Hobbyking Radjet 800mm motor?

[Pilot Ben 28]

So I'm having trouble finding a motor for my radjet. I'm hoping someone can point me the right direction. I think it's time for a crazy plane so here I am, but I do genuinely mean ballistic missile fast!!!

I'm looking at the https://www.overlander.co.uk/tornado-thumper-v3-3542-05-1250kv-brushless-outrunner-rc-motor.html  for the plane. I plan to use a https://www.overlander.co.uk/speed-controllers-ubec/speed-controllers/overlander-xp2-speed-controllers/overlander-xp2-60a-sbec-brushless-speed-controller-esc.html for the ESC and a 4s 1300mah LiPo.

Now for the maths.

1lb is 450g give or take and with my uprated bits and bobs I'm thinking 500g flying weight so for simplicity's sake that's a plane that weighs 1lb

 

The rule I've always employed is;

Watts/lb

30 to 50 for ROG

40 to 60 for sport aerobatics

70 to 100 for 'good' aerobatics and EDF jets

180 to 300 for some serious speed

350+ for the RAF to be called about a ground to air missile heading to heathrow coming from a field in Croydon.

 

The motor I have picked out will run on a 4s with a 9x5 prop and should deliver 710 watts. 

 

Now I want something the will reach triple figures and keep on going. 

To put this in perspective my wot 4 foam e delivers 303 watts and it weighs 2.4lb. That motor punches it skyward at a fairly decent rate. The radjet weighs under half that and the motor gives me over twice the power.

The question I have is; will 710 watts rip the plane apart? 

Needless to say I have modded the plane greatly with many carbon rods, carbon strips, glass tape and double tape on the hinges! And I have seen radjets on youtube do this kind of speed so I know its possible!

 

Any input would be greatly appreciated guys!

 

B

 

P.S. Can anyone recommend and servos? They should be 9g size. Will these be up to it? https://wheelspinmodels.co.uk/i/109052/

 

 

Edited September 10, 2017 by Pilot Ben

[Bravedan 106]

MMMM, well, having gone the "how fast can you go" route several times over the years, inc getting as far as customising home wound epoxy potted and balanced large inrunners, a few comments generally first. Where do you intend to fly this plane?

1.The CAMFC site boundaries are quite tight/close, and there is that ever present height limit. A truly fast plane covers so much ground so quickly that you really don't get ANY decent running.

2. It is a noise sensitive urban site near school and housing and don't for a moment think that ultra fast high power pusher electric is acceptably quiet. Front prop pylon or hotliner shape is MUCH preferable and significantly quieter.

3.There is a point where operating at real speed at CAMFC becomes a problem rather than fun, as you are forever lifting and turning fighting to keep in bounds and not let the speed decay. The slightest issue and you will be WAY out of bounds.

The last super fast pusher wing of mine (triple spar FW Slipstream with above motor) was far too fast for the site and too noisy. If I lifted off suddenly there was a triple bounce echo off the trees, and, like a Jet Ski, it was an annoying high pitched howl. Note that any conventional noise test will not discover this issue, its a feature of the airspeed and the prop chopping the trailing edge, so I quickly "banned" it myself, the only sensible thing to do! The wing now has a much less powerful and lower revving motor, revs limited by a high pitch prop and part throttle use, still quick for the site but no speed demon now, more a fast cruiser, flies most of the time at half throttle or less, and carries FPV.

Now to your specific plane and planned spec.

A 1250kV motor is very low for an ultra fast wing, I would not contemplate less than 1800 (std Radjet is 1850) and I use 2200-2500kV.  The penalty of low kV is that you have to swing a bigger prop, and torque roll becomes a serious issue esp at launch. If you cannot accelerate quickly due this you will never reach the top speed possible as before it gets "onto the step" you will be lifting to turn or do a reversal. Don't think its an issue?  My Slipstream even with a small dia prop needed an appreciable amount of aileron held to counter torque not just at launch but also as speed rose above three figures. And it covered the boundary to boundary limits in well under two seconds.

Also, have you done the maths on the current needed against what the battery can TRULY deliver, not the mythical claimed "C", and how long you will get?  No way would I contemplate a battery as small as 1300mAh. I was running 2300mAh 60C/120C 10 seconds surge and that was minimal!

Have to say though that my biggest concern is that you are heading towards exceeding the practical capabilities of the site. In any event, you will get much more fun from a less aggressive and more controllable set up, they are great fun with ridiculously fast roll rates and in a flat spin but be careful of one from inverted as they really don't want to exit once in!

