995    0.2126
                             0.2269    0.1424   0.1892   0.2151    0.2286
                             0.2443    0.1565   0.2054   0.2323    0.2464
                             0.3491    0.2537   0.3298   0.3755    0.3983
                             0.5236    0.45     0.585    0.666     0.7065
                             0.6981    0.7      0.91     1.036     1.099
                             0.8727    1        1.3      1.48      1.57
                             1.0472    1.35     1.755    1.998     2.1195
                             1.2217    1.5      1.95     2.22      2.355
                             1.3963    1.57     2.041    2.3236    2.4649
                             1.5710    1.6      2.08     2.368     2.512
                          </tableData>
                    </table>
                    <property>gear/drag-coef[1]</property>
                    <property>gear/drag-coef[2]</property>
                    <property>gear/drag-coef[3]</property>
                    <property>gear/drag-coef[4]</property>
                    <property>gear/drag-coef[5]</property>
                    <property>gear/drag-coef[6]</property> <!-- Amphibious gear extended -->
                </product>
            </function>

Now there are a couple of issues with this...

At 90 degrees the wing is basically in flat plate drag mode... this value should be 1.28... not 1.6 or 2.08 or 2.368 or 2.512
https://www.grc.nasa.gov/WWW/K-12/airplane/shaped.html

Also this isn't a table of just induced drag.. it's shape and skin and induced drag combined. Yet ground effect which only effects induced drag is being applied to all..

this is wrong

Simon

Posted: **Mon Jul 13, 2020 4:04 pm**

bomber wrote:So you're flying in the area where ground effect takes place... usually defined through a table such as..

Here's something to think about, and we're only thinking about ground effect on LIFT.... but if you're flying at a positive AoA generating LIFT, ground effect has an effect... But if you're flying at a negative angle of AoA, sufficiently so as to not be generating lift but instead SINK (if that's the correct term) ground effect shouldn't have an effect.

Granted if you do do this within one wingspan of the ground, you're not going to notice this effect for very long.

Ground effect is very airframe specific - so the 'generic' tables you refer to should not be used when accuracy is important.

Lift is usually a signed quantity - sink isn't a term that I've ever heard used.

As you say it is highly unlikely that any effects caused by negative AoA would have time to take effect before the inevitable impact.

For the F-15 I'm using the following data; that I put together from NASA TM-104278, extract here https://1drv.ms/b/s!AvbsHHF00E7FhuE8RTj ... w?e=muIdbH full document here: https://ntrs.nasa.gov/search.jsp?R=19950026588

p228

which I converted to the following tables

This is the ground effect data that I have computed with OpenVSP for a Douglas A-4 (spreadsheet available on request), but be aware that this model is still in early development.

==============

bomber wrote:Got no problems with this... as you get closer to the ground the drag gets less...

At 90 degrees the wing is basically in flat plate drag mode... this value should be 1.28... not 1.6 or 2.08 or 2.368 or 2.512
https://www.grc.nasa.gov/WWW/K-12/airplane/shaped.html

I agree that the way kCDge is applied seems wrong; but without references to check and verify I simply wouldn't use this data as it could be wrong in all sorts of different ways.

Also remember that hb-mac-feet isn't actually in feet but instead (as I understand it) the height of the chord line (MAC) above ground expressed as a ratio of wingspan. (Height over B).

A flat plate normalized by its surface area would be around 1.2; however the Cd is for the entire aircraft in which case using the wing surface area to normalize drag force is going to result in a higher value than for the wings alone.

Posted: **Mon Jul 13, 2020 5:45 pm**

Richard wrote:A flat plate normalized by its surface area would be around 1.2; however the Cd is for the entire aircraft in which case using the wing surface area to normalize drag force is going to result in a higher value than for the wings alone.

And that''s the point I'm making.... you can't apply the ground effect to the whole plane Cd.... just to Cdi

Simon

Posted: **Mon Jul 13, 2020 6:18 pm**

what is the CDi if you dont mind clarifying?

Posted: **Mon Jul 13, 2020 6:51 pm**

don't mind....

Co-efficient of incidence drag

https://www.grc.nasa.gov/WWW/K-12/airplane/induced.html

Posted: **Mon Jul 13, 2020 7:15 pm**

Richard wrote:
Lift is usually a signed quantity - sink isn't a term that I've ever heard used.

I second Richard.
(See? It can happen!)

I undestand lift to be a vectorial quantity. The direction being specified by the sign on the vector. a "sink" is really still correctly termed, lift. I think.

Posted: **Mon Jul 13, 2020 7:18 pm**

I just don't like writing negative lift.... especially as it's a positive force.

Posted: **Mon Jul 13, 2020 7:22 pm**

ok .
be the force with you :twisted:

The magnitude of all force is the absolute value. The signs reflect directions.

Posted: **Mon Jul 13, 2020 7:23 pm**

yes.... with negative being up

Free Flight, Free Speech

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