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Old May 21, 2004, 06:27 AM
HeliAP'er!
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Surrey, UK
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Thrust from Sp400, 7-cell pack and 5.5x4.5 prop?

I would use Motocalc but my version has expired ... I still don't know how to use the flipping program!

If someone could either physically measure the thrust from this setup, or tap the specs given above into Motocalc to get the predicted thrust I would be most grateful.

I'm thinking of building a CDROM motor to provide this amount of power if not slightly more!

Thanks for any help
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Old May 21, 2004, 07:04 AM
RIP Ric
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http://brantuas.com/ezcalc/dma1.asp
..a
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Old May 21, 2004, 10:15 AM
HeliAP'er!
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Well according to that site (thanks ever so much for posting it!!) a 5.5x4.5 prop on a Graupner Speed 400 6v on a 7-cell 0500AR pack, produces 7oz or 198g of thrust at a current of 12.7A. If that is correct then I'll be over the moon!!!!!

I had this setup running ages ago which produced 220g+ at about 6-7A!!!



That was a 12 N45 magnet, 12 turn CDROM motor driving a 5x? prop (Ikarus Piccolo tail rotor!)!

I'm going down to the Sandown Model Symposium tomorrow and there'll be a stall there selling CDROM motor bits so I'll scoop up a handful of things then! .... bit short of spare drives ATM!

Could someone check that prediction please? I'm not entirely convinced that I did it right!

Thanks again! Oohoo! I'm gettin excited now!!!
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Old May 21, 2004, 04:07 PM
HeliAP'er!
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Oh ... I was looking at the wrong reading on that site - I was looking at the inflight thrust instead of the static thrust! That 220g I measured was a static thrust. The S.T. of a S400 on the setup above is 14.8oz = 419.5g!

Oh well, looks like I will have to buy a motor for it!

Of course all this may change if I see something more tasty on offer at the Sandown Model show tomorrow!

... I can hear my wallet shaking with fright ......
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Old May 21, 2004, 11:44 PM
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Don't believe implicitly everything you read. To get 14oz out of a Speed 400 direct drive with a 5.5x4.5, is, I reckon, going to need close to 200W.

I don't have test stand data for a 5.5x4.5, but with a 6x3 which should produce more thrust than a 5.5x4.5 I get:

7-cell KAN 950, 6x3 Graupner folder, 6v Speed 400:

12.0A, 6.4v, 80w, 11110 rpm, 246g (8.66oz) thrust.


Even using 3s TP 2100, 6x3 Graupner folder, 6v Speed 400, you need to draw TWICE as much current as a Speed 400 should be subjected to to get <14oz.

18.0A, 9.1v, 170W, 13250 rpm, 380g (13.38oz) thrust.

You'd need a LOT more even than that to get the 14oz with a 5.5x4.5!

Cheers, Phil
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Old May 22, 2004, 03:14 AM
HeliAP'er!
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Yeah that did pass my mind ..... 420g is a large amount of thrust for a little 6" prop to produce ....

Here's another little question:

If you want good (but not ballistic) performance from a plane such as a the Hunter, would you need the thrust from the motor to equal the weight of the plane? Presumably if the thrust was greater than the weight then not only could it prop hang but it could climb vertically, ya?

Based on your results:

7-cell KAN 950, 6x3 Graupner folder, 6v Speed 400:

12.0A, 6.4v, 80w, 11110 rpm, 246g (8.66oz) thrust


(which frankly I trust more than that website's) the Hunter should fly sweetly on approx 240g of thrust, ya? Well with the results I got (shown in that image above) I reckon I'm well on the way to being able to make a CDROM motor that can provide the same thrust as a S400, 5.5x4.5, 7-cell setup, which is EXACTLY what I'm trying to do!!!

Excellent! I feel I'm making progress at last!

Thanks for doing those tests for me Phil!

Cheers, David
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Old May 22, 2004, 09:37 AM
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A problem which many prop calculator programs seem to have is that they do not take into account the fact that a propeller with a pitch/diameter ratio greater than 0.6 is stalled statically and the results given for thrust will be higher than reality.

You can compensate for this by determining the thrust for a propeller with the same diameter turning at the same RPM, but choosing a pitch of 0.6 times the diameter.

For example:

Your 5.5 X 4.5 prop has a pitch/diameter ratio of 0.81 and is therefore stalled.

The thrust it delivers statically would be approximately the same as a 5.5 X 3.3 prop at the same RPM.

The 3.3 comes from multiplying 5.5 X 0.6.

If the program told you that the static thrust was 14.8 oz., in reality the thrust is probably closer to 2/3 of this value or approximately 10 oz.

You can determine this from the calculator by entering data for a 5.5 X 4.5 prop and noting the RPM.

Then change the pitch to 3.3 and lower the battery voltage in the calculator until you get the same RPM as you got with the 5.5 X 4.5 prop.

The result will approximate the real world thrust for a stalled prop.

