Consider a plane that has, with respect to a datum line, – 3 degree thrustline, +2 degrees wing incidence, and +1 degree stab incidence.

Now consider the same plane, same datum line, but we change all the angles. Now it has 0 degree thrustline, +5 degrees wing incidence, and +4 degrees stab incidence.

The last example actually describes the S.E.5a (in the picture – note the stab is shown at 0 degrees, but it was adjustable, and rigging diagrams had it set to +4).

These two planes have exactly the same downthrust, not with respect to the datum line, but in actual flight. If we don’t agree on the simple geometry of this comparison, there is nothing more I can say.

So let's take this same S.E.5a and change the angles.  Set the engine to -5 degrees, wing to 0, stab to -1.  Should fly the same, right?  Except the fuselage will move through the air at a different angle.  <div>
</div><div>Anyone looking at the plane after changing the angles would immediately say it has downthrust, even though all the relative angles are exactly the same as they are in the picture above.  Of course, in the picture above, the plane doesn't look like it has downthrust to most people.</div><div>
</div><div>Now here's the take home message:  A scale modeler who is using a lot of down trim to keep his plane from climbing too much might be tempted, or advised, to reduce the wing incidence.  But if he did that, he would reduce the downthrust, unless he changed that too.  You could build an S.E.5a with 0 wing incidence, but then it wouldn't have any downthrust.  Even worse, it wouldn't look like an S.E.5a anymore.  Wing incidence is a very noticeable characteristic.  Why bother building a scale model if you're going to undo the effect by changing the wing incidence?</div><div>
</div><div>With reduced wing incidence the plane would indeed use less down trim, but that's not the real problem.  Who cares if you are carrying down trim?  The real problem is the difference between trim at low throttle and high throttle.  </div><div>
</div><div>Of course full scale airplanes have this "problem" too.  That's why pilots are taught that the throttle, not the elevator, is to control climb and descent.  But many people expect their scale models to fly like their sport models.  That's the real source of the problem.</div>

Hello everyone this is my first post.

I've been a bodyman for a good long time. We use a Datum line when repairing frames unibody and such. It is a imaginary line usedfor measurments, this can be used for repair or manufacturing. For instance locating the lower controll arm mounting bolts. Also used for specific heights. Now change controll arms for landing gear strut mounting bolts, turbo prop mounts, wing spars and ribs while loaded or unloaded,etc. you get the idea.

Yes, the planes are rigged identical.Rigging theSE5 at(-3,+2,+1), or (0,+5,+4) or even (-4,+1, 0) is doing the same thing to the plane. The only difference is that the Datum Line that the numbers are referenced to has inclined in angle. The airplane doesn't care. But notice that in all of the examples that the angle between the prop thrustline and stab is 4 degrees. That number will determine the dynamic balance of the fuselage regardless of where the wing is set.</p>

An example. If we take a 10 pound iron bar and tie a stringat the exact center and hang it by the other end of the stringto the ceiling we can closely model an airplane fuselage. The string tied to the ceiling represents the wings lift. If you lightly touch one end of the bar it will rotate in pitch about the center where the string is attached. This is exactly what an airplane fuselage does in flight. The only difference is that the engine and tail group are different weights so the "string" has to be tied to the fuselage with a forward bias to accomodate the heavy motor. Visualizing this heavy bar on a string idea is critical to understanding dynamic balance.</p>

In your SE5 example (-4, +1, 0) the wing (+1) is just along for the ride. The number that the plane cares about is (-4, ,0). If we make the runway on the surface of the earth our Datum Line thefuselage pitchin level flight will be a balance between the downward force at the nose of the plane and thelift force at the tail (could be + or -). At 3/4 throttle, level flight might be (-2, ,+2) but full throttlethe plane now flies at (-5, ,-1). The -4 degree difference between thrust and stab of your SE5 waschosen based onengine powerand stab liftso that the plane would pitch forward just enough to maintain level flight as airspeed increased.</p>

So now if we look at the SE5 (-4,+1, 0) we can see that changing it to (-4, 0, 0) doesn't decrease the downthrust 1 degree, it decreases the wing incidence by 1 degree at all airspeeds because the difference between thrust angle and stab is still 4 degrees. Alternatively, changing it to (-3, +1, 0) is a 1 degree reduction in downthrust because the difference between thrust and stab is now 3 degrees. The reason for this is that the Y component of the thrust is reduced but the tails ability to generate lift at different angles of attack has not been.

