Postby pprop » Thu Feb 04, 2010 11:12 pm
Propellers that have been represented in SA for longer than 5 years.
This is by far the most difficult decision that I had to make. Please read it through and ANY SANE SUGGESTIONS ARE WELCOME!
Local made: – P PROP (Pieter de Necker), NC ( Nigel Coots), HM ( Hennie Malan), Geo Killey ( George Killey) thus 4 models in total.
Imported: - IVO, ARPLAST, WARP DRIVE, AERO PROP, SENSENICH, DUC, BOLLIE, KIEV, AEROSAIL, JABIRU, GT, WOODCOMP and BRENT THOMPSON thus 13 models - (so far) - in total.
Total number of propellers to be tested = 17 propellers (so far.)
PLEASE HELP ME IN LOCATING THE AGENTS FOR THE ABOVE PROPELLERS. THE REASON IS THAT WE SHALL NEED AT LEAST 2 FULL SETS OF EACH PROPELLER SINCE AT LEAST ONE OF THE TESTS SHALL LEAD TO TOTAL DESTRUCTION OF THE BLADES!!!!!!!!!!!!!!! THE AGENTS MUST PLEASE CONSIDER DONATING NEW PROPELLER SETS. WE CAN NOT HAVE ANY SANE, RELIABLE, BASED ON SCIENTIFIC PRINCIPALS AND METHODS RESULTS IN THE END IF WE DON’T TEST ALL THESE PROPELLERS REPRESENTED IN SA.
I am willing to venture R24 000.00 worth of propellers (3) of which I am willing to have at least 2 destroyed.
TESTING VENUE
1) Alanmack is graciously offering his engine for a test bed to test as many propellers as needed. He is also offering to publish the findings of the test. Well done!
2) The owners of Petit airfield must still be contacted for permission to use the field.
IN VIEW OF SAFETY – always!!
The condition of the test area should:-
1) Be at least 50 meters from any obstacle standing taller than 100mm
2) Be at least 99% horizontal.
3) The surface should preferably be flat smooth concrete of 5 meters x 5 meters. (To protect the guys involved from getting hit by flying debris).
4) A very sturdy support and safe tying down methods and strong points on the aircraft should be identified beforehand.
ADD YOUR SUGGESTIONS HERE GUYS!!
IN VIEW OF FAIRNESS AND ACCURACY = SCIENTIFIC METHODS
The condition of the aircraft
1) The engine should be tested beforehand by say ROTAX engines or GIDEON maybe, and then be certified to be putting out X amount of HP From say 6200 to 6600.
2) The wing should be “as factory approved” SUGGESTION:-Alanmack can approach Manfred Springer to maybe come out and see if the wing is set up correctly. I am sure Manfred would be just as interested in seeing what goes for what.
3) The tyres should be inflated to the suggested tyre pressure.
4) I suggest that no add on paraphernalia be present i.e. like – saddle bags, or any add on object that will result in constriction of airflow to the propeller – REMEMBER SHORT PROPELLERS WILL SUFFER FROM THIS EFFECT!!.
5) ADD YOUR SUGGESTIONS HERE GUYS!!
The envelope of a propeller
After sifting through theory, designs and myriads of parameters I have come up with the following suggestions on testing a propeller for SA conditions (in Africa)and in general the overall conditions of Microlight aircraft runways. Please guys any more testing parameters or suggestions will be welcome as long as it’s sane and scientific.
1) Durability = 2 Categories a) On the surface and b) in weather
a) At the least a propeller should be durable enough to withstand, or at the least stay intact a number of take off and landings on dirt runways (excluding accidents AND, aborted TAKE OFFS).
b) Should be able to withstand raindrops, and should at least stay intact when hail is encountered on the fringes of thunderclouds.
THE DURABILITY TEST.
DEGREE OF DIFFICULTY:- Fairly easy – could get complicated – will destroy the test propeller.
CATEGORY 1 : - On the surface.
Gentlemen this test will probably destroy the blades of the test propellers. The test, however, CAN NEVER BE SEPERATED FROM THE DESIGN ENVELOPE OF ANY PROPELLER EVER DESIGNED!
Aids to be present at the test site:-
At least 8½ cubic meters of RIVERSAND. The grain size should be from 3 mm up to and including 10 mm in diameter.
A JCB front end loader / tipper that can load ½ cubic meter of sand. An operator that cam control the bucket over a certain time period – say over 5 minutes.
The method is to slowly drop the river sand into the arc of the propeller while at full rpm. (I did this test with 5 cub. Meters of sand some 15 years ago – it blew the prop to hell and gone!) so ½ cube meter sand per set of propellers should represent a lifetimes worth of taxi and take offs on dirt runways.
CATEGORY 2:- Weather durability
Here I have no experience at all. I haven’t ever done such a test. However, TREVOR DAVIES ENCOUNTERED LIGHT RAIN SOME 25 YEARS AGO FLYING A WOOD PROPELLER THAT DID NOT HAVE A LEADING EDGE NOR CARBON FIBER OR GLASS FIBER. The time span he was flying in the light rain was from Midrand to Tarlton approx 30 minutes. About 10 % of the propeller disappeared from the leading edge on both blades. It’s the best equal sided removal of material from a propeller I have ever seen!
I then suggest that some type of device be made that will simulate raindrops – light rain – and that this device should spray these rain drops onto the turning propeller for about an hour. This hour should represent at least 20 times worth of encountering light rain – 3 minutes per encounter. Is this too much or is it too little
ADD YOUR SUGGESTIONS HERE GUYS!!
Centrifugal forces or “THE PULL” test.