The rest of your results, current, power etc., should be determined from the data given by the calculator using the 5.5 X 4.5 information.
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Old May 22, 2004, 10:23 AM
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Chattanooga, Tennessee, United States
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Yesterday I happened to acquire a 5.5 x 4.5 APC SF prop, so I thought I'd try to avoid the theoretical calculator route and try the real thing.

I fitted an ancient Speed 400 to a not recently charged 7-cell KAN 950 pack (8.6v initial voltage) and the 5.5 x 4.5 prop to my test stand to get an idea of what you can expect:

9.9A*, 6.4v, 66W, 11400 rpm, 158g (5.56oz) thrust, 2.39g/W

Now a fresh pack might get you a little bit more, but 14.8oz is in fantasy land!

I also tried 2s TP 2100 Lipoly and got essentially the same numbers:

9.6A*, 6.3v (61W), 11250 rpm, 152g (5.35oz) thrust, 2.49g/W

Hope this helps.

Cheers, Phil
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Old May 22, 2004, 10:33 AM
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For fun, I entered your data into the calculator in post #2 above, making sure that the voltage applied to the motor was the same as yours.

The results were comparable to yours, except for the thrust which was unrealistically high.

Compensating by changing the pitch to 3.3 in. and lowering the applied voltage to get the same RPM as with the 4.5 in. pitch, resulted in a thrust result much closer to your real life example.
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Old May 22, 2004, 04:59 PM
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Wow! You guys are great! Thanks for the time you put into writing those replies!

Well, as I mentioned earlier:

Of course all this may change if I see something more tasty on offer at the Sandown Model show tomorrow!

.... I did find something more appealing than the Robbe Hunter!

It's called the V.I.P. and it's made in the Czech:

http://www.shredair.com/pylon.html

Small, made of glass fibre, extremely aerodynamic, almost entirely prebuilt (pushrods, torquerods, control surfaces, servo mounting plates, motor mounting plates - all included and prefabricated), it flies extremely well on a S400, and it's fast! .... how fast? About 80mph with the S400 and up to 200mph on serious brushless! The only downside is the price - 119 ... but you really do get what you pay for - the quality of the build is outstanding. I was lucky enough to hold one in my hands at the Sandown Model Symposium and it's smoother than glass - no wrinkles, no imperfections - just the most beautiful pylon racer you've ever seen! I'm planning to run it on a 2s1p pack of 10C 2100mAh Thunder Power Lipos (at least I think that's what the bloke showed me) which should give me on the way to 10 minute flight times, if not more when using careful throttle management! I wanted something fast, and boy am I getting something that's fast! I've been browsing around on the 'High Performance' part of RCGroups and it seems there've been a fair few threads there about this plane and the general concencus is that it's an awesome plane! Gotta save up for it first though . Just splashed out on a FF9 from CyberHeli in Hong-Kong (why HK? I got a Tx, Tx & Rx NiCds, PCM Rx, and trickle charger for the same price as the Tx and Tx NiCd on its own in the UK!!!).

Please don't think to yourselves that it was a waste of time you replying to this thread - I've learnt a lot just reading your replies, and I will no doubt use the info in them in the future.

Many thanks again!

A bientot
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Old May 22, 2004, 05:12 PM
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I've always found it fun sharing information and ideas.

This thread was valuable to me because of the data posted by yourself and others.
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Old May 22, 2004, 06:06 PM
Motors beat engines!
Milwaukee Wisconsin, United States
Joined Feb 2001
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If yo can fit a 3stp2100 pak in there, use a 7.2V 400 instead and watch theat sucker scream! About 15k rpm with an apc 5.5x4.5 prop, and about 12 amps.


Dean in Milwaukee
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Old May 22, 2004, 06:48 PM
HeliAP'er!
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If yo can fit a 3stp2100 pak in there, use a 7.2V 400 instead and watch theat sucker scream! About 15k rpm with an apc 5.5x4.5 prop, and about 12 amps.

You kidding?! The bloke on the stall which sells them said I'm looking at about 20A on a 2s LiPo and a S400! .... although true he didn't say which version of the 400 that was with ...

On a 3s of 2100mAh LiPos I could get 10 minutes full throttle!!!
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Old May 22, 2004, 07:38 PM
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United States, WI, New Berlin
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Deans right.....That's how I was flying my split 280, or should I call it a speed 280 since with that setup it flys about 60 mph

I was using a sp4007.2v to keep the amps down and a 3s 2100tp and a 5.5x4.5 prop.

I found an even better setup by just switching the prop to a 5x3.
Motor barely gets warm...speed only drops to 52mph...thrust stays the same at 9oz with flight times at 20 minutes.
AUW of my plane is 12 1/2 oz.

Randy
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Old May 22, 2004, 08:46 PM
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"The bloke on the stall which sells them said I'm looking at about 20A on a 2s LiPo and a S400! .... although true he didn't say which version of the 400 that was with ..."

Hogster, believe me, you CAN'T expect to put 20A thru ANY Speed 400 (6v or 7.2v) and have it survive more than a few nanoseconds! As Dean says, 3s on a 7.2v 400 at 12A may be okay for pylon stuff, but 20A - who are you kidding! For long term survival 10A is about the limit.

Cheers, Phil
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