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Bobcat Conversion

Old 05-30-2019, 12:30 AM
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djmp69
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Default Bobcat Conversion

I know there's another forum about conversions, but its all people talking about converting an electric to glow. Plus, they are all talking about propped motors. I want to go the other way around. I have a Bobcat that had a .60 on it, and I want to put a fan on it. What better place to come to for advice than a forum about jets? So my problem is everywhere I have gone for research, people are always talking about volts and amps and what not. I know that's important, and I understand you have to match up ESCs, motors, BEC capabilities and all that. What I need to know is thrust, and no one talks about it. How many amps a motor/fan combo draws, while important in its own right, is not going to tell me if it's a good combo for a *lbs plane.
I have seen a coupla videos, and the common setup seems to be anywheere from 90mm to 115. So I figured on a 90mm. Now if I can get away with 6s, great. From what I've learned over the years, I think 6s, if it works, may just get the plane up, some slow scale flight. Intermediate, seems more like an 8-10s, and for balls out, 12s. But, what motor/fan/esc combo? Has anyone had any experience with a good setup that's not going to cost $1000? Plus, an 8s or 10s I;m sure is a bit heavy, and I'm not well versed on running two smaller packs in series and the pros and cons of that.

So. Hopefully I've explained my situation clearly enough without too much gibberish. I apologize in advance if not. Anyone have any suggestions on what would be a good place to start with this plane? I weighed it w/o engine and it came up 6.5 lbs. After a 6s battery, fan and esc, but minus a servo and tank, Im guessing the weight at around 8lbs. ish. Not thinking of retracts yet. Plus I plan to build a nacelle for it with a bit of thrust tube.

Any ideas?

I've looked at the Freewing 8s 12 blade, the Dr Mad Thrust and others. All the current draw and volt specs are all over the place, but no idea of thrust. I know that's hard to say without all things considered, but I need to know where to at least start. I don't have the time nor do I wish to spend unecessary money just experimenting for nothing.

Thanks in advance!
Old 05-30-2019, 01:32 AM
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Joseph Frost
 
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Have you got a pic of the model, what is the wing span and full flying square area?, good thing to know before making decision about power unit. Guessing about AUW is never good enough, you got to know precisely these figures to decide on the set up to make it successfully flying model.
Old 05-30-2019, 11:15 AM
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djmp69
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Name:	silverbobcat1.jpg
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ID:	2264289

Wing Span: 51 in / 1300 mm
Wing Area: 666.5 sq in / 43 sq dm
Fuselage Length: 60 in / 1525 mm
Engine Required: 2c 0.46-0.60 cu in or 4c 0.52 - 0.70 cu in
Old 05-30-2019, 03:06 PM
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Joseph Frost
 
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Sleek looking, I would expect 5 kilo AUW all fitted out with 90mm/8S power set up. Still reasonably light wing loaded at some 115g/dm/sq. Your challenge will be to fit the power unit to the frame.
I would modify the rear with semi exposed fan nacelle attached, pretruding slightly backward.
Forget about cheap chines crap like Dr. Max, go for something decent like Jet Fan, etc.
Should move like a missile with the right set up, mate of mine got a larger turbine powered version, he did 409km/h pass at one of our jet meets with his.
Good luck.
Old 05-31-2019, 10:06 PM
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Originally Posted by djmp69 View Post
I've looked at the Freewing 8s 12 blade, the Dr Mad Thrust and others. All the current draw and volt specs are all over the place, but no idea of thrust. I know that's hard to say without all things considered, but I need to know where to at least start. I don't have the time nor do I wish to spend unecessary money just experimenting for nothing.

Thanks in advance!
I would stay away from the cheap Chinese fans, the rotors are knockoffs and not all that well engineered, they make thrust on the bench but have poor velocity in the air. Also heavy especially the mad thrust fans which are just lander anyway, their 8s fan is almost 200 grams heavier than the Jetfan combos we do, pull more amps and produce less thrust.

Your model is a perfect 90mm size jet, I wouldn't go bigger as you are going to need to clear the deck at the rear of fuse anyway in order to create a scoop area in front of the fan to allow clean flow. I would look at cutting from a little behind the canopy in the shape of the tip of a spoon, like a curved naca duct, around and down to the back where the motor is sitting, then you can create a concave shaped piece that fills this area up, with the fan mounted where the motor is.

I would suggest a fan like THIS as it gives you the best combination of thrust and performance with one of the lowest amp draws for the power. I sell a ton of this 8s version, all the Blackhorse jet fliers go for this as its easy to install, light and powerful, 4.5kg thrust on 8 cells with strong batteries.

You will need to make up a tailcone, tapered down to around 80mm for the Jetfan, around 6 inches long is ok. I would suggest THIS speed controller, its compact and very tough, you can mount it in front of the fan with the heatsink sticking out of the new paneling you install at the back of the fuse. It would all look really neat and work very well. I have had a few customers convert bobcat style models over the years, great performance with the right fan.
If you need any help shoot me an email, [email protected]
Old 05-31-2019, 11:37 PM
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djmp69
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Really good practical and relevant info, thanx! The plane is actually already designed like you describe, I think they knew people would do this kind of conversion. I wanted to keep costs down, especially not wanting to spend hundreds just for 3-4 minutes of flight at a time, but it seems theres a give and take here just like everything else in this hobby, lol. Good stuff, confirms my suspicions, thanks! Now, if only someone had some thrust numbers...

