PRINCIPLES OF STABILITY AND PERFORMANCE

            Hi-Speed vs. Low-Speed Forces by Ron St. Jean

 

The basic idea is that some adjustments or offsets are mainly effective at low speeds, while others have a greater effect at high speeds. When we understand which is which, and the interaction between forces, it not only allows us to explain why we may be having a problem in flight, but this knowledge permits us to take effective corrective measures. Adjustments and offsets may be categorized into the two types. This list is not necessarily all-inclusive:

 High Speed - rudder offset, wing wash in/out, decalage (relative inci­dence) and wing cocking (changes wash-in/out).

 Low Speed - stab tilt, downthrust, side thrust, spiral prop wash effect, CG location, weighted wing tip, drag flap and engine torque.

A few examples should serve to provide the understanding needed to utilize this principle of high speed and low speed adjustments. Although they are probably most useful in free flight power events because of the vast speed difference between climb and glide speeds, they should be of equal use in HLG and some use in rubber events, as well as in RC. The following il­lustrations generally assume a power model:

 1. Condition: Model tends to dive when first launched, but noses up as speed is gained.

 Explanation: At low air speeds either downthrust or a nose heavy condition (low-speed) will nose the model down. Incidence, being high-speed, noses the model up, overcoming the low-speed factor as speed is gained.

 Uses of Condition: The combination of downthrust and incidence produces a stabilizing force in the longitudinal mode. Should an under powered model tend to power stall (e.g. a 1 /2 A Texaco) the downthrust would predominate at low speeds approaching the stall, lowering the nose before the stall is encountered. The nose lowering would then cause speed to increase, and eventually the incidence would take over. Oscillations would eventually be replaced by a condi­tion of constant speed and angle of climb. Once equilibrium is established, power changes will mainly change climb angle.

In the VTO era, vertical takeoffs were facilitated by downthrust working against incidence. With 10° down and normal incidence on the moderately powered models of the era, models could be launched straight up against a moderate breeze without looping in. In those days, the model had to stand on three pegs for takeoff. The low speed effect of the downthrust would cause the nose to pull into the wind while speed was increasing.

 Incidence and CG location are balanced for each airplane to produce the same kind of longitudinal glide stability dis­cussed above in regards to a low powered model in the climb mode. Here, however, a forward CG is the force opposing incidence so as to establish equilibrium. But as the CG is moved aft and the incidence simultaneously reduced to produce a slower glide and less drag due to incidence, a limit is reached where incidence is no longer effective. Stability is then totally lost, and the model "zeroes out." For each design, then, there is an optimum CG location. At such point, there will be sufficient decalage to provide minimum stability and maximum performance. Moving the CG farther aft of this point will require decalage reductions in order to maintain a stall-less glide, and thus increase performance at the sacrifice of stability. Conversely, a forward CG shift will require more incidence to compensate, and stability will be increased, but at a sacrifice of performance. Models that are too nose heavy need excess incidence in order to glide, and therefore will loop under power.

 2. Condition: Model tends to go strongly to the right when launched (perhaps crashing), but assumes normal climb turn
after picking up speed.

 Explanation: Two low-speed factors can cause a model to bank sharply to the right upon being launched-right thrust and the effects of spiral prop wash. In the case of the latter, the rotating prop wash impinges upon forward fuselage areas creating both roll and yaw forces to the right. It continues, though somewhat diminished, to similarly impinge upon the fin, creating a compensating left yaw force.

 Uses of Condition: In some cases a model will loop before going into a normal climb turn. When this happens side thrust may be added to help establish the turn before the model loops. Or the design may be changed to utilize spiral prop wash and thus avoid side thrust:

Assuming single engine tractor models with normal prop direction of rotation, a stronger right hand tendency may be had by

 (a) deepening the forward part of the fuselage (or raising the pylon),

(b) moving the thrust line toward an extreme position (either very high or very low),

(c) reducing fin area,

(d) moving the fin forward to make its area less effective,

(e) moving part or all of the fin area to the stabilizer tips to free it from prop wash impingement, and

(f) increasing dihedral, which works similarly to decreasing fin area.

Conversely, doing the opposite of any of these will reduce an excessive low-speed right hand tendency.

 One of the major reasons for the success of the Ramrod design in the 50s was its strong natural right-hand tendency. It was so strong that left rudder and left wing warp (right wing washed in) was required to compensate and cause a normal climb turn to the right. This produced what is called top rudder by full-scale pilots. That is, the left rudder helps hold the tail down in a right turn, thus preventing spiral dives. These left offsets then aided longitudinal stability by forcing the model into a tight left glide circle at the bottom of a stall recovery. The high speed encountered at the bottom of the stall makes rudder and wing offsets strong enough to result in about a 45° bank to the left. This prevents further stalls. At the same time, the stability provided by ample dihedral and incidence prevented spiral dives in the glide.

 3. Condition: Glide circle is nearly independent of rudder or wing offsets.

 Explanation: A low-speed factor is causing glide circle.

 Use of Condition: Prior to the discovery of low-speed adjustments with which to control glide circle, adjusting a power model often required much dangerous trial and error if there was no natural glide circle. One method of achieving the needed circle was to add rudder offset while compensat­ing with opposite side thrust. Quite often the two adjust­ments were not balanced, and a crash resulted.

Today we can control power and glide circles almost independently of each other. Rudder is typically used for the power pattern, while one of several low-speed adjustments may be used for glide, stabilizer tilt being the favorite. But drag flaps and weighted wing tips will work just as well.

 4. Condition: A typical full scale light plane requires right rudder offsets during climb and left rudder offsets in the glide, in order to maintain straight flight.

 Explanation: Torque is an in­significant factor in most FF models because it is over­powered by the effects of spiral prop wash.But because of vast airframe differ­ences, most full scale air-craft are especially suscep­tible to the effects of engine torque.

Some manufacturers have compensated for this low-speed torque factor by introducing fin offset, a high-speed factor. But it is normally only in the cruise mode that the two are balanced to produce hands-off straight flight. During climb, speed is reduced, but torque is increased due to added RPM. In this condition the torque easily overpowers the small fin offset, requiring right rudder to compensate. In a similar manner, torque disappears in the glide, the fin offset takes over, and left rudder is required.

 Problem Solution: If right thrust were used to compensate for torque instead of right fin offset, the effect of one low-speed factor could be balanced with another. In addition, the effect of the thrust offset would grow and diminish with the torque, as both are functions of RPM. The Beech Bonanza for example has had right and down thrust since 1964, when the horsepower was increased from 225 to 285.  

 Ron St. Jean, P.O. Box 149, Yerrington, NV 89447.