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Brodjack
04-15-2006, 09:48 PM
Running roughly 1400W with a 9-9 prop in static at approx 17000rpm.
Should i use a 'glow' prop or APCE prop,or what?The APCprop makes a scary,grunt noise at speed.
Do guys use the stronger 'glow'ones?
I,ve tried a Graupner Cam prop,but the nearest size is 10-8,which is a bit quieter and seemed smoother.
Been advised the 'E' props are not suitable for these power levels?

watt_the?!
04-15-2006, 10:44 PM
ive asked the question many times of glow and e ''experts'' and do not have a satisfactory answer from them to date.

i sent an email to APC asking the same and got no reply.

here's my take on it.....fwiw. A glo engine pulses power from the maximum surge at top dead centre, then maximum acceleration (at top dead centre and bottom dead centre) and then return.

the ''power'' from these engines is an average output per stroke, which includes the high power bit and the turnaround bits......so, at one point in the cycle the power is actually as high as 10 times the rated power of the engine...yadayadayada.

so, my guess is that a glo prop needs to be very stiff to deal wit that vibration, and also strong enough to deal with the pulsating loads.

alot of people have said to me that they are not efficient on e planes, and vice versa. I personally think that is BS and just parrot talk as they cant explain why.

So the thought is that for a smooth power application like electric, the prop only needs to be stiff and strong enough for the output load, which is easily measured.

the other side of the coin is that you can use glo props, but expect them to be heavy.

There are other props designed for pylon racing that you may want to look into also.

if you have a 9-9 prop from APCE and you are running it as directed (i.e. at or below their specced rpm), then you'll be fine. I am thinking that the growl you are hearing is the noise of severe cavitation from the sqaure prop.

Tim

Brodjack
04-15-2006, 11:05 PM
Thanks for the quick reply.
The growl could be as you say,by the time the plane starts to really 'take off' its right at the far side of the field.I only realy notice it as fast close flybys.It doesnt make the growl in a static WOT.
The plane is a glow conversion,the growl wouldnt be noticable with a 2 stroke screaming.
Like you stated,not a lot of info regarding prop type performance.

watt_the?!
04-15-2006, 11:52 PM
another thought...have you got the right prop pitch for your plane? pitch speed is nearly 150mph for that rpm....is your plane doing 150mph?

if it isnt, it could still very well be cavitating.

i could be wrong.

Tim

rcers
04-16-2006, 12:06 AM
You will be more than fine at 17k. Per APC the limit for 9" "e" props is 21,111 rpm. You should not be exceeding that even in unloading. And even then it is likely "good" to a bit past that.

Source:
http://www.apcprop.com/html/rpm_limits.html

You should try the "glow" prop, the shape is a bit different, and no question it is stiffer. In fact you should try a variety of props and if speed is what you are after you might want to try the pylon APC props too.

I guess I should ask what type of ship is it on - and what is the goal? (I am assuming speed).

Remember our props don't technically cavitate - you need water for that as it is the creation of air bubbles in the water....

They do however "stall" (remember they are wings) and until they catch up you are wasting energy there.

Another point to make your mind twist - you may actually exceed the max pitch speed by 10-15%. Very possible on slippery ships. Pretty cool.

Mike

watt_the?!
04-16-2006, 12:14 AM
Remember our props don't technically cavitate - you need water for that as it is the creation of air bubbles in the water....


Mike

lol...Mike....i should know better shouldnt i?...

you are correct. I was looking for a description that was simple and described an equivalent effect..considering air as the working medium.

whilst not completely different in pragmatic terms, it is, as you point out, not cavitation....ill watch my Ps and Qs now knowing you are on the board!..lol.

good to have you here mate.

Tim

rcers
04-16-2006, 12:24 AM
O no biggy - I get it!

I just like playing with props - I am surprised that more modelers don't spend more time on them, and picking the right one for the right application. I have used 10+ props on a model before before I settled on something!

They all act so different.

In fact one of my "fun" planes right now is powered by the inexpensive Skatty 400 motor and a APC (glow) 7x8 prop! Draws 11.5 amps (a tad high, but I know it unloads) on an Ace Simple 400. It scoots along at near 65ish MPH pretty darn close to the Mega 5 turn I had it in that ran 18-19amps at 80mph.

So you can see that last 10-15mph takes almost twice the power!

Life is good! ;)

Mike

Brodjack
04-16-2006, 02:08 AM
Thanks for the replies.
This is the plane;http://www.sussex-model-centre.co.uk/shopexd.asp?id=4794
Powered by a Neu 1907-1.5Y 6S 3700 20C.
I think i'm just under the 'glow'weight.
She flys well,nice and slow too.Definely not like a trainer though!
The acceleration with all 3 props is incredible.
i am just a bit concerned about the grunt noise,especially with APCs'.Thinking their bending under the power.
Its speed I'm after,its a dot in the sky after the blink of an eye,great stuff.
My projeti with a HET3W spinning a 5.2-5.2 dosnt make the same noise?
Could it be the larger prop on the Tigershark just makes it more noticable? ie.louder.
Perhaps the 'cavitation' noise? is because of the motors ability to spin the prop fast so quickly.a glow would probaly never do that?It does seem louder when i hit the juice.Then dies a bit.Quiet as a mouse just cruising around.
perhaps a 8-8 would be more appropiate.
if it was 'cavitation' noise wouldnt it be more pronounced at static blasts?
The 9-9 at static is the quietest,followed by the G.cam10-8 and then an APC 10-7.
In flight the Graupner Cam 10-8 is the quietest,followed by the APC 9-9,and noisiest the APC 10-7.There the only ones i got.

Unknown logical territory for me here......

rcers
04-16-2006, 03:03 AM
Any square props will get you quite a bit of noise. The other element that creates noise is the tip speed. At 17k on a 9" prop the tips are very close (if not over) supersonic and that is LOUD...

Try the 9x9 glow prop and see if it goes away. If so the noise is likely the prop flexing (bad - very bad) the issue is they don't maintain their shape (they "flatten").

When this happens though you have ample warning, I have only seen one undamaged prop let go, thankfully it was a GWS on a low power system. What will happen is you will begin to see stress fractures and "white" areas that look like you have creased the prop. Take an old one and bend it, you will see the whiteish lines form. That is how they fail. Usually those appear near the tips (the weakest area of the prop).

You need to go prop shopping and do some test in the air - that is where it matters. You will likely find one that you are happy with, and that will not make that noise.

I have used apc-e props at 17k+ on a belly lander for over a year before I got enough stress lines to replace the $2 prop. They are tough little dudes....

Mike

Brodjack
04-16-2006, 03:21 AM
Thanks Mike,i'll order some glow ones on tuesday.There not dear so a few left over wont matter.The glow props have to be stronger.
just had alook at the APCE 10-7,it does appear to have white discolouration at the tips! The thing just looks so skinny.
Must admit the fear of a prop coming apart is pretty frightening taking static amp readings.I only run them for a short 5 secs.The power of these Neu's with the 20C bats.is truly scary stuff.I dont know whether to look at it or not!

Brodjack
04-19-2006, 01:14 AM
well,bit of a disaster tonite.Bought a few glow props to try,first a 9-9,seemed much sturdier.I launch with bungee type platform,the plane rockets about 10 ft forward,upwards,then i hit the juice,no real probs.
When i hit it tonite,it went into a massive torque roll,quess the rest!
It must have been the extra weight of the heavier prop creating an opposite greater reaction,luckily only lost the prop,and a bit of pride.
Went to my flight box,i'd left the others on the kitchen table at home!!
Good 20 mins. drive.Just packed up and retired for the nite,i'll try again tommorrow.
I will be well ready for the increased torque roll tommorrow!!

chute42
04-19-2006, 02:48 AM
When i hit it tonite,it went into a massive torque roll,quess the rest!
It must have been the extra weight of the heavier prop creating an opposite greater reaction,luckily only lost the prop,and a bit of pride.
I will be well ready for the increased torque roll tommorrow!![/quote]
An other possibility is the stiffer prop doesn't flatten out at high rpm and moves more air. Try adding power slower just to get it going, after it builds some speed kick it in the butt and see what happens:)
Chuck

Brodjack
04-21-2006, 12:04 AM
The torque roll was overcome this past day or two by increasing throttle a bit smoother,possibly getting better at flying this beastie of a plane.
The grunt i was experiencing wasnt helped by using only 2 bolts to mount the motor,stupid me!,it seemed tight until i put another 2 in and realised the motor wasnt perfectly flat against my home made mount.
The noise is still there but not half as bad.
Gone back to the 10-8 Graupner for a week or two to get used to its bullet like feel.The plane has a totally flat plate wing with zero lift.All i do is aim it and she goes there.Motor off at about 10' to land covers about 100yds to land slowish!
Power has to be kept on all of the time,no power off gliding and then power on,she dosnt like picking up speed from a crawl.
Run time with the 6S 3700 bats is about 4-4.5 mins tops.
Neu Motor comes down cool.
Proper adreneline stuff,she covers the field in seconds,well over 100mph.
It certainly looks a bat out of hell tearing up the sky,with a frightening roar,scares everybody to death, great stuff!!

Dr Kiwi
08-28-2006, 04:21 PM
Any square props will get you quite a bit of noise. The other element that creates noise is the tip speed. At 17k on a 9" prop the tips are very close (if not over) supersonic and that is LOUD...


Mike

By my calculation ~910mph!