Last edited by djmp69; 05-31-2019 at 11:40 PM.
Old 06-02-2019, 01:28 AM
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Originally Posted by djmp69 View Post
Really good practical and relevant info, thanx! The plane is actually already designed like you describe, I think they knew people would do this kind of conversion. I wanted to keep costs down, especially not wanting to spend hundreds just for 3-4 minutes of flight at a time, but it seems theres a give and take here just like everything else in this hobby, lol. Good stuff, confirms my suspicions, thanks! Now, if only someone had some thrust numbers...
Regarding the thrust, 3.5-4kilos with decent 8S set up will be good enough to zoom around your model at 5 kilos, AUW. With the cheap china set ups it will struggle.
If you keep it simple, very light, as bungee launcher, no landing gear you might get away with decent 6S power around 2.5-3.5 kilos thrust.
This are just some rough figures based on my experience.
Old 06-02-2019, 02:22 AM
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Sounds great, seems to confirm my suspicions, as I said before. After I convert all this metric kilos stuff to something that makes sense to me, lol, I'll have a better idea. Actually, I think I have a good idea, but I am wondering WHICH fan/motor setup will give it to me. I am not trying to spend $800 to make a $99 plane fly. So as you can see, I am trying to get to the meeting point. I guess what I'm missing is that everyone keeps saying stay away from the cheap Chinese stuff. How does it differ? 90mm is 90mm, and 8s is 8s. 10s is 10s, and so on. Are we talking reliability, longevity? Please forgive me, I am an expert with gas and glow, but am still getting clues when it comes to electric setups. I figure if a setup can statically move an 8lb plane, then it should be able to dynamically fly the 8lb plane without too many problems. Am I thinking wrong here? If not, WHICH setup can accomplish this?

Thanx again!
Old 06-02-2019, 03:41 AM
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are ya old enough to remember the old plastic grish "tornado" props. the ones that would explode if ya ran em at too high an rpm level?

imagine 10-14 blades doing that.

really, each fan has its own blade pitch and width factors that determine how it performs and at what rpm it works best, just like props of different pitch/diameter do.
jetfan,stumax and a few others have done the homework to get it all pretty much right.

add in
https://www.rcgroups.com/forums/show...Jetfan-100-PRO
posts here with data charts #24 and #34 have additional info.
somewhere over there is a thread for the 90mm as well

not so much for the cheep chinese knock offs.

gary, at eflux r/c here in the states has links to information on motors/kv/cells/thrust for the jetfan stuff.

Last edited by mongo; 06-02-2019 at 04:02 AM.
Old 06-03-2019, 12:07 AM
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Joseph Frost
 
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Originally Posted by djmp69 View Post
Sounds great, seems to confirm my suspicions, as I said before. After I convert all this metric kilos stuff to something that makes sense to me, lol, I'll have a better idea. Actually, I think I have a good idea, but I am wondering WHICH fan/motor setup will give it to me. I am not trying to spend $800 to make a $99 plane fly. So as you can see, I am trying to get to the meeting point. I guess what I'm missing is that everyone keeps saying stay away from the cheap Chinese stuff. How does it differ? 90mm is 90mm, and 8s is 8s. 10s is 10s, and so on. Are we talking reliability, longevity? Please forgive me, I am an expert with gas and glow, but am still getting clues when it comes to electric setups. I figure if a setup can statically move an 8lb plane, then it should be able to dynamically fly the 8lb plane without too many problems. Am I thinking wrong here? If not, WHICH setup can accomplish this?

Thanx again!
Hi Body, you have probably heard the old saying, "I'm not wealthy enough to buy rubbish"! Unfortunately you pay more for quality, but if your budget is limited , shop around in classifieds, you can buy great set ups from 2nd. hand at bargain price. There's so much of stuff avail, it's shoppers paradise. Offer always lot less, most of them take it!
As Mongo mentioned, Gary from Eflux is great to deal with if you go for quality product.
Old 06-04-2019, 10:20 AM
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Hi: Djmp69
I had the 90mm FMS F-16 super scale and the battery died in flight..... Lucky for me the F-16 body protected the Fan and electronics during the crash.
I moved the 12 blade 90mm fan and 130amp ESC onto a Bob Cat 50 and that jet can really move. I run a 6 cell 5200 mah pack.

Wayne
Old 06-04-2019, 10:22 AM
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The BobCat 50 is between the A-4 Sky hawk and the Evel Knievle Falcon 120 at the bottom of the photo.

wayne
Old 06-04-2019, 03:31 PM
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Nice line up, and what a magic flying strip. Down here we have to put up with 'goat tracks'! Interesting to see the amount of down thrust required for these models, with all my deltas it's just opposite.
What's your total weight with that 6S set up? Any flying footage?
Old 06-04-2019, 04:34 PM
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Originally Posted by djmp69 View Post
Really good practical and relevant info, thanx! The plane is actually already designed like you describe, I think they knew people would do this kind of conversion. I wanted to keep costs down, especially not wanting to spend hundreds just for 3-4 minutes of flight at a time, but it seems theres a give and take here just like everything else in this hobby, lol. Good stuff, confirms my suspicions, thanks! Now, if only someone had some thrust numbers...
The issue with most of the Chinese fans is they are copies of western designs, usually missing key elements which affect their efficiency and performance. I have had many of the Chinese fans on the bench including the best of them, the all alloy JP90 fan unit. The JP90 pulls around 2800 watts for 3.95kg thrust, compare that to the GRP Jetfan90 pulling 2900 watts for 4.5kg thrust, its a big difference in output! And the JP is the best of the Chinese I have seen.

Most of them pull a LOT more amps than the Euro fans and its because the rotors are loading up the motors too much, and the motors they use are cheap to keep the cost down. Dr Mad Thrust (lander) use oversize motors to make sure they don't burn up, that's why their fans are 150-200 grams heavier than the European ones, and their knockoff rotors while showing good thrust figures static (albeit at a much higher amp draw), lack the dynamic thrust to give you speed in the air. Most of the copies lose a lot of thrust due to the ducting in the models too, something you wont have to worry about as you have no ducting! The hi blade count copies do not like complex or long ducting, they exhibit a lot of losses whereas fitting a German Wemotec you wont see the same losses.

There are a lot of thrust figures on my website, I test most of the setups I list I don't rely on manufacturers specs like most sellers. I have been doing custom hi performance fans since 2005, send them all round the world. This link will take you to the EXTREME GRP Jetfan page, every listing has performance specs, amps, watts, thrust.

There are two 8 cell setups listed, power levels are 87A for 4.2kg thrust or 100A for 4.5kg thrust. The mild fan is very popular with guys wanting to run in sports models with smaller batteries, you can use a pair of 4s4000mah packs no problem. With the hotter fan you really need 5000mah packs.

If you are after a factory carbon Jetfan then all the combos are listed HERE. Each listing has the performance figures, amp draw and thrust are what you want to know so you can choose a combo that gives you a balance of weight and thrust.

You also have the choice of the 11 blade Wemotec 90mm EVO, its a little cheaper option, highly engineered made in Germany, runs the same motors as the Jetfan and presents almost the exact same load hence why they often share the same Kv for a particular setup, 6s, 8s, 10s etc. The wemo like the Extreme GRP Jetfan will bolt straight down to rails, their mounting tabs are in the same position, housings are essentially the same size. The WEMO is also very light, one of the lightest combos you will get, about 20 grams lighter than our GRP jetfan units. The Wemo 8 cell combo for example works out to around USD$198.00.

You don't need to spend 800 bucks on a power system for the model, here is an example of a great 8s setup:
Extreme GRP Jetfan 8s 3Kw USD$225.00
HW V5 110A 14s Opto ESC USD$130.00

Order yourself a pair of 4s5000mah 45c batteries locally and you are good to go. Use an Rx pack or UBEC to power all your radio gear, I use a 5A HV-UBEC in my 90mm jets as it will run off the main packs, anything bigger I use an 8AUBEC with a 2s1800 feeding it.
I have set up a lot of power systems for guys converting old pusher models like the bobcat, and have done plenty of fans for models like the multiplex funjet and all the copies out there, guys have done the same thing as you are planning only in foam, cut the rear deck down and mount the fan up the back exactly as in your photo of the BC50, it works really well and performance is always great if a quality fan is used. Bear in mind once you have this sort of quality fan combo, it will drop into any 90mm jet down the track, it will last for years, it comes dynamically balanced (I have been offering dynamically balanced ready to install fan units since 2006) and if you damage any part of it you can order ANY part to fix it!

Last edited by Extreme_RC; 06-04-2019 at 04:38 PM.
Old 06-05-2019, 09:10 AM
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Hi: Joseph
The all up weight on my BobCat is 10lbs.
On all the BobCat , Falcon 120's , King cats........ect... type jets. The 5 degrees of down thrust keeps the jet from lifting at high speeds.
I have down thrust in my Boomerang and it helps keep it level at high speeds.

If the turbine is sitting level on its mount, The jet will lift like its out of trim.

wayne
Old 06-05-2019, 03:14 PM
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Joseph Frost
 
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Thanks, I expected some 4.5kg on 6S, so 8S set up will be easily at 5 kilo AUW. I have seen number of models with exposed power units having lot of down thrust in relation to the fuse line, very similar tho strait wing my "A-10Xs" have it strait in line with no weird tendencies. Fuel tank level makes a slight difference, being nose heavy on full to avoid being tail heavy on 1/3 of the tank towards the end of flying. No such issues with E-power but unfortunately landing at same weight with limited juice.

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