Sorry to be harsh, but what servo you run is for now the least of your concerns!!

BTW, my own design delta "Rasp" is a broadly similar but site capable and noise sensitive design and set up, and that has a 2212-6 2100kV Suppo with 6x5, 80A ESC, 2200mAh 60C 3S.

[Pilot Ben 28]

Right thanks Dave lots of food for thought. So 710 is too many watts I gather!

I was hoping to fly it at CAMFC but then move it to Sevenoaks for the speed demon.

I went for the low KV in the hope that it would reduce noise?

I didn't really plan on flying at the speed of sound all the time, just flying around at a decent speed but well below triple figures and then occasionally go from one far side of the field to the other at full chat.

I was hovering between 2200mah and 1300mah but decided one 1300mah for weight.

 

Are there any motors you can suggest that run on 3s or 4s that would give me the turn of speed I want, but meets noise regulations and is flyable at CAMFC?

What about this? Runs on a smaller prop has less power and has a much higher KV? https://www.overlander.co.uk/bm400-2845-09-2100kv-brushless-inrunner-rc-motor.html 

Is an inrunner or outrunner better?

 

Thanks Again,

 

B

 

It's also worth saying I don't want a full time ICBM! I was hoping for a generally fast plane (like equaliser plyon racer) but has a big turn of speed on full throttle....

Although preferably if it runs on a 3S that would be best so I can just use my 2200mah....

B

[Emma 47]

16 hours ago, Pilot Ben said:

350+ for the RAF to be called about a ground to air missile heading to heathrow coming from a field in Croydon.

Thanks Ben i needed cheering up.

Any techies recommend a good way to clean coffee of a laptop screen and keyboard

Edited September 11, 2017 by ChasePlane
sticky keyboard !

[Bravedan 106]

Watch this...................

 

https://youtu.be/qwuY-YsCtOg

 

Sucked in yet????    !!!!!

[Bravedan 106]

Ben, the issue is not really the motor size, but is linked to the prop position. Pusher props are inherently much noisier than tractor. This follows into real world, the Piaggio Avanti P180 which comes in and out of BH sometimes is WAAAAAYYY noisier than anything else (It's turboprop but pusher twin). Props reflect noise forwards, as said above "chopping" the wing trailing edge. This is immutable and limits the sensible to how far they push the boundary!  (Piaggio have allegedly spent huge sums on trying the quieten the Avanti, bit like Porche spending 40 years making a rear engine car handle, when the answer was staring them in the bonnet)

People seem to think my Superfly and Rasp are quite fast and they only have this motor:-  

http://www.ebay.co.uk/itm/Practical-A2212-6T-2200KV-Brushless-Outrunner-Motor-for-RC-Helicopter-AircraftUK-/162399956343

Lots of suppliers inc UK stock, just random picked this one. Not suggesting you choose it, but it works for me giving the best combo I can find of noise against speed in pusher ( or prop in slot) config. And cheap as chips.

Neither plane named above are quiet, though acceptable during mainstream times. DEF not for electric only days or after the watershed!!

Personally I think the days of the Radjet and its MPX progenitor the Funjet are well past, and yes I know that's a shame when you are part way through a build, but that's how it is!!

I'd aim for 250 - 300 watts MAX on 3S and have fun.

BTW, please please PLEASE do NOT launch it like a lot of the brain dead iggits on the Internet so your hand comes close to prop on follow through. SERIOUS crippling accident waiting to happen (and has lots of times!!). Hold a wing out of the prop arc and reverse discus throw, do not hold the fus!!!

BTW BTW, you mention Sevenoaks, but while less trees, they have the same height limitation issue, don't they?!

One of THE best fast planes is the HK/Dynam EFRacer and its higher powered newer model the EFXtra, and by comparison they are almost silent! Even the slower bog std EFRacer on only 3S (they can run 4S with a prop change) can out pace an Equaliser. What's more they are genuinely easy to handle, great fun and versatile, the Racer even able to do passable pattern aerobatic and slope soar well.

Said all I can say now, Good Luck!

[Bravedan 106]

For anyone following, I replied to Ben on the EFRacer/EXEtxtra by PM, Yes, I have both. Yes, I'm REALLY glad I have.

OK, simple talking over, demo time.       Necc explanation first though.  

I tried to find archive video of my various 2212-6T motored models, but could not quickly, Google would not let me into one of my accounts (BOO) , so yesterday Ivan filmed me flying the "Rasp" own design EPP delta (HURRAH!).

Now the Rasp is NOT the fastest