Nose drops when you use ailerons too. For a bunch of reasons...our planes tend to have roll rates that, in a full scale aircraft, would be downright lethal, you compensate without realizing it after years of experience, etc...it's just not as obvious or dramatic.

Specifically, however, it's really nothing more than a function of drag, and the airplane doing what it can to offset the effects. Consider this:

The key to EVERYTHING an airplane does (I'm paraphrasing Wolfgang Langewiesche here) is understanding that at any given angle of attack the airplane will fly at a constant airspeed of X....regardless of power (we'll come back to that in a minute).

Apply rudder, change the drag, airplane tries to re-find the SAME AIRSPEED because it seeks to maintain the SAME ANGLE OF ATTACK (all other things being equal).

There's only one way it can do this...point the nose down.

Now, our problem here is a couple of things. First, the drag isn't equal along the sides of the airplane. That is to say, you roll with ailerons, and for most practical purposes (this isn't technically accurate, but it'll work for this conversation) you've applied the same increased drag to each side as one aileron deflects up, the other down. As such, the application of elevator (and thus the change in the angle of attack of the aircraft) is "consistent" along the length of the aircraft. (Again...something of a gross oversimplification, but it'll work for now).

With rudder, the induced drag is greater on the side the elevator was deflected to. Here's where the real "difference" is from the airplane's point of view. Since it yawed its nose, say, left in this case, it has changed not only the angle of attack of the wings, but also changed THE AMOUNT OF AIR THEY'RE SEEING. In a very real sense, you just gave the airplane one wing that's smaller than the other.

The only option the aircraft has, in its constant, unchangeable, and inarguable effort to ALWAYS maintain its current AoA/Airspeed is to make life easier for itself...point the nose down.

If you REALLY want to "prove" this to yourself...that it is ENTIRELY a function of the aircraft trying to maintain AoA and thus airspeed....try this. When you're out flying rudder/elevator only, give the aircraft a couple clicks of throttle in the turn. Watch the "nose drop" flatten out considerably.

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Now for the boring "more on this later" stuff *heh* This will seem off topic, but really it isn't...it's a long winded pet peeve that directly applies to what airplanes do when you change their attitude, and thus...it applies to how and why the rudder behaves in certain ways. The reader is cautioned that if he/she does not wish to spend the next 5-6 hours reading boring drivel, he/she should probably move on to the next thread

At risk of starting another of RCU's famous "You're wrong, I'm right aerodynamics arguments" the whole "at any given angle of attack the airplane will fly at a constant airspeed of X" has been, for me at least, THE single biggest key to understanding why an airplane does what it does in ANY situation. My maneuvers got cleaner and smoother in full scale aircraft, my full scale landings improved dramatically, my precision RC flying took huge strides, and I even figured out how to fly a heli...ALL nearly "overnight", simply by grasping this.

First, as I've done in several threads before, let me HIGHLY recommend [link=http://www.amazon.com/Stick-Rudder-Explanation-Art-Flying/dp/0070362408]Stick and Rudder: An Explanation of the Art of Flying by Wolfgang Langewiesche [/link]. It will, imo, be the best $17 you EVER spend in aviation...scale, rc, or otherwise. Pretty much everything I'm about to say comes directly from that book.

You've probably heard people say that, when landing (RC or scale), you use the THROTTLE to control altitude, not airspeed, and use the ELEVATOR to control airspeed, not altitude. That is a true statement.

Now..the airplane doesn't KNOW it's landing. It's not self aware, it doesn't say "oh, look, here comes the runway, I must be getting ready to land". So, it's not as if the airplane suddenly becomes this "throttle=altitude, elevator=airspeed" machine. If that correlation is true for landing, it must be, by definition, true for EVERY phase of flight.

And it is.

Whether we accept to believe it or not, the LEFT stick...that is, the THROTTLE...is our up/down stick. The RIGHT stick...the elevator...is our speed up/slow down stick.

Yeah, I know...bull-****. I pull back on the right stick, airplane goes up. I push the left stick forward, airplane goes faster.

Well..yeah...for a brief time. Of course, the problem is that, except for those guys who follow their airplane from the back of a truck...a "brief time" is all we RC guys have. Before we ever have a chance to see the truth of the matter, we're at the other end of the field, and making a turn.

If you have the chance, however, try the following in a scale bird:

Establish straight and level flight at airspeed X. Trim for hands off.
Now...increase power slightly.
First, the airplane will speed up a bit...
Then, for some 'unknown" reason, it starts slowing down....and climbing.
This sequence will repeat a couple times...climb, stop climbing speed up...climb again...each time the "change" becoming less dramatic.
Within a few moments (say, 15-20 seconds or so in most small trainer-type airplanes)...the airplane will have settled down. And what will it be doing?? Flying at PRECISELY the same airspeed as it was before...in a very gentle climb.

End result : You increased POWER, and airspeed stayed the same. ALTITUDE is changing.

Try the same experiment, only instead of changing power, change trim a bit. Bump in a notch or two of up trim.
Plane goes up a bit...slows down...up a bit...slows down...finally settles down. What will it be doing? Flying STRAIGHT AND LEVEL at a LOWER airspeed.

End result : You applied UP ELEVATOR, and the airspeed DROPPED, and the altitude is CONSTANT.

Elevator is a fast/slow control, throttle is up/down.

Now, clearly, we can sit here and go round and round about this "engineering stuff", and we all know that, for PRACTICAL purposes at the RC airfield, if I want my plane to go up, I haul back on that elevator, and up it goes. Quite simply, we just don't have the ROOM or TIME to see the "oscillations" happen that the airplane will go through to regain equilibrium.

But what can help us is knowing that it will do it. If we understand that at any given angle of attack, the airplane will maintain a constant airspeed, we can not HELP but understand that as we change one thing or another, the airplane will ALWAYS try to get to the airspeed it needs to fly at its current AoA. it may take it a few tries, but it'll get there.

When does this really matter? Simple:

What is the SINGLE biggest cause of RC crashes, by far? Stalls. They may have been brought on by any number of factors, but bottom line, the airplane stops flying because we got its angle of attack too high, and it falls like a rock.

And where do we see these, ESPECIALLY with new pilots? landings and takeoffs. Why? Simple:

We taught them to stall their airplanes.

We said, with all confidence, that this right stick here is the elevator. It makes the plane go up and down. Pull back, and it'll go up. Push forward, and it'll go down. And then we 'proved" this to them during flight training.

So new guy goes out, flies around, is coming in for a landing...and the airplane starts to come down "too fast". What does he do? Every "instinct" he has spent the past few months learning and demonstrating to his instructors satisfaction says "Airplane must go up...PULL BACK!"...and it "inexplicably" rolls on its side, falls to the ground, and another thread appears in the Beginners section.

Guess what...if we'd taught him (or, for that matter, could convince ourselves) that elevator is speed control, and throttle is altitude control...his INSTINCT (and ours) would be to do EXACTLY what we'll all tell him in his "I crashed my trainer thread" ANYWAY...use power to control altitude, elevator to control airspeed, when on final approach.

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So, anyway, enough of my rant. Buy Langewiesche's book, and go take your favorite trainer to the field and do some slow flight "experiments" like the ones I mentioned above. If that book...and the single realization contained therein, isn't the most important lesson you've ever learned in RC, come back and let me know, and I'll...oh...I dunno...I'll eat my copy, and post a YouTube video of me doing it.