MiG-21 MF (Fishbed J) Scratchbuilt - Page 7 - RC Groups
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This thread is privately moderated by Herb, who may elect to delete unwanted replies.
Mar 20, 2012, 02:16 PM
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I would say yes ! I remember the JHH F-86 flies fine on a 90mm WeMo, and this fan is only slightly larger.

I still think this is a very good fan for the money, bit it absolutely needs to be balanced !!
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Jun 25, 2012, 04:56 AM
SoCal, Year Round Flying!
bmiller's Avatar
For fairviews short dirt runway how about more blades to pull air past the radar and thru that loooong duct!
Should give it some extra low speed thrust for the takeoff!
And it'll have that "whoosh" sound too!LOL
Remember your sapac foam hawk shubie vs wemo test flights?
The wemo kicked ass!

Note the time of my post! I got the retracts/struts drilled and assembled for the F100.
Thanks for those 4mm strut wires!
Jun 25, 2012, 02:32 PM
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Yes that sharp intake lip (needed for scale) robs thrust at low speed.

More flight maybe tomorrow (Tuesday) ... ?

I did some maintenance work & cleanup on the nosegear ...
Jun 25, 2012, 02:46 PM
SoCal, Year Round Flying!
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Ok for tuesday flying.
Always amazed how stout those springairs are.
btw Probably flying friday at apollo w/Gregg. you in?
Jun 27, 2012, 12:27 AM
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Pictures came out nice! thanks for posting them on the F-4 thread.
looked at hobbypartz for gens ace packs...out of stock for the size and cell counts I use...
Glad the Mig is flying well after I screwed up the trim sunday....sorry bout that...
Jun 28, 2012, 12:57 PM
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Yes a few pics came out nice

Jun 28, 2012, 11:08 PM
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bmiller's Avatar
Missed you at the field today. Habu32 instead of F-4 this time.
Vince had a flyfly F-100 which he flew so I got to check out his 6S setup and pester him with build questions/mods. Thing flys great on 1800W and floats in. Inspired to try finish mine by this weekend!
Jul 06, 2012, 12:11 PM
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Too bad I missed the F-100 ... Sprayed another can of tamiya red on the MiG-21
Jul 08, 2012, 02:39 AM
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Here's the battery IR "true C" calculator website I use in conjunction with my ESRmeter.
Jul 08, 2012, 02:20 PM
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Thanks for the link ... the guy is a bit confused about the relationship between pack impedance Z (IR) and C rating.

It's not what he claims, it's significantly more complex.

IR is a completely well defined number: The battery's internal resistance Z you can just compute from the battery voltage drop,

V0 - V = I * Z

V= voltage under load, V0 voltage with no load (eg. 4.20 V on a charged 1S pack), I current under load, Z = battery impedance (at 0 Hz) or pack "internal resistance" . This is basically a modified Ohm's law: http://data.energizer.com/PDFs/BatteryIR.pdf

As an example, if on a fully charged 6S pack your voltage drops from 25.2V to 22.2V under a full load of 80A as measured by your Watt meter (each cell is then dropping from 4.20V to 3.70V, so that 0.50V* 6 =3.0V) then the internal resistance is

Z = 3.0V / 80A = 0.0375 Ohm = 37.5 m Ohm ( per cell that 6.25 mOhm).

It's as simple as that. This is what your device (that grey box) measures, but a pocket calculator will give you exactly the same number ...

(In fact the watt meter / pocket calculator combination will give you a more accurate number, relevant for an 80A load as opposed to the mA load your grey box uses, simply for the reason that the IR is weakly dependent on current ...)

C rating on the other hand is more complex and subjective. What current can the battery sustain w/o damage? That depends on the quality of the electrolyte, by how thick the copper film is inside the battery, how thick the pcb's and cables are etc. That is the reason why C ratings are all over the place. e.g Some just slap on a thicker wire and calls it 40C ...

Nevertheless a battery with a lower Z/IR will get less hot, delivers a bit more punch (more current), blows your motor and esc earlier and hopefully also lasts a bit longer because it gets less hot ...

Personally, if I need more power I just use a bigger battery instead of screwing around with IR's
Last edited by Herb; Jul 08, 2012 at 02:35 PM.
Jul 08, 2012, 11:08 PM
SoCal, Year Round Flying!
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Thanks for the detailed info..very informative!
Unlike most other gadgets out there, the ESRmeter applies (IIRC) a 30A draw for it's pack numbers to give a more real world IR reading.
It may apply a mA load for individual cells...I'll have to check that.
Anyway it's a great aid for us non physics professors!LOL
Regardless I still believe in under 20C draws from any battery so I TRY (but sometimes fail) to use that as my guide for pack size.
Jul 13, 2012, 01:07 PM
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I should really finish up the Mig-21

Clearcoat applied :

Jul 20, 2012, 09:26 PM
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F-100 ready for tomorrow!
Mar 17, 2013, 08:31 AM
Registered User
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Thhanks for posting your build, and viedos. I have enjoyed it a lot.

Bought the plan several years ago, and not sure how air worthy it was.

The first landing was perfect....

Mar 17, 2013, 01:34 PM
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Hi Thanks

The MiG-21 is a nice flier once dialed in properly (CG, throws, expo). It's just not an aerobatic plane ...

The main thing about building from plans is to build it LIGHT: select components with a postal scale, weigh every part and use lightweight glues, fillers, paints and fiberglass cloth where you can. This is absolutely crucial in an edf.

Large turbine MiG-21's fly nicely too, see youtube under Airworld MiG-21

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