1.What are they?

What I call the “hypotwisted” design for propellers has been deployed with excellent results since the late 1920s, least within the sphere of rubber-powered Scale. That it remains esoteric, with helical props standard, must be due to its relatively difficult conceptualization and manufacture, and more choices to make and sub-types to try.

I arrived at “hypotwisting” as a way of increasing propeller-efficiency, by my own lights in ignorance of previous efforts. My carefully performed tests confirmed the validity of my, atypical, forms of the design in the context of “spatula” and “fan” planforms, fine airfoils and rubber-powered Scale and quasi-Scale applications.  They have also added evidence of the efficiency-increasing effect of tip-ward P/D-washin in-general. A tethered car, initially deployed for greatest accuracy in measuring Work, traveled 7%-farther on-average, and my flying models stayed aloft about 10%-longer, than with strictly comparable helical props. – What flew for 110 seconds now broke 2 minutes. Not bad.

The standard, helical, propeller’s pitch and P/D do not change from one blade-station to another. Departing from that, the “hypotwisted” prop’s pitch and P/D increase from hub to tip. Consequently, angles of attack are steeper above the station of nominal P/D and shallower below it, than are those of a formally comparable helical prop, that is, one of same nominal P/D, that of 75%-station.

“Hypotwisted” is an accurate descriptive term because upward graduation of P/D tip-ward requires less angular difference between the chords at any two blade-stations -- particularly between the 50%- and 100%-stations, where almost all the prop’s work is done. Again: the comparison is to a formally comparable helical prop.  [See Diagram 1, below.]

Note that the common modification of a helical prop, tip-ward “P/D-washout” and reduction of angular incidences to plane-of-rotation – tip-ward reduction of “static angles of attack” -- increases the preceding difference.

It should be understood that the angular incidences of airflow to the undersurface of the airfoil when the prop is moving the plane through air -- “dynamic angles of attack” -- cannot be accurately predicted. What we can know is 1) per the design’s intent, the dynamic angles of attack of a hypotwisted prop are steeper above nominal station and shallower below it than are those of a helical prop and 2) given same nominal P/D, the less the blade-twist, the greater the angular divergence from helical configuration.

2. How are they specifically designed?

A certain nominal P/D is stipulated and, normally, the parameters of a helical prop of that P/D are altered accordingly. That is: a helical blade whose P/D is lower than the stipulated nominal one is “advanced” to the stipulated nominal P/D. [See Diagram 1 and section 4.1., below]

In the extreme case there may be no blade-twist at all. That is, chords at all stations are perfectly parallel.  The blade is set on its hub so that at 75%-station it has the static angle of attack of the stipulated nominal P/D. [See the indicated formula in Diagram 1, above.] If stipulated P/D is 1.5 -- common for expert-built outdoor rubber scale models, that angle is 32.5 degrees.  The “all-32.5-degree” prop then has a P/D of 2 at 100%-station and a P/D of 1 at 50%-station – 1.5 P/D on-average. A helical 1.5-P/D prop’s static angle at the tip is 25.5 degrees, so there the hypotwisted prop is 7 degrees steeper; and at 50%-station the helical prop’s static angle is 43.7, so there the hypotwisted one is 11.2 degrees shallower.  Thus, between 100% and 50% stations this prop is 18.2 degrees less twisted than is the comparable helical one.

3. Why are they better?

The professional view is that modest upward graduation of pitch from hub to tip comes closer than does helical pitch-uniformity, to providing for no variation of dynamic angle of attack from one to another station. This presupposes that some dynamic angle is most efficient regardless of airspeed at one or another blade-station, and pretends that the angle can be accurately determined, as equal to the “advance”.

My different, independently formed, rationale reflects the basic facts of trimming a model airplane for best glide. The steeper the angle of attack, the slower the airspeed and the greater the efficiency, or “L/D” – ratio of lift to drag, until, at too-little speed for angle, stalling occurs or is closely approached. By virtue of better ratios of dynamic angle to airspeed and less partial blade-stalling, then, L/D for the prop as a whole is greater, as dynamic angles of attack are steeper tip-ward and shallower hub-ward. This may also be seen to follow from the facts that lift – here, thrust -- and induced drag both increase as does angle of attack while thrust increases with square of speed and induced drag decreases with speed. That is: the faster the airspeed the greater the L/D, so let the tip-ward section do more of the work.

Taken alone my reasoning called for an “anti-helical” blade – what to Superman would be a “Bizarro-World” perversion of a natural standard. Static angles of attack would increase from hub toward tip as much as do stations. But that would impractically concentrate thrusting (and “torque-over”-drag) near the tip, so I went only as far as to use no twist at all, as on the fans propelling my experimental, torque-over-neutralizing, Embryo, the “Transcender”, photo’d elsewhere in this site.

The “Transcender”’s skyrocketing climb, attended by the audible air-pumping of what in Standard Aero-Engineering’s view are “theoretically unsound” fans, tends to support my hypothesis that tip-ward steepening of dynamic angles of attack increases efficiency.  Note that in contrast to a “kiddie-prop” normally used only when rules forbid twisting, static angle was 45 degrees -- as made 50%- P/D 1.57 and tip-P/D 3.14, and that planform was fan-shaped and of very low aspect-ratio with very-short span.

4. Selecting and Making Your Own

4.1. Re-set helical blades. Here is what I’ve found to work best as a “starting-point” or best final guess for Scale and quasi-Scale rubber-powered models with normal ratios of drag to weight, per average climb-steepness, in their prop-loads. As indicated, it involves more aggressive hypotwisting than has been professionally deemed optimal. 1) Select a normal nominal P/D for a given type of model, and “cut it in half”.  If it’s 1.5, the blades then are those of a .75-P/D helical prop. 2) Using a music-wire or carbon-rod joint and a jig, set the blade at its 75%-station at stipulated (here, 1.5) P/D.

The difference between the static angles of .75-P/D and 1.5-P/D props at 75%-station is 14.82 degrees, so the blade must be advanced that much, relative to its setting in a .75-P/D prop, to reach the 32.5 degrees of 1.5 P/D. P/D then climbs from 1.33 at 50%-station to 1.69 at the tip – again averaging 1.5 P/D within the “working sector”. [Again see Diagram 1, above.]

By the same principle, if, say, a ready-made Peck blade were to be used, where “Peck-P/D” tends to be negligibly below 1.0, it would be selected a priori for a model which promises to like, or currently does like, a 2-P/D prop.

Nominal P/D may be experimentally varied just as with a helical prop. A 10%-percent increase is a smart first move. Beyond that, for the “cut-out half” may be substituted one-third by way of reducing hypotwist and its effects on dynamic angle of attack – a 1-P/D blade (such as that of a Peck prop) is used for nominal 1.5 P/D, or (more interesting) two-thirds by way of increasing them – a 0.5 P/D blade is used for nominal 1.5 P/D.

4.2. One-piece hypotwisted props.  The block to be carved is proportioned via mathematical formulation to correspond to the dimensions of an “advanced” helical prop, as detailed and shown in Diagram 2. The easiest way to effect this is to vary block-width instead of block-height. The block is plotted at every 25% of radius. Note the resulting “hour-glass” block-planform.