Some Results for current Propellers for F3D Models
Engine Data
As realistic performance data for current pylon engines are rarely published,
some tests were performed with a Cyclon pylon racing engine. This engine
represents the average pylon racing engine, similar to a standard Nelson engine,
maybe with slightly less peak performance. Nevertheless it can be used as a good
example for a typical engine, used during the late 1990s in F3D models.
Power curve for the Cyclon pylon racing engine, showing test
results and the polynominal approximation.
Additional information on this engine and the performance data can be found on
my Cyclon page.
| To simplify the following analysis, the power curve was approximated by
a polynominal of the 5th degree
.
The coefficients for the power curve are listed in the table on the
right. The rotational speed n must be entered into the equation
in [1/min], the resulting power P is in [W]. The approximation is
used mainly in the rpm-range between 23000 and 30000 1/min and may be in
error outside of this band. |
| Approximation |
| a = -1.36E-16 |
| b = 1.41E-11 |
| c = -5.47E-07 |
| d = 0.00945832 |
| e = -61.1950565 |
| f = 0.00042227 |
|
Propeller Data
To find useable data for model propellers in general is very difficult. Even
more rare were any data for pylon racing props: they did obviously not exist at
all. I had created numerical results for existing propellers as well as new
designs for pylon racing propellers several times during the last 10 years, but
never had the chance to lay my hands on experimental results for these props.
There are some experimental data available for commercially produced propellers,
but the selection is very sparse and most of these results were obtained in the
1970s and it is not clear, whether these propellers are identical to the ones in
the current catalogues of the manufacturers. Also, all these propellers were
completely out of question for application in pylon racing models (would you
like to try a Graupner 11x7 prop?).
I was lucky to find (with the help of these web pages) a student, who was
working on his diploma thesis and who wanted to test several folding propellers
for electric gliders in the wind tunnel of his engineering school. The proposed
experimental setup was cleverly designed and professionally manufactured, and I
could persuade him to extend his tests by including 5 additional propellers,
which I had collected from various sources. These propellers can represent a
cross section of typical propellers for pylon racing models. All propellers were
molded and consist of carbon fiber reinforced epoxy resin. Unfortunately I had
no opportunity to test one of the plastic props from the APC series for Quickie
500 models. Currently, I have no detailed data on the geometry of these props
(chord and twist distribution, airfoils), so the following table lists the
diameter only. Therefore, the results cannot be used to examine details of the
individual propellers, but indicate the bandwidth of typical propellers instead.
The wind tunnel tests provided a lot of data - several ten thousands of data
points were taken for each propeller, covering a whole range of operating
conditions. To make that huge amount of data useable and better suited for my
poor brain, I created approximations for the power- and thrust-coefficients for
each propeller. This involved some guesswork, sorting and extrapolating, as the
data was spread over a wide band of Reynolds numbers, slightly below the
in-flight values of modern F3D models. But, as the results showed, Reynolds
number effects were covered well enough to extrapolate to flight conditions I am
quite confident in the results. This time, the approximation was based on a
polynomial of the order of 4, with the factor b set to zero. The
coefficients given above can be used to approximate the coefficients as a
function of the advance ratio 
.
Working Together - good not only for Boeings 777 ¹)
The approximated propeller coefficients and the polynomials are shown in the
table above: a first look shows, that the maximum efficiencies fall between 70%
and 80%, which is quite good, considering the difficulty to create airfoil like
shapes on the small propeller blades. The top runner seems to be the A1
propeller, followed by V1 and F1. But: what counts in the end, is the thrust of
the propeller-engine combination. A good efficiency is nice to have, but when
the power consumption of the propeller forces the engine to turn the propeller
at a speed far away from the optimum v/nD, the performance of the propeller
cannot be used. One propeller may work perfectly well on a certain engine, but
the results on a different engine may be unsatisfying.
| Together with the performance curve of the Cyclon engine, it was
possible to calculate the thrust T versus flight velocity v
relation. These curves, as shown on the right hand side, indicate, that
the F1 propeller outperforms the A1 propeller, because it "fits the
engine" better. The promising V1 propeller looks even worse - a
deeper analysis shows, that this prop cannot consume the power offered by
the engine, resulting in high engine speeds above the peak power. It would
work better on an engine with less power. The propeller S1 shows the
highest thrust at low speeds, which will result in good results for the
first lap, but during the following laps, it will be outperformed by most
of the remaining propellers. |
 |
Remark:
Similar to the approximation of the engine power curve, some errors are
introduced, but I feel, that the data is still representative enough to
deliver realistic results during the following simulation steps.
Results of Flight Simulation
During a race, the propeller-engine combo has to pull the airframe through
the straights and turns of the racing course. It is not clear, where in the
advance ratio regime the propeller finally operates and what the velocity of
rotation of the engine will be in flight. Therefore I wrote a simulation
program, which calculates the flight speed and its variation while the model
"flies" around the course. Results from these simulations include
flight speed and lap times as well as details on airframe aerodynamics (lift and
drag) and the operating points of propeller and engine.
Here, I present some results of the simulation: lap times, calculated for
each one of the five propellers.
 |
The graph on the left side shows the racing time versus laps
for each propeller. We can see, that the times diverge with increasing
number of laps. The lines are finally ending at race times between 76 and
68 seconds. As all these results were calculated with identical data for
engine and model, they represent the performance of the individual
engine-propeller combination. |
 |
Here we see the time for each lap, which makes it clear,
that there is a different order after the first lap (e.g. propeller V1 is
slower that R1 after lap 1), but starting from the second lap, the
differences are nearly constant. Thus propeller F1 performs faster by
nearly 1 second per lap, compared with prop R1, which finally adds up to
the large difference in the end, as shown above.
The net result makes it clear again, that it is still worth to
experiment with different propellers, especially when engine and model
seem to be performing already fairly well - but that's what you already
knew before reading this paper... |
____________________________
¹) Boeing introduced the slogan «Working Together» during the development
of the Boeing 777 - I don't know, how they built their planes before... (if you
want to read the interesting story of the Triple Seven, slightly blurred by the
propaganda department, see [32]).
Last modification of this page:
21.05.18
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Table
of Contents
Propellers
for F3D pylon racing models
Introduction
