Vanmorgen kwam ik op m'n werk en daar bleken weer 4 antwoorden binnengekomen te zijn.
De eerste bleek geen fabrikant te zijn maar alleen een verkoper :roll:. Hij vertelde het bekende verhaal met de vergelijking met de versnellingsbak van een auto maar gaf geen echt antwoord op m'n vraag.
De tweede meldde dat hij de vraag had doorgestuurd naar de technische afdeling en hoopte dat zij mij spoedig antwoord een antwoord zouden sturen.
De derde stuurde mij het onderstaande antwoord (wel in het Engels):
Hello,
You ask a lot of intelligent questions about propellers! First, you might want to read the technical bulletin about propellers at our web site,
www.masterairscrew.com.
I would like to send you a prop article that may answer some of your questions and you can share this with your club. It can help to answer the more technical questions.
As to noise, there are basically 2 approaches. All agree that higher revs equal higher noise. Our scimitar props reduce noise by reducing rpm for a given thrust (reaching a certain thrust at lower rpms).
It is important to note that much of the airplane noise originates from the engine, not the prop.
As for tip shape, the debate over square, round, or scimitar is a long one. Square and round tips are good for flight characteristics that are varied, some climb, some speed, some aerobatics. The scimitar shaped undercambered props are good for thrust and speed.
The trend right now is to put a larger diameter and lower pitch prop on an engine and to try to rev up as high as possible. The increased blade area and flatter pitch act as an air dam and limit rpms.
Sincerely,
Teresa McTernan
Windsor Propeller Company
Ik heb deze dame gevraagd het artikel naar mij toe te mailen. Helaas beantwoord dit verhaal nog niet de vraag over het deel van de prop dat stalled bij lage voorwaartse snelheden :cry: .
Als laatste kreeg ik een antwoord van Les Bollenhage van Bolly props.
Hij stuurde het volgende:
Dear Ernst,
Thank you for your questions. To answer them fully would take too long, but I am happy to add a few lines. Also see the theory section of the Bolly Book on our web site.
At 01:13 PM 20/03/03 +0100, you wrote:
Dear Sir,
For many years I am a model aircraft pilot and member of a model flying club. For some time there is a discussion going on at our club and on a model builder's forum about the working of an aircraft propeller. We use fixed pitch props, usually fine pitch for slow flying models and models that are used to pull banners or gliders. Coarse pitch is used for fast models.
correct
The problem is that we disagree about why a coarse pitch prop does not pull as good as a fine pitch prop at low forwards speed e.g. at take of. I think it is caused by the fact that large parts of the prop blades stall because the angle of attack at low forwards' speed is too big. The part that stalls does not provide thrust only drag. When the forwards' speed goes up the angle of attack decreases so more of the blades produces thrust. The engine can run faster (rev up) because the drag becomes much less. At the same time the increased thrust causes the aircraft to speed up even more.
Is this right or is there more to it?
The above is partially correct, but it can be described simpler than that.
1) compare to gear ratios in your car. Acceleration is best with low pitch, speed best with high pitch.
2) with blade size and shape being equal, the lower pitch prop will have more rpm - or if the same rpm it will have more diameter or blade area. Diameter is the best friend of slow models.
Does the rpm of a prop influence the angle of attack (higher rpm = higher
angle of attack)?
No.
The noise a prop produces is also a discussion point. When a model with a
coarse pitch prop starts its take-of run the prop produces a lot of noise
when the engine is at full power. This noise gets much less once the model
is in the air and at a speed where the blades are no longer stalling. Once
in the air and unloading the engine e.g. in a dive the rev's can go up so
high that the airflow around the tips reach the speed of sound. This is very
noisy and even dangerous because it can damage the prop. Because the engine does not reach these rev's at take-of it seems impossible that the noise is produced the same way but it sounds much the same.
Again partially correct. The velocity of the air leaving the prop is higher, indeed a 6" pitch prop is about 50% higher velocity of a 4" pitch propeller. Props are under less load when in the air - on the ground are often more noisy as they often flexing all over the place. The big danger at elevated rpm is flutter.
Can the turbulent airflow around the stalled blades reach the speed of sound and cause the noise or is the noise produced by something else?
Not likely
Also what is the influence of the shape of the prop tip on the noise
production? At our club some people use a scimitar prop to reduce noise.
Others use square tips and again others use rounded tips. All claim that the efficiency of the shape they use it best and that the noise is the lowest.
See the bolly book. We regard our Clubman propellers as having about the best shape.
Maybe it is a bit out of your line but I hope you want answer my questions.
Looking around on the Internet gave me an awful lot of info but nothing that answered my questions. I found an in-depth lesson about the theory of the prop with lots of calculations but this didn't help me much either. The calculations where also a bit over the top for me but it did give a bit more understanding about the airflow around the prop blades.
I hope this helped. Certainly most of the theory to be found is very confusing and forgets that the prop is a very complicated thing - and I find most of the theory forgets that it is a dynamic thing prone to flexing and twisting under load. One of the reasons our props work so well is that we have 25 years of experience and usually know what not to do.
Thanks and Best Regards
Les Bollenhagen
Bolly Props - The Propeller Company Pty Ltd < http://www.bolly.com.au >
For more technical information re our products, Props, Pipes, Engines or Model building, check out the BOLLY BOOK section our web site at, < http://book.bolly.com.au >
Please be aware much of the product data is out of date (written in 1998).
Hier geldt helaas hetzelfde hij beantwoord DE vraag net niet. Ook in het BOLLY BOOK kon ik het antwoord niet vinden. Er staat wel vreselijk veel in dat verhaal, jullie moeten zelf maar eens kijken :idea:
Er hebben nog een stuk of 4 niet geantwoord en ik mag nog twee antwoorden verwachten dus WORDT VERVOLGD.
. Enkele hebben dat al terecht opgemerkt.
, het luid als volgt:
