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Old Jan 22, 2004, 03:59 PM
Registered User
Ocala, Florida
Joined Sep 2003
113 Posts
Question
Can someone explain how pitchspeed should be used in determing which motor to get?

I am trying to decided between a Hi-max 2025-5300 in a GWS C gear box (5.33 to 1) using a 9x7.5e prop versus a Axix 2212-26 direct drive using a 9x7.5e prop in my Formosa.

I plan on using a 2 cell TP 2100.

The Hi-max numbers are the following
Amps: 11.0
Thrust: 13.7 oz
Pitchspeed: 45.7 mph

The Axix is the following
Amps: 9.6
Thrust: 14.3
Pitchspeed: 36 mph.

I would prefer using the Axis, because I won't have to deal with a gearbox. But I am concerned the airspeed of the Formosa would end up being quite slow, because the pitchspeed is a lot lower. Is this or a correct assumption? I guess what I am trying to figure out is the correlation between pitchspeed and airspeed.

I should note, my priority is not to have a fast plane, same or slighter faster than stock would be fine.

Thanks in advance

Gatorfan
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Old Jan 22, 2004, 04:37 PM
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meteor's Avatar
Ontario, Canada
Joined Nov 2002
1,568 Posts
More choices...

Well, since you haven't bought the motor yet, you have more choices...


1) If possible, go one step lower on the AXI wind, (higher Kv)

2) Jump up to a 9x9 prop

3) look at other motor choices: Mega 16/7/x on DD


It's odd though, the Axis must be turning very slowly to only have a 36mph pitch speed with a 9x7.5...

With a slippery ship like your Formosa, your static pitchspeed should be pretty close to your max. straight-line airspeed as the motor will unload a little in flight thereby making the zero-thrust "pitchspeed" point a bit higher that the static one.


Lastly, gearboxes will save your motor in an "impact" situation!!




Hope this helps.
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Old Jan 22, 2004, 04:40 PM
FLYER spelled I-squared-R
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Grand Rapids, Michigan
Joined Sep 2001
1,623 Posts
The old rule of thumb is that the pitch speed should be stall speed x 2.5
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Old Jan 22, 2004, 05:10 PM
Motors beat engines!
Milwaukee Wisconsin, United States
Joined Feb 2001
4,564 Posts
Somethings wrong here. If your using the same 9x7.5 prop for both setups, pitch speed is a direct indicator of rpm.

The one with the higher pitch speed MUST have more thrust, yet your figures indicate the opposite.

Somethings very wrong......................


Dean in Milwaukee
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Old Jan 22, 2004, 05:50 PM
Registered User
Ocala, Florida
Joined Sep 2003
113 Posts
Nice catch Dean,

I was using numbers for the APC eletric prop on Hi-max and APC slowfly with Axis.

with 7x5e, the Axis has the following
Amps: 8.5
Thrust: 10.9oz
Pitchspeed: 37mph

with the 9x6e, the Axis has the following
Amps: 6.4
Thrust: 12.8oz
Pitchspeed: 32mph

By using the Axis with the 9x6 instead of the Hi-max, I will be losing some thrust (~.9oz), but be pulling a lot less amps too (6.4 versus 11.1). But I guess the trade off will be the pitchspeed again. The Axis' pitchspeed is 15mph slower.

I am a fairly new pilot, so top speed of plane is not too important to me. Therefore I am leaning towards the Axis. But at the same time, I don't to want end with the Formosa that is all thrust and no speed. Needless to say, I am stuck and don't know which way to go.

Gatorfan
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Old Jan 22, 2004, 05:56 PM
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Wayne, NJ
Joined Apr 2001
473 Posts
As previously mentioned pitch speed should be around 2.5 x stall speed. Stall speeds around 4 x sqrt of wing loading. A plane without sufficient pitch speed is sluggish, kinda drags itself around. Not a lot of fun to fly.

Mike
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Old Jan 22, 2004, 06:21 PM
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East Anglia, UK
Joined Sep 2002
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Don't forget that APC do an 8x8 prop as well - a good swapout for a 9x6 if you want more speed.

Also, depemnding on drag, the fact that a given combo has teh pitch speed may be irrelevant, if it doesn't have the thrsu to get to teh pitch speed.

Motocalc isn't teh best thing at estimating drag, but you can play around a bit and see that the highest pitch speed sometimes isn't the fastest model.

I have my graphing function set to thrust and drag versus aircraft speed - where they cross is teh fastest the plane will do in level flight.

It's instructive to see the relationships develop between static thrust, rate of climb, pitch speed and ultimate model speed.

If you have a draggy model, teh drag xcan go up so fast with speed that its like running into a brick wall. It doesn't matter what you do, it has a to[p speed that is almost impossible to get past. Tuning for pitch speed there is a complete waste of time and may reduce the actual speed, sincve a small prop whizzing fast may maintain its thrust up at high speed, but there isnn't enough thrust to get you there in the first place!

Conversely, unless you want vertical climbouts, a slippery model doesn't need a lot of thrust but it needs it to stay there all the way up to 80-90mph or more. That means a steep pitched small prop, or a LOT of RPM, or both.

You need to think about how you like to fly. 3d and pattern style aerobats are not fast. Want you want is lots of low speed thrust to pull them through manouvers, and only enough speed to get the model comfortably above the stall.

Sport planes, most scale planes, and trainers need a little MORE speed IMHO, becauseyou tend to run at higher wing loadings and use the speed to carry them through loops. This is even more true of warbirds and suchlike, where half the thrill is doing 70mph inverted passes at 1 ft off the deck...and ducted fans are even more extreme case where its almost impossible to get low down thrust, but the pitch speed is generally VERY high.

If you look at the flight envelopes of real planes, they are just the same. A Jet airliner may look fast at 500mph, but if you look at the size of it - you realise its just a big sailplane with JUST enough power to get airborne, and waft along on a big light wing at - er - 500mph :-)

That's why they use turbofans on em. To get the jet efflux velocity DOWN and the thrust UP. Even fighters use turbofans these days for efficincy, using nastuy things like reheat to get the much faster efflux velocities they need to go and stay supersonic.

Its a general rule that a wing that stalls at speed X is at its most efficient something like 2X mph. Your lightplane prpbably needs 50-60mph to get airborne, and cruses in teh mid hundres. I am not sure what teh stall speed of a big boeing is at 40,000 ft, but 250mph seems reasonable. They seem to climb out at around 300mph. And to land them they need a LOT of flap to get them down to 120-150mph landing speed.

The relevance of this? Don't get hung up on pitch speed. As long as there is ENOUGH of it - generally 2-3 times stall speed - its better to concentrate on low down thrust, particularly for aerobats and high drag planes like biplanes. And fast climbing sailplanes. Only when you are willing to trade off climbrate for speed, do you want to go chasing pitch speed.
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Old Jan 22, 2004, 06:22 PM
Registered User
Ocala, Florida
Joined Sep 2003
113 Posts
The upper end of wing load for the Formosa is 8.4oz/sq.ft.

So to figure out the stall speed, I would do the square root of 8.4 oz times 4. This results in a stall speed of 11.59mph. This times 2.5 equals 28.98mph. Therefore the Axis should be sufficient at 32mph, but it will be a close call. Whereas the Hi-max at 45 mph, would make the plane pretty fast.

Thanks for your help.

Gatorfan
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Old Jan 22, 2004, 08:14 PM
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Ocala, Florida
Joined Sep 2003
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I totally understand what you are saying Vintage.

Ideally, I should buy both motors and the decide which one I liked best on the Formosa. But since my budget is limited, I have to make a choice. I guess, a safe choice would be the Hi-max, because then I won't have to worry about the plane flying to thrusty with not enough airspeed. At least with the Hi-max, I can throttle it back.

Thanks for all your help and any other advice would be greatly appreciated.

