prop questions

[john haz]

Hi Guys,

 

It wasn't going to be long before i needed more input was it LOL

 

why do flying wings etc tend to use smaller props running at higher speed ?

 

F'rinstance,   on my Roo I have tried 5x5 and 6x4, giving me around 170w / 200w, which is fine on a 2200kv 28A motor.

 

What would be the disadvantages of running maybe a 7 or even 8" prop on a lower kv motor ?     Was thinking maybe 1400 - 1600kv ?

 

No real reason i must do this other than as an experiment & from what i understand, maybe the bigger prop would give me more punch at launch time ?

 

Most flying wings / delta's seem to use smaller props, there must be a reason (loads of room on a Roo to put larger props.

 

 

[Bruce Richards]

It a simple one word answer Speed. Flying wings normally are for fast flying and if you want to go fast you need a small prop spinning fast. A bigger prop would be more efficient so you could fly for longer but more slowly.

[john haz]

Hi Bruce,

 

Thanks for that simple answer to my question   

 

Let's assume I had a 5x5 prop on a 2700kv motor pulling 250w  & got 60mph,  

 

 if i changed to a 7x5 / 1400kv / still pulling 250w does that mean it would be slower  ?

 

  (my figures are plucked out of my empty little head, but you can see what I'm getting at).

 

 

[Wingman]

John,  the second number in 7X5 is the number of inches the prop will screw itself through in one revolution so its 1400 x 5 inches versus 2700 x 5 inches 

[john haz]

Does that remain constant throughout the diameter Wingman ?

 

If it does, that makes calcs so much easier to guess    I had (probably wrongly) assumed that a 7x5 would give more speed than a 5x5 because of it's larger diameter.

 

 

[Wingman]

Well a rotating aerofoil that is 7 inches in diameter is going to push more air past it than one that is 5 inches in diameter for the same pitch so you get more thrust but the speed of the air would be the same - I think that makes sense 

[Simon Chaddock]

john haz

In your example you state a 5x5 and a 7x5 each drawing 250W. The only way this could happen is if the bigger prop is going round slower. (All other things being equal bigger props need more power)

Even at lower revs the bigger prop is likely to produce more static thrust (it has twice the swept area) but the lower revs will reduce the maximum speed achievable.

 

As a prop & motor combination is a performance compromise what you like in your model may be different from someone else.

.

[Bob Cotsford]

as the motor driving the larger prop is lowe kV it will run slower for the same battery, in simple terms 1400/2700 of the speed of the small prop setup (ignoring slippage etc, just keeping it very simple)

IE for example at 10v the 7*5 will try to turn at 1400 * 10 (kV * V) =14,000 rpm.

The 5*5 would try to turn at 2700 * 10 = 27,000 rpm.

Both props are trying to move forward by 5" for each revolution (that's a definition of pitch), so the smaller prop would try to go half as fast again as the bigger prop.

 

[Mowerman]

Is Bruce telepathic ?

I've seen cases where questions  are posed then 'Bruce' is thanked for his prompt reply but this reply does not appear.

Surely the answers are of interest to many readers not just the original poster.

[DH 82A]

 

     "   Pitch speed "  is the theoretical airspeed of the model in MPH, according to the pitch of the prop, and the RPM.

 

   It is calculated thus--  RPM  X  PITCH (in inches)  divided by  1056 = MPH  airspeed.

 

     So, a  10 x  6  prop at   10000  rpm =  56.8  MPH.   

 

        On the same motor/battery  combination  a larger dia. prop. at the same pitch, will spin at lower revs, so the  pitch speed will be lower, but will produce more thrust, and also draw more amps.

[Biggles' Elder Brother - Moderator]

Posted by Mowerman on 18/11/2010 10:17:29:

Is Bruce telepathic ?

I've seen cases where questions  are posed then 'Bruce' is thanked for his prompt reply but this reply does not appear.

Surely the answers are of interest to many readers not just the original poster.

 

Hi Mowerman,

Can you not see Bruce's post? Its the second in the thread.

 

If you can't is it possible you might at some stage have accidental pressed the "ignore member" option on one of his posts?

 

Let me know if the problem persists and I'll "look into it" as they say!

 

BEB

[john haz]

Many thanks chaps, got it now.   where i was going wrong was thinking the second number was an "angle" of the blade, i.e. x degrees, which i assumed would have meant more forward speed on a larger prop.

 

I understand now that it's a rate of forward movement which remains constant regardless of diameter & also that a larger prop might / will load the motor more & reduce the max rpm.

 

I have just bought quite a selection of different props for the Roo & am going to do some testing tonight, will post results for the sake of interest  

 

Appreciate ALL your comments & help, I'm a little step closer   

[john haz]

Last night didn't go as planned !   I took the wing & the Roo to do some testing & burnt out one of the leads on my meter !     Home at 9pm, repaired it & decided to bring them to work today for a test     ('er indoors doesn't like the roo spinning up in the house, noisy thing apparently LOL)

 

All props from giantcod ....

 

2200KV  SP 28A motor   /  3s 2200 20c flightmax, fully charged.

 

 

4.5 x 4.5  165w   14A

6x3e      227w 20A

5x3e   196w  17.4A

7x3e   230w @ 75% throttle (decided to leave that one)

5x5e   190w 17.3A

 

I think I will try it with the 6x3 & see how I go, if I need more speed I suppose I could try a 6x4 but thinking it will be ok for me as it is.

 

 

The Swift wing has a 1100kv 13A motor, 1000/3s lipo & only had 80w on a 7x3 prop (apc),  not very fast but I still have a lot of fun with it ---  got a couple of other 2200 motors I could try on it

[Mowerman]

Hi BEB.

Bruce is still invisible.

[Bruce Richards]

That explains why my wife ignores me all the time

[john haz]

Posted by Bruce Richards - Moderator on 19/11/2010 11:16:19:
That explains why my wife ignores me all the time

 Who said that ??  

[john haz]

Update...

 

Today was a quiet day for me at work, only one customer to see at 10am & a fitting job at 3:30, so i decided to leave the girls in peace doing their thing & disappear to the flying field.

 

The Roo is better than i had ever hoped for, now I have the COG right, the motor working as it should & a 6x3 prop on the back it's a treat to fly !

 

hand launches at half throttle, cruises at 1/4 throttle & it comfortably quick at half to 3/4.   Landing seemed a bit weird after flying warbirds, it's more like a high wing trainer, one touch of "up" & it wants to glide another mile !!

 

All my concerns about launching & needing loads of power for it were unnecessary  !!

 

Thanks again to all the offered me advice, hoping to get another go tomorrow if the weather is kind to me  

[Biggles' Elder Brother - Moderator]

Posted by Mowerman on 18/11/2010 10:17:29:

Is Bruce telepathic ?

I've seen cases where questions  are posed then 'Bruce' is thanked for his prompt reply but this reply does not appear.

Surely the answers are of interest to many readers not just the original poster.

 Mowerman,

 

I've PM'd you on this.

 

BEB

[Mowerman]

Thanks BEB and Bruce.

Now I see all

[Biggles' Elder Brother - Moderator]

All part of the service

 

Actually, it was Tim that fixed it - but we'll keep that a secret between us!

 

BEB

 

[Mowerman]

Many thanks to Tim.