Ok, I am not familiar with the FMA FS8 copilot and had started to answer your post based on what my experience has been with antennas. Then, I thought that, perhaps in my ignorance of the FMA copilot, there was something I had missed. So, I wandered over to the FMA site, pulled down the manual and read a bit.
Not sure where the 18 inches comes in and that would definitely detune a radio (talking 72MHZ here), however I use the Revolution base-loaded whip on my Intrepid and the instructions say to measure and cut the receiver antenna wire exactly four inches from the receiver. This is based on the fact that 39 inches (the length of a standard 72MHZ wavelength) is duplicated with four inches of antenna wire and the base-loaded whip (the base-load representing the balance of 1 wavelength). For a quick explanation, the receiver looking at the antenna for a signal coming in must see one wavelength for the best signal to be accepted. Anything more or less and you start to have what is called a "mis-match" in impedance or resistance and the full strength of the signal will not be received and range will either be degraded or lost. This holds true for both reception and transmission although transmission is a bit more critical and, in higher power transmitters at HF frequencies, this can actually generate a dangerous situation.
Bottom line here is that anything less than a 39 inch antenna or a base-loaded compensated antenna for the specific frequency being used will result in some loss of signal and range at the receiver. We all know that there is some trade-off as evidenced by a car radio receiver. Obviously the antenna for an auto radio is not tuned for just one station but for a band of stations, so, depending on the frequency of the station, the antenna will be either more or less receptive. Our radios are single frequency transmitters/receivers, therefore there need not and most certainly should not be any trade-off. Personally, I advocate with my fellow flyers that if, their antenna gets cut, worn or damaged in any way, that it be replaced and for less than $3, that is insurance well worth buying.
Enough said about the dynamics of wavelengths. Bottom line is that anything less than 39 inches or a compensated base-loaded antenna tuned for the specific frequency represents a mis-match and reception will be degraded. Yes, your receiver will still probably receive some signal, however, in our hobby, any signal degradation is unacceptable. I couldn't find the reference to 18 inches, however there must be an alternative explanation. FMA Direct is a well-known, well-respected RC hobby company, so I am at a loss on this one.
Also, interestingly enough, I have flown both Dean's and Revolution's base-loaded whips and, personally I prefer the Revolution. The Revolution instructions give a definitive 4 inches of antenna while the Deans says between 4 and 10 inches (reference the above dissertation on wavelenth propagation). And, yes they do, in electronic terms, "put back the length," however they put back precisely the length they told you to remove, so draw your own conclusions. One other tip I might add is that I strongly recommend that the joint between the receiver and whip be soldered and covered with heat shrink. I was the victim of not soldering that connection and paid rather handsomely for that mistake.
"There are those who have...and, those who will" IRCHA #2117, AMA #70068, Turbine Waiver #105