DEGREE OF DIFFICULTY:- VERY EASY – CAN NOT BECOME COMPLICATED EASILY – WILL DESTROY THE TEST PROPELLER.
Centrifugal forces on the blades or the effort it will take to pull a blade completely away from the hub in KG’s. When will the blades say TA-ta dearies?
Aids to be present at the test site:-
An “A frame stand” with block and tackle that can withstand at least 10 000kgs pulling force..
At least 5000kg worth of sand in sandbags.
A scale that can measure the amount of sand needed - in Kg’s - to strip/ remove the blades from the hub.
ADD YOUR SUGGESTIONS HERE GUYS!!
Bending resistance or “ the resistance to deformation” test
This test is to determine the resistance of the blades to deform, contort, twisting and eventual failure of the blades (probably at the root).
DEGREE OF DIFFICULTY:- Very easy – can not become complicated easily – will most probably destroy the blade
Aids to be present at the test site:-
A 3 meter length of robust metal tubing of say 100mm x 100mm square.
Sandbags in 10kg units.
Thrust testing.
Degree of difficulty: -Very difficult – can get complicated in seconds – will not destroy the propeller.
VERY, VERY EXPENSIVE TO DO IN A SCIENTIFIC MANNER!
VERY INTRICATE TO EXECUTE = THE CSIR WILL HAVE TO GET INVOLVED TO MAKE SENSE OF THIS TYPE OF TESTING
PLEASE READ THIS, GENTLEMEN OF THE FORUM, BEFORE JUDGING ANYONE OR ANY STATEMENT MADE BEFORE ON THIS SAME SITE!
DO YOU UNDERSTAND THIS CLEARLY?
YES = YOUR COMMENTS ARE WELCOME –
NO = GO PLAY SOMEWHERE ELSE PLEASE!
Static Thrust of Propellers
The thrust of a propeller is not constant for different flight speeds. Reducing the inflow velocity generally increases the thrust. A reduction of the aircraft speed down to zero tends to increase the thrust even further, but often a rapid loss of thrust can be observed in this regime.
That is why the static thrust of a propeller is not such a terribly important number for a propeller - the picture of a propeller, working under static conditions can be distorted and blurred.
As long as an aircraft does not move, its propeller operates under static conditions. There is no air moving towards the propeller due to the flight speed, the propeller creates its own inflow instead. A propeller, with its chord and twist distribution designed for the operating point under flight conditions, does not perform very well under static conditions. As opposed to a larger helicopter rotor, the flow around the relatively small propeller is heavily distorted and even may be partially separated. From the momentum theory of propellers we learn, that the efficiency at lower speeds is strongly dependent on the power loading (power per disk area), and this ratio for a propeller is much higher than that for a helicopter rotor. We are able to achieve about 80-90% of the thrust, as predicted by momentum theory for the design point, but we can reach only 50% or less of the predicted ideal thrust under static conditions.
Static thrust depends also on the inflow, influenced by the environment of the propeller (fuselage, crosswind, ground clearance). Measurements of static thrust can be easily done, but the theoretical treatment is very complicated and only possible with a lower degree of confidence than calculations in the vicinity of the design point. Due to local flow separation, the behavior of propellers under static conditions can be very sensitive with respect to blade angle settings and airfoil shape.
To get a picture of the bandwidth of static thrust, several older NACA reports and some publications from model magazines have been examined. The results are combined in the following graph.
Static thrust parameter (units are [(kg^(1/3)/m] versus blade angle for different propellers, having 2, 3, 4, 6, and 8 blades. Given power P and diameter D, an approximation of the thrust T can be calculated. The density of air has been set to 1.225 kg/m³ (for a description of the coefficients see: aerodynamic characteristics of propellers).
Of course, the real world static thrust depends on planform and blade angle of the blade and the generic graph gives you a rather wide band of results. One important aspect seems to be the observation of a critical blade angle around 25°. For increased angles, a large part of the blade seems to stall. This effect can be seen on some propellers for high speed model aircraft with large pitch values. After launching the model, it takes some time for the propeller to »catch on«, even when engine and exhaust system are properly tuned. For high static thrust values, a smaller number of blades seems to be better, because (for the same power consumption) they have a wider chord, creating a stronger circulation, being less prone to separation.
As the expression for the propulsive efficiency of a propeller breaks for the static case (the efficiency becomes zero), it makes more sense to use a simple figure of merit like "thrust per input power" if you are interested in static thrust only.
Remark: A hovering helicopter would have a very small blade angle (maybe 5°) resulting in large static thrust values.
Example: We have got two different propellers with a blade angle of 10° and 25° respectively. The first one has a diameter of D = 200 mm, the size of the second one is D = 300 mm. Which one would be better suited to build a VTOL aircraft model? How much thrust can we expect using an .60 engine of 2000 W (assuming a suitable gearbox)?
From the diagram above we read a static thrust parameter of 0.32 [kg^(1/3)/m], respectively 0.1 [kg^(1/3)/m] around the center of the blue band. To calculate the thrust we have to multiply these values with the power P [W] and the diameter D [m] to the power of 2/3. Performing the calculation for the first propeller (10° blade angle) yields T = 0.32*54.288 [N] and thus a static thrust of 17.4 N, whereas the second, larger propeller delivers 0.1*71.138 = 7.1 N only. Using the same engine in a helicopter with its large rotor of 1 m diameter and low pitch angles, would give us a lifting force of more than 55 N !
This example shows, that the diameter of a propeller is as important for static thrust, as it is under flight conditions. But, for static thrust the blade angle is also very important - probably even more important than for the design point, where a gearbox can match almost any propeller pitch and flight speed quite well.
