timstoro

I have 2 complete kits in the box. I'm in need of any info or pictures of this plane before I start building. I have only one engine package which is a K&B .81 w/pipe and Dynomax fan unit don't know if this will work.

JET FX

I think I know the kits you have, I believe manufactured by a company called "Knights of the Air" they have been out of production for some time now, last I saw them advertised was in the early to mid 90's from memory. I would suggest trying to source more information by using to the RCU "search" feature, try typing 'Knights of the Air' OR Mig-25 and see what comes up. You may also try starting a thread in the main RCU jet forums with a request for any info and likely someone will offer further assistance. I think the K&B 81 and Dynamax will be rather marginal for power. If going D/F I would suggest an O.S. 91 engine and the Dynamax. Another option maybe even to consider electric D/F as current technology is even more powerful than the glow powered systems......Contact Jet Model Products for further information on ideal Dynamax engine and pipe power combinations. Sorry I could not be helpful, good luck with the project.

Brg- Marty

watt_the!

Another option maybe even to consider electric D/F as current technology is even more powerful than the glow powered systems

umm, i wish. unfortunately lipo tech is still a long way off glow/ic powered systems and i'm certain that as long as RCers climb over each other to get so called "high c" lipos the technology wont advance too fast either.

we're in a bit of a catch 22 where lipos are the best weight/energy cell choice and high C lipos drive down the price of low C lipos but the demand for high C means low C packs are becoming difficult to get.

you might have a case for paralleling up low C lipos as they work out alot cheaper and deliver the same performance at much lower weight than the high C lipos. But economics is basic supply and demand ad the public get woo'd by this "high C" label- also at the expense of low numbers of battery cycles...sigh.

anyways, the weight trade off, cost of electrics etc is still miles behind even ICDF for big power. I dont recall exactly but a single dynamax/os90 setup runs at about $400US or so, plus about 2lb of fuel/tanks etc. so without any research i'd guess 6-7 lb of ICDF total for 12-13lb thrust..per fan/engine combo.

To get that from an electric youd be looking at about 2-2.5kW which is about 28 x 2500mah cells depending on what motor etc you use..maybe say a 8S8000mah pack to get 4-6 mins flight. A standard 8S8000mah lipo weighs about 1.3kg (3lb) and will cost you around $700US.

add in the high voltage ESC and the motor and allow for another $300US...at least

so the electric route is going to cost at least $1kUS per motor setup- without the fan unit.

e rc has a long way to go yet.

and this is april 2008.

JET FX

Well sorry to disagree but in my opinion EDF is still a very viable "option" to glow D/F power for .90 size D/F model designed airframes. Yes cost is another factor of that there is no doubt but as far as the performance advantages go!!..... 20 lb thrust from such units as the new EVF 6215 or 6416 is hard to ignor. One thing for sure its nice to have more options nowadays anyway

http://www.bvmjets.com/Pages/electric.html

watt_the!

wow if those prices are inclusive of batteries then that looks like a good deal.

but im not so sure...visiting the battery pages it's clear that the packs themselves cost as much as the EDF units...approx.

the 14lb thrust 10S (EVF4010) system comes in at $2,000US total and weighs 2.5lb (batteries only)...the unit must weigh at least another 1.5lb i would think...

so thats 4lb of electric system and $2kUS cost. At about 100 cycles per pack minimum, thats $10 per flight approx from the batteries.

i just cant see how the pricings at the BVM site are inclusive of batteries..the cells alone are as pricey as the EDF systems..so the Systems themselves would be FOC if the price was inclusive.

Tim

JET FX

Hi Tim, I agree the battery cost is the factor that puts many off at present... On the flip side some people are looking for a turbine alternative for other reasons because noise or fire hazard restrictions. Living in Australia as we both do its all to familier living with fire restrictions etc. My local club here in Sydney is slap bang in the centre of Kurringi National Park so that alone stops the turbine option due to the bushfire risk. D/F's well yes they are a cheaper option again but of course you really need to know your stuff with two stroke engine and pipe tuning to get the most performance wise.... Give it another year or so and I reckon EDF battery prices should start dropping to more acceptable levels for many of the fence sitters, myself included


Cheers- Marty

watt_the!

yep i agree..the edf is attractive wrt noise,environs etc.

its interesting though- i havent done the sums etc...but i'd be interested to see what the cost was using low C packs with that BVM unit..or even one of those el cheapo chinese 12 inch fans..

something else doesnt match either from that bvm site...it states 5000W for 12S and a max of 50A. By my calcs that's "only" about 2100W maximum. and more likely "only" around 2000W.

so i guess im sceptical about the thrust claims also now as the OS90 fan on dynamax and ramtec etc delivered about 13lb at 22000rpm and power output was around 4-4.5hp (2,900-3,400W)

the plot thickens when you look at the motor used...its a great motor but it is over 1lb on its own and has a max power rating of 3000W. So forget the 5000W claim...well i wouldnt be putting a $330US Neu motor 60% over max rating thats for sure.

going from that motor, kv is about 780rpm so at 12S- about 42V max, thats 32000rpm!

considering that the OS90 delivers 4.5hp (3400W) and can only turn the fan at 24000rpm and get 13lb something aint right....

IF the fans are the same then a 1/3 increase in rpm needs a 2.2 x power increase...or around 10hp!!!...and thrust at 32000rpm would be around 22lb.


so im thinking that its all BS...OR the fan has adopted less aggressive and/or fewer blades.

ill have to get excel out one day and do some math...i wonder what the cost is for the low C batteries....


a bit off tangent i know, but very relevant to these big DF jets. I'd be inclined to get hold of a couple of OS91s and dynamax fans...no tuned pipes. OR ditch the BVM and go for as big a fan as possible to maximise efficiency. 4+ hp into a 5inch fan will waste alot of power.

are there any bigger than the ramtecs/dynamaxes?

JET FX

Bigger D/F than the Dynamax/Ramtec.... hmmm not sure? none that I can think of at least commercially available anyway.
I do not know if the thrust claims of 20 lbs for the BVM EDF or for that matter the claims of 13-14 lb D/F's. I certainly have never seen either systems or any models flying that would back those claims up!

There is one EDF system that is worth checking out. Its is designed and produced by an Australian Stuart Maxwell of Stumax fame based here in Sydney. Performance of Stu's EDF is at least on par with what else is out there at present Iam hearing and looks the goods to power many of the existing .90 D/F sized airframes and even some of the turbine designed models. We will be testing one of Stu's fans in a Fei Bao F-5 soon I hope with other airframes to follow including a Kangaroo and maybe an Fei Bao L-39 as well. Just waiting for battery costs to lower now....

Cheers- Marty

watt_the!

yeah i know the stumax well.

actually he seems to have been trumped by turbofans and the tf8000 delivers 26lb thrust!