PS: Welcome to RCU1texasgolfer. I think you'll like it here.</p>

Indeed full scale airplanes are rigged to climb at full throttle so the pilot doesn't have to pull the stick back for 20 minutes while climbing to cruise altitude. Our models absorb a considerable portion of out attention just keeping them within visual range. Thats why they are rigged to be altitude neutral with throttle setting. While your comments on full scale craftareinteresting Buzzard it just confuses this discussion. Lets try to use R/C examples ok?

Let'sdesign a very simple .60 size low wing sport plane. It will weigh 7 pounds, with 60" span and 11" constant chord. I will make the stab parallel with the fuselage angle for simplicity. It will use a NACA0012 airfoil. I want it to be altitude neutral at both half throttle (V = 50 mph or 73 fps) and full throttle (V=80 mph or 117 fps). How should I rig it?

Weight = Lift (full throttle)

mg =( Cl ) A (r) V^2 (1/2)

Solving for Cl at full throttle, Cl =2mg/rAv^2 = 2(7)32.2/(.0745)4.58(117)^2 = 0.097

Looking this up on a lift chart for NACA0012 we discover a Cl of 0.1 corresponds with an angle of attackequal to 1 degree.
We now know that if we want the fuse straight at full throttle the rigging needs to be ( ,+1, 0). We still don't know the thrust angle we need so it is left blank for now. We could easily tune the plane at this point through trial and error by simply adding a degree of downthrust at a time until it flies level but we can still get closer before we have to get our hands dirty.

Weight = Lift right? so, Lift (half) = Lift (full) also. Writing the lift term for each throttle and speed setting we find that all the constant terms drop out and leave us with a ratio:

Cl v^2 = Cl v^2 so, Cl(half) = .097 (117^2/73^2) = 0.249

Again, looking this up on the NACA0012 chart we see that a Cl of 0.25 corresponds with 2.5 degrees angle of attack. So we're only talking about 1.5 degrees difference in the angle of the fuselage between half throttle and full throttle! We could do a free body diagram and determine the lift generated by the stab, thrust relative to the throttle setting etc but its probably easier to just go the field and start at (-2, +1, 0) and change the first number up or down until it maintains altitude at full throttle.

CrateCruncher, Your last post seems to run away with this thread.  My question has been sufficiently answered, so have at 'er. 

Interesting how different people view the use of referrence lines</p>

On the latest superlight electric aerobats (rc types of 400 sq inches ) the entire model is simply laid out 0-0-0</p>

why?</p>

because amodel this size may weigh -ready to fly - 6 ounces (not a misprint) and have thrust of 18 ounces (not a misprint.)</p>

with these relationships ,the angles of attackneeded are extremely small for even fairly sharp maneuveing</p>

fo r th wild n wooly flailing</p>

the model can actually turn in it's own length with litle or no noticable loss of altitude</p>

We can't post videos here (thank goodness) but there are a number of videos of models which do this very easily and smoothly.</p>

http://slowflyworld.de/sfw-yak-54.WMV
here is one which is quite revealing.</p>

Tho this type flying may seem preposterous to some - the technical advances which have made it possible are very interesting
Better yet , by adapting the methods learned , many new RPV (pilotless craft) can do things totally impossible with piloted vehicles
In models we have adaptedeight to power and wingloadings, totallyremoved fromthose when we first flew RC in1970
much of the data considered necessary then , is simply not needed for this apprach .stalling-,
banking required for turns- take off minimum speeds-, etc., are not relevant
the model flies at ANY speed because power (thrust) and lift generated by all surfaces of the model can be played with at will.
Tho it aint relevant - I ambuilding aNeuport at the moment
just because I like to play withdifferent types of flying
The Neuport won't be expected to do any of the tricks the YAK does in the video..</p>