You may have noticed that the helicopter guys install their short base-loaded antennas exactly like you describe. I've mounted mine both ways (i.e., pointed up and pointed parallel to the ground) and, based on unscientific range tests, the antenna doesn't seem to care in the airframes I'm using them in. One thing to remember: there are signal nulls (wrt both Tx and Rx RF paths) when there's a "look angle" oriented straight on to (or off of) the antenna.