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Old Nov 27, 2011, 05:02 AM
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Originally Posted by kensp View Post
Luke2000

The thrust developed is only half the story. Just as important is Pitch Speed. This is how fast a propeller of a particular pitch will propel an particular model. If the pitch speed is not high enough then the model will be flying too close to its stall speed. If so then it will be tricky to fly and prone to tip stalling in turns.

I have found that any thrust that is greater than half the weight of a model is sufficient to give good flying provided that the pitch speed is greater that 2 1/2 times the stall speed

Ken
Thanks Ken,
I need a remedial class version of that! Do you mean the tip speed of the props or the "slippage" experienced by the prop moving forward? Can pitch speed be measured?
I was expecting that someone would suggest that the figures at half or two thirds throttle would be more important, which is where most flying is done. I'm still amazed at how the R2 motors would seem to be more efficient than the TP ones!
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Old Nov 27, 2011, 05:51 AM
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Originally Posted by Luke2000 View Post
Thanks Ken,
I need a remedial class version of that! Do you mean the tip speed of the props or the "slippage" experienced by the prop moving forward? Can pitch speed be measured?
I was expecting that someone would suggest that the figures at half or two thirds throttle would be more important, which is where most flying is done. I'm still amazed at how the R2 motors would seem to be more efficient than the TP ones!
Luke

When the propellers pitch is measured in inches Pitch Speed in MPH is found by using the formula

RPM x Pitch / 63,000 X 60.

i.e. At 8000 rpm a propeller with a pitch of 6 inches would have a pitch speed of
8000 x 6 / 63000 x 60 = 45.71 MPH

This is the theoretical speed at a given RPM before slippage is taken into account.

Ken
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Last edited by kensp; Nov 27, 2011 at 06:09 AM. Reason: Correct formula
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Old Nov 27, 2011, 06:17 PM
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Originally Posted by kensp View Post
Luke

When the propellers pitch is measured in inches Pitch Speed in MPH is found by using the formula

RPM x Pitch / 63,000 X 60.

i.e. At 8000 rpm a propeller with a pitch of 6 inches would have a pitch speed of
8000 x 6 / 63000 x 60 = 45.71 MPH

This is the theoretical speed at a given RPM before slippage is taken into account.

Ken
Gee, thanks Ken. I used to see all those HK reviews mentioning prop speed and couldn't figure what that had to do with anything. That's very informative, I'm gonna work on that now as I have a digital meter that can read the prop revs.

Luke
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Old Nov 30, 2011, 03:47 PM
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Quote:
Originally Posted by kensp View Post
Luke

When the propellers pitch is measured in inches Pitch Speed in MPH is found by using the formula

RPM x Pitch / 63,000 X 60.

i.e. At 8000 rpm a propeller with a pitch of 6 inches would have a pitch speed of
8000 x 6 / 63000 x 60 = 45.71 MPH
63000x60? What is that?

The equation I use is ( RPM x Pitch ) / 1056

1056 is a constant.

I see you got the right pitch speed answer, but I don't think you were using the 63000 x 60. Did you mean 63000 / 60?
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Old Nov 30, 2011, 07:19 PM
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Originally Posted by mattyhawk View Post
63000x60? What is that?

The equation I use is ( RPM x Pitch ) / 1056

1056 is a constant.

I see you got the right pitch speed answer, but I don't think you were using the 63000 x 60. Did you mean 63000 / 60?
In my formula pitch is stated in inches as this is the normal way of measuring model aircraft propellers. i.e. 8 x 6

RPM multiplied by pitch gives the distance in one minute measured in inches. (Inches per Minute.)

Divide by 63000 converts this to the distance in one minute measured in miles. (Miles per Minute)

Multiply by 60 gives the distance in one hour. (Miles per Hour)

I did some research and found that there are actually 63360 inches in a mile so the correct formula is:-

RPM x pitch / 63360 x 60

Ken
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Old Nov 30, 2011, 09:15 PM
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Ah, i see what you are doing, the equation was confusing without units.
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Old Nov 30, 2011, 10:54 PM
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Originally Posted by mattyhawk View Post
63000x60? What is that?

The equation I use is ( RPM x Pitch ) / 1056

1056 is a constant.

I see you got the right pitch speed answer, but I don't think you were using the 63000 x 60. Did you mean 63000 / 60?
I think it should actually be 63360, which is the number of inches in a mile.

So it's (RPM x Pitch) / 63360 = miles travelled in a minute x 60 minutes = MPH

EDIT: Never mind, I didn't refresh the page and missed the answer above...
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Old Dec 01, 2011, 10:12 AM
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Nice one Ken,

Any idea of how I'd figure the stall speed of an aircraft?

Luke
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Old Dec 01, 2011, 02:24 PM
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Originally Posted by Luke2000 View Post
Nice one Ken,

Any idea of how I'd figure the stall speed of an aircraft?

Luke
I use MotoCalc to get all the predicted parameters for my models. You can download a one month trial of MotoCalc from here. http://www.motocalc.com/motodown.htm. After one month you must purchase it if you wish to continue using it

I have been using MotoCalc for over 10 years and find that it gives me a good idea of what motor and propeller combination is most suitable.

Here is the MotoCalc predictions for my CL-215

MotOpinion - Hobby King CL 415 Water Bomber
Sea Level, 29.92inHg, 59F

Motor: Turnigy Bell 2410-12Y; 1000rpm/V; 0.9A no-load; 0.205 Ohms.
Battery: Zippy1600 (20C); 3 cells; 1600mAh @ 3.7V; 0.0093 Ohms/cell.
Speed Control: Hobby King Blue Series12 Amp; 2 controls (separate); 0.0013 Ohms; High rate.
Drive System: GWS 9 x 5 DD; 2 motors (parallel); 9x5 (Pconst=0.86; Tconst=1.336) direct drive.
Airframe: Hobby King Cl 415; 493sq.in; 38.4oz RTF; 11.2oz/sq.ft; Cd=0.049; Cl=0.47; Clopt=0.68; Clmax=1.24.
Stats: 76 W/lb in; 54 W/lb out; 15mph stall; 21mph opt @ 59% (38:52, 85F); 25mph level @ 69% (31:04, 89F); 1082ft/min @ 36.2; -168ft/min @ -5.2.

Power System Notes:

 The full-throttle motor current at the best lift-to-drag ratio airspeed (8A) falls approximately between the motor's maximum efficiency current (6.8A) and its current at theoretical maximum output (26.4A), thus making effective use of the motor.

Aerodynamic Notes:

 The static pitch speed (40mph) is within the range of approximately 2.5 to 3 times the model's stall speed (15mph), which is considered ideal for good performance.
 With a wing loading of 11.2oz/sq.ft, a model of this size will have very sedate flying characteristics. It will be suitable for relaxed flying, in calm or very light wind conditions.
 The static thrust (37oz) to weight (38.4oz) ratio is 0.97:1, which will result in very short take-off runs, no difficulty taking off from grass surfaces (assuming sufficiently large wheels), and steep climb-outs.
 At the best lift-to-drag ratio airspeed, the excess-thrust (22.5oz) to weight (38.4oz) ratio is 0.59:1, which will give steep climbs and excellent acceleration. This model should be able to do consecutive loops, and has sufficient in-flight thrust for almost any aerobatic maneuver.

General Notes:

 This analysis is based on calculations that take motor heating effects into account.
 These calculations are based on mathematical models that may not account for all limitations of the components used. Always consult the power system component manufacturers to ensure that no limits (current, rpm, etc.) are being exceeded.

Ken
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Old Dec 01, 2011, 03:48 PM
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That looks very interesting Ken,

I'll download and have a go.

Thanks, Luke
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Old Dec 01, 2011, 05:19 PM
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Originally Posted by Luke2000 View Post
That looks very interesting Ken,

I'll download and have a go.

