[gbanger]

OK ...What's the story with the Chevron/sawtooth on the trailing edges?

[secant0give]

I think part of the idea may be to reduce turbulence behind the blade so the next blade passes through nice clean air. Although it should do so mostly, as it's advancing through the air and the infeed air should be pretty clean.

Also any time you reduce turbulance you reduce wasted energy so I guess it's an attempt to make as much of the air leaving the TE of the prop blade to travel rearward and thus generate the maximum ammount of thrust. When you have turbulance a certain ammount of the the air in the props wake is moving in random directions. Some of this will add to the thrust and some not. if you can get it to orderly travel in the right direction it should all add to the thrust.

High reving props fly in a funny regeime, even tho' the Renolds number is not super high, (this is due to the the chord of the airfoil being quite short), the tips still can get fairly close to the speed of sound. So you have an odd situation, in that you have a low Re at a transonic airspeed.

Propellers and exactly how very work is a very interesting and complex subject and I don't fully understand the greater part of it.

[gbanger]

I like to think I'm a pretty smart fella... but I would have thought that an irregular shape like that at the the trailing edge of the prop would have generated more turbulance not reduced it... just goes to show, how important it is to think outside the square.
Here's one to make you think a little. What about a 4 blade prop with 2 blades at say 10" pitch and 2 blades at 12"pitch. I know they will bleed energy because they will work against each but propellers spinning in the air are only about 70% efficient, so will the 10's help by giving you that initial thrust and then the 12's help with top end as speed increases or do they both cancel each other out and give you 4 inefficient 11" pitch blades?

[kizza42]

http://en.wikipedia.org/wiki/Chevron_(aerospace)
http://www.nasa.gov/topics/aeronautics/f...vents.html

[gbanger]

As expected Kieran ...you D man!
I knew you would have your finger on the pulse.
Might not be to long before we see big mother Chevrons on Heli blades, Blue Thunder's wisper mode make not be science fiction after all!!!

[Claudius]

No probs Claudio I'll lay some up for you

Thanks Jason!

[secant0give]

I like to think I'm a pretty smart fella... but I would have thought that an irregular shape like that at the the trailing edge of the prop would have generated more turbulance not reduced it... just goes to show, how important it is to think outside the square.
Here's one to make you think a little. What about a 4 blade prop with 2 blades at say 10" pitch and 2 blades at 12"pitch. I know they will bleed energy because they will work against each but propellers spinning in the air are only about 70% efficient, so will the 10's help by giving you that initial thrust and then the 12's help with top end as speed increases or do they both cancel each other out and give you 4 inefficient 11" pitch blades?

I don't know how well that prop would go.
Back about 20 years ago I was doing some contract work at comalco
On some big fan forced annealing furnaces. These furnaces had two big fans to blow the Inert atmosphere around and keep the temperature even. One fan rotated clockwise and the other anti clockwise. These were expensive fans as not only were they big but they had to move a lot of Air around and it was hot too at 400 deg
Well to save having two spare fans someone had the bright idea of having a special fan made with two left hand blades and two right hand blades so it would work equally badly in either direction.
I don't know if they ever used it but it was sitting there in the store sure enough I thought they were pulling my leg until they showed me. Brand spanking new. I wonder what the company who made it thaught when they ordered it

[kizza42]

Just as an aside, Apple's latest laptops use different size fan blades for their cooling:
http://appleinsider.com/articles/12/12/2...etric-fans

They claim it keeps the noise down (Less waste energy) and they don't get a loss in performance.

[secant0give]

That's very interesting Kizza, Looks like they are reducing noise by unequal spacing of the fan blades so that you don't get a reinforcement of the sound at the same frequency as the blades go past. by changing the angle between the blades they pass at slightly different times and that reduces noise. It's a clever aplication of an old trick used when making multi tooth machine tool cutters for aplications where all the teeth are in cut at the same time. Things like good quality reamers and countersinks also have a prime number of blades and the cutting edges are spaced around at slightly uneven angles to each other. It stops them from chattering when they cut. Certainly worth a patent to apply the same principles to fan blades, very clever indeed. I wonder if the inventer had any experiance with tool and cutter grinding or weather the idea was totaly independant ??

[secant0give]

The results from Nowra have been posted on nswpylon.org

Claudio and I gave Jeremy and Ian a hurry up in FFC on day 2, We gotta be happy with that !!!

Here's the link

http://nswpylon.org/2013/13_01_Results.htm

I'll have to see if I can get my RareBear just as fast with the APC prop. since the Graupner has been black listed.
Perhaps the aileron linkage Fairings will help make up for up it, they seemed to help at Maitland
but it was difficult to be certain as the course was shortned and the turn lights were not comming on 'till very late, or something was going on

[secant0give]

I'm going to try and get down to the field on wednesday arvo around 3 or just before to set up the pylons for some practice.

Just looking at the weather, If it doesn't rain that is

[smuzz]

Well, being the most recent inductee to the Rare Bear club I was wondering whether the dynamic loads on the control surfaces required MG servos or whether the usual HXT 9g plastic types are sufficient?

[Claudius]

smuzz, I've got both configurations. Can't tell the difference...

[smuzz]

Thanks mate ... that's just what I was hoping to hear

[wingtipper]

cant make Wednesday. but have a couple of newbies just waiting to go !!!

[secant0give]

I'm using one HXT in one of mine for elevator and the cheapo servos they come with for the other control surfaces.

They're Great "tipper !!

[smuzz]

Thank you too Jason - all good then.
Cheers,
Steve

[samste]

Frank they do Look The Part, Nice Job, great color Scheme.

We can tell That someone likes Rare Bear Racing

Steve

[gbanger]

Yes indeed... very nice job Frank...

[smuzz]

Nice work 'Tipper! What sorts of paints did you use?...

[wingtipper]

the green is water based acrylic gloss modelling spray paint , which i had left over , was out of black and secantogive said epo can handle solvent enamal . so i tried it on a test piece of epo , and as long as you do light coats which dry quickly he was right . so i used water acrylic ,and solvent enamel . i sanded the whole plane with 400 grit wet n dry 1st , if you dont do this the paint wont take too well . i recommend hobby water paint .

[smuzz]

Thanks mate, sounds good.

[Claudius]

Thanks mate, sounds good.

And if you'd like a properly awful result, try gold gloss enamel spray paint (like I did on my most recent bear). You're welcome to the rest of the can...

[smuzz]

And if you'd like a properly awful result, try gold gloss enamel spray paint (like I did on my most recent bear). You're welcome to the rest of the can...

... I see - duly noted

[secant0give]

Scores and times from Illawarra

   


Pitt Town is comming up, Satterday and Sunday the 1st weekend of next month!!

If your thinking of buying a rarebear the I'd do it soon as they seem to be allways be sold out, very shortly after a race day.

[gbanger]

Am I allowed to use my FF Trojan or is it for Rare Bears only?

[secant0give]

You can use the Trojan or any of the HK funfighter series of planes
Ian sometimes flies the sea fury or the mustang and there are a couple of p40 that race as well

[secant0give]

Pitt Town is Next weekend Just received this from Jeremy so I thought I'd post it here.
If your coming just email him and let him know the classes you want to fly.



Jeremy's email is

jag at acfr dot usyd dot edu dot au

Just remove the spaces and replace the "at" and "dots" with the correct charictors

Hi Guys,

So just one week to go before the next race at Pitt Town. Hope the weather clears so you can all get some practice (except for Mark ;-) ) this weekend. As usual, can you please drop me an e-mail to let me know if you are coming (or not).

Couple of quick reminders

NOISE - Please try and fly towards the West (river end) when practising

NOISE - Please remember you silicone (or similar) extension tubes for the Nelson / Jett / DKT type engines (ie/ anything of higher performance than a Q500). We need to keep our hosts happy.

Practice Sat afternoon - This is a normal flying day at CKSMAC so please be considerate of local members wanting to fly

LUNCH - Stevo has kindly offered to do a BBQ with sausages, bread rolls and drinks. Please bring your gold coin donations

Look forward to seeing you all there.

Cheers,
Jag


--
Research Engineer
Australian Centre for Field Robotics
University of Sydney
Australia
Mob: 0418 390 446

[secant0give]

Some more video from Illawarra
http://youtube.com/watch?v=-iSf8LRMYTY

[secant0give]

"Pitt Town II" is comming up on the 3rd and 4th of August !!

I'm looking very much forward to it after the wash out of "Pitt Town I" and even more so after the following race day "Nowra II" was also canceled due to the inclement weather.

Lets hope that when Scooter threw his dart at the calender, It landed on a sunny day this time !!