Effect of landing gear on pitch stability

[deh6]

Does the landing gear have the same effect on pitch stability as the airfoil Cm?

My reasoning is that the drag of the gear makes a pitch-forward moment, and is proportional to velocity squared, and the Cm (pitching moment) of an airfoil (usually negative) is also proportional to velocity squared (for a given wing). Therefore, the gear drag has the same effect of adding some negative Cm to the airfoil. My focus is on flying wings, so even a small amount of pitch forward is important.

Assuming the foregoing is on-track, then is there a way to make an estimate of the drag of a landing gear with wheel?

[JetPlaneFlyer]

I think you are correct... the LG will tend to produce a 'de-stabilising' nose down piching moment. However I'd doubt the effect was something to worry too much about unless you had especially large wheels on very long legs.

The size of the pitching moment would be easy enough to work out using the drag equation: http://i3.photobucket.com/albums/y65/Rapier_HP/be6b583aeb2d71e75a2dd1dfd745f59b.png I'd guess that the Cd of an average wheel would be in the order of 0.3

[vintage1]

Don't neglect the fact that the gear also lowers the CG, which tends to have a beneficial effect on pitch stability.

[nmasters]

The CD of a cylinder is about 9X the CD of a symmetrical airfoil of the same thickness. So to make a rough estimate of the drag of your gear calculate the drag of a fairing 9 times as thick as the actual gear. Better yet put a fairing around it and reduce the drag by a factor of 9

[deh6]

However I'd doubt the effect was something to worry too much about unless you had especially large wheels on very long legs.

Putting numbers to the plane (picture in this thread)--
http://www.rcgroups.com/forums/showthread.php?t=563973&highlight=Lightning+Dart
-- From my calculations I estimate that at 80 mph the gear requires about 12% of the available thrust, and has a pitching moment that is 25% of the pitching moment the twist/elevons must supply to offset the weight of the plane times the distance between the CG and (calculated) AC (and assume a zero or near-zero pitching moment airfoil).

So getting rid of the gear, or fairing nicely (only to be ripped apart upon one of my landings!), only will allow the top speed to increase around 6%. Not a big deal. (Removing the gear would eliminate 5.6 oz out of 92, so that, too is not large.)

I'm not sure what the effect of removing the pitch moment would do, however. On a tailed plane the forward pitching moments are offset by horizontal stab which supplies a down force times the distance to the NP, so the calculating amount of down-lift & associated drag, plus additional lift and drag of the main wing is straight forward. How might one estimate that for a swept wing?

[Work in Progress]

The tailplane is a much more efficient way or providing a given pitch moment than the undercarriage is, even if it's a well faired undercarriage. So while you will get small gains from being able to re-trim the tail (loss of trim drag and a marginally reduced lift requirement from the main wing), it's not going to make anything like as much difference as the loss of the parasite drag from the gear. If you reckon 6% speed improvement from the profile drag improvement, it may be about another 1% from the rest.