Gatorfan
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Old Jan 22, 2004, 11:28 PM
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tone's Avatar
Atlanta
Joined May 2003
4,297 Posts
Gatorfan
If your evryday field is large, then the 45mph setup is going to please with big loops and higher speed.
If you fly in a tighter area then the 30mph setup will be very thrilling and powerful.
I use a razor 400 and on 2s the 11x8 gives me about 30mph and ok thrust, fun flying, easy but not unlimited.
on 3s and 11x4.7 i get unlimited vertical at the same 30mph
but on 3s and 10x8 i get 45mph and plenty of power.
the razor is 3700 rpm/volt and in a gws C at 5.3:1
thunder power 3s - any old 1200 2s
hope this is usable info...
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Old Jan 23, 2004, 01:38 AM
Cogito Ergo Sum
Joined Dec 2003
485 Posts
my brain hurts

after reading this informative discussion, I am wonering what prop would be correct for my electrostreak. I am not worried about wing loading at all...it is not a crucial factor for my flying. I bought the electro because it is slippery design. I am not going to be pulling high G 3d manuevers.....I want big loops and diving speed. I will run the phasor 15/4 witha 40 amp speed control. 9 cell 3300 mah pack. I was going to use a 9-6 apc but what do you guys think might be more appropriate??? I could use a folding prop but i am not sure how efficient they are. let me know what you guys think
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Old Jan 23, 2004, 04:51 PM
Registered User
Wayne, NJ
Joined Apr 2001
473 Posts
My brain hurts too!

with 7x5e, the Axis has the following
Amps: 8.5
Thrust: 10.9oz
Pitchspeed: 37mph

with the 9x6e, the Axis has the following
Amps: 6.4
Thrust: 12.8oz
Pitchspeed: 32mph

Numbers don't make sense 7x5 @ 8.5A, 9x6 @ 6.4A

Would have run this myself but Motocalc doesn't have an Axix 2212-26. I would go with the lightest setup, whichever that is.

Generally when I'm asked to help out with a poorly flying electric the problem's a lack of pitch speed. A lot of time gets spent worrying about thrust, but pitch speed is generally ignored. Doesn't only apply to elctrics either, my favorite is the typical glow powered trainer with an OS46FX in the nose swinging a 10x6 at 13,000. Got a beginner at the controls of a plane capable of 80mph if it's clean.You've got a good handle on the pitch speed you're going to need, none of this is cast in concrete so you're going to have to prop it to suit your flying style. Once you get it ballparked you can change the prop to better suit your needs.Vintage makes a good point about not getting lost chasing pitch speed, but you don't want to ignore it either. Plotting the thrust vs. the drag is interesting but it's only half the story. The full picture is the power curve which applies to any aircraft.In a Cessna I need just about the same power setting for straight and level at 40 knots and 80 knots. At low speeds (high angle of attack) the induced drag is high, and at higher speeds (low angle of attack) the parasitic drag is the limiting factor. If the pitch speed is too low the plane spends all its time on the back side of the power curve and never gets up on the wing. On the other hand, if the pitch speed is too high the planes top speed is limited by the parasitic drag, not the pitch speed. My nieces Teddy was a good example of this, Speed 400, Gunther 4.9x4.7, on 8 cells the motor was turning around 13,000, that's about a 65mph pitch speed, no way was it flying at 65, maybe 45. So all the power applied to the motor above 45 was basically just wasted. I switched it to a 4.43 geared Speed 300 with a 7x6, top speeds about the same, climb performance is greatly improved and the flight times have increased.

High wing loading on an Electrostreak will get it real snap happy, a 9x6 on a 15/4 will put you right at 40 amps, might want to go to an 8x6 and drop a cell so the Jeti doesn't throw its rotor.

Mike


Power Curve
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Old Jan 23, 2004, 05:13 PM
Registered User
Ocala, Florida
Joined Sep 2003
113 Posts
I didn't even notice my typo until you mentioned. Instead of 7x5e, it should say 9x7.5e.

I get these numbers from
http://www.flyingmodels.org./index_en.htm

Thanks for your great explantion Mike. You and Vintage have definitely giving me a better understanding of the relationship of thrust and pitchspeed.

Gatorfan
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Old Jan 25, 2004, 12:29 PM
Registered User
Wayne, NJ
Joined Apr 2001
473 Posts
I'd probably go with the Hi Max, efficiency seems to be better than the AXI. Also gives more flexibility with the gearbox 6.6/1 with a 10x8 looks pretty good.

The nice thing about understanding pitch speed is that once you get a handle on it you can pretty much eyeball a plane and know where it should be. For a 4oz wing loading you're looking at 8 to 20 mph and for a 25oz wing loading its 20 to 50mph. Not a lot of rocket ships in that spread

Have fun!

Mike
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