Trimming F1D's
by Bernie Hunt
The new 55cm F1Ds fly with a much wider range of power than the old 65cm class. This can cause some problems in trimming so I have attempted to set out my own understandings.
Surprisingly, a normal rigid indoor model (like a braced F1D), properly trimmed, flies at a near constant speed throughout the flight. The relative incidence of the wing and tail are fixed and chosen so as to give the minimum power requirement at cruise and hence the best duration. If a model were to fly faster for any reason (high power or launched flat),then the wing will generate more lift and the model will raise its nose and slow down (component of weight offsets thrust) until it reaches equilibrium. So at high power, a model climbs quickly but its flight speed is no more than at cruise (in fact it should be slightly less). You should always try and launch a model at the correct (ie equilibrium) attitude and speed. If it does set off too flat or too fast then structural distortion (aeroelasticity) may prevent the return to a stable pattern.
You can get indoor models to fly faster than their cruise speed but you need one or more of: 1. downthrust to counteract the extra lift 2. a variable incidence tail (via stick bow) to reduce the incidence and lift 3. a spiral climb. Successful models usually use 2 and 3. Really fast flying indoor models are probably not a good thing because aeroelasticity can cause fatal outcomes (dive in, roll or deep stall) and also our models are very draggy.
A spiral climb is the classic way that a rigid model can be trimmed successfully for fast climb with a big range of power. Free flight power (gas) models (those without VIT) use this method. The model is allowed to loop on high power but it is rolled at the same time so it stays the right way up and flying stably. How is this achieved in practice for indoor? I am not exactly sure of all the factors for braced models but for my unbraced models PLENTY of left wing washin and a LITTLE stick bow are the key. Fortunately, on unbraced models (like floppy EZBs and unbraced F1Ds), the stick twist caused by the high initial motor torque does naturally induce a lot of washin at the critical launch phase . A stick that is too stiff in torsion between the wing posts definitely harms the climb pattern. Some of the old braced 65 cm F1Ds were able to achieve lovely spiral climbs but braced 55cm F1Ds and 35cms are tricky. It must be something to do with the ultra low aspect ratio of these classes.
Now onto props. Props are a bad thing for model stability and the bigger the diameter and pitch the greater the problems they create. One textbook I read said that a prop behaves like a destabiliser equivalent to half its area. The ultra high pitches of VP props are particularly bad because they are often partially stalled and distorted. So for the easiest trimming model, choose the smallest diameter prop with a very conservative P/D ratio. A nice rigid 16" dia x 26" fixed pitch prop is a dream to fly but it will cost you time in a low site against VP props.
In the light of the above here are the my suggested steps in trimming an unbraced model like 55cm F1d:
1. Assemble the model with A. ½" tail tilt, B, a little fin offset and C. 0.2" negative on the tail. Start with no wing warps and 2-3 deg sidethrust and a touch of washin on the left tail panel.
2. Fly the model on low power (0.15 oz in)and adjust A, B and C until you get a nose up attitude without stalls and a 30-40' circle - do not exceed 1" tail tilt. If the model has inadequate or excessive cruise wing washin the alter the warps now - the ideal is about 0.25".
3. Apply more power (0.25 oz in) and check that the circle does not open up - you may need more washin or sidethrust. Recheck and adjust the trim as seen in the calmer air higher up.
4. Now fly on full power (0.4-0.5 oz in ) on a 1/3 or ½ motor + ballast, being careful to launch at the climb angle (about 30 deg). If the model stalls whilst turning OK to the left you need to reduce the stick bracing tension to get more stick bow. Achieve this by shortening the bracing post height 0.030" at a time - you will need to increase the negative on the tail to compensate. Obviously, if the model goes flat you need to increase the bracing posts height (add a little block on top). If the elevation seems OK but the model does not execute a spiral climb then you will need to add more washin and go back to 2. If you just cannot get good spiral climb try a smaller dia prop.
5. Finally, fine tune the trim of the model by altering elevation, wing warps and stick bow till you get the trim perfect both on the way up and the way down - you always have try too much of each adjustment to know where the optimum lies.