Thanks, Luke
MotoCalc gives a large volume of information and it took me a long time to fully understand how to interpret it. Also its accuracy is dependent on making accurate measurements to establish the input parameters.

Do not hesitate to PM me if you meed help with MotoCalc.

Ken
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Old Dec 04, 2011, 07:30 PM
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Renton, WA, US
Joined Oct 2007
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Had another mishap with my 415 today. I was 3 or 4 minutes into my 3rd battery, landed smooth as can be on glassy water and went to take off again. Got her going on a nice scale takeoff run, slowly feeding in throttle. Once on step, I advanced the throttle a little more to about 3/4, and began to pull up. I was about 4 feet above the water when, without warning, the plane snap rolled to the right and dove into the water inverted at a very steep angle I cut throttle immediately, and thankfully the plane popped back to the surface upright and in one piece. I was able to taxi back to the shore, which was really lucky because the air temp was in the upper 30's(F), and the wind blowing gently away from shore with at least 5 miles of water to the other side of the lake. Damage was limited to a clean crack in the fuse under the wing. Looks to be just a 5 minute repair

But now I am left to wonder what caused this

My suspicion is that one of the ESC's went to LVC as I advanced the throttle. I had seen this happen a couple of times on the bench, but only using an older battery at close to full throttle.

The battery I was flying with today was once one of my favorites, a Mystery 3S 25c 2450 mAh, but it is probably going on 3 years old now. It does not have the pop that it did when it was new, but I never really worried about it since I mostly fly this plane at 1/2 throttle.

I think I will reprogram the ESC's for a lower voltage cutoff point. And I think this particular battery will also be banned from the CL-415
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Old Dec 04, 2011, 11:20 PM
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That is exactly why I use a seperate BEC on my multis. I like the Castle Creations 10 amp. Russ Farris
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Old Dec 05, 2011, 04:39 AM
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Originally Posted by fly time View Post
Had another mishap with my 415 today. I was 3 or 4 minutes into my 3rd battery, landed smooth as can be on glassy water and went to take off again. Got her going on a nice scale takeoff run, slowly feeding in throttle. Once on step, I advanced the throttle a little more to about 3/4, and began to pull up. I was about 4 feet above the water when, without warning, the plane snap rolled to the right and dove into the water inverted at a very steep angle I cut throttle immediately, and thankfully the plane popped back to the surface upright and in one piece. I was able to taxi back to the shore, which was really lucky because the air temp was in the upper 30's(F), and the wind blowing gently away from shore with at least 5 miles of water to the other side of the lake. Damage was limited to a clean crack in the fuse under the wing. Looks to be just a 5 minute repair

But now I am left to wonder what caused this

My suspicion is that one of the ESC's went to LVC as I advanced the throttle. I had seen this happen a couple of times on the bench, but only using an older battery at close to full throttle.

The battery I was flying with today was once one of my favorites, a Mystery 3S 25c 2450 mAh, but it is probably going on 3 years old now. It does not have the pop that it did when it was new, but I never really worried about it since I mostly fly this plane at 1/2 throttle.

I think I will reprogram the ESC's for a lower voltage cutoff point. And I think this particular battery will also be banned from the CL-415
O dear. If you look back a bit, same happened to me but it was the battery. Sounds a bit similar, it was the sudden "request" for full power that did it I think. It lost power just after lift off and due to the wind became inverted. Def wasn't the ESC in my case as the control surfaces all still worked.

Luke
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Old Dec 05, 2011, 08:09 AM
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Originally Posted by pookielips1954 View Post
That is exactly why I use a seperate BEC on my multis. I like the Castle Creations 10 amp. Russ Farris
I'm not sure that would have helped in this case with a twin engine.. If the right side ESC failed and that motor was lost at full take-off power, then the plane would do exactly what it did and roll over very quickly to the right. Unfortuneatly the plane wasn't high enough where it could possibly be recovered.

I had it happen on my Catalina 2 years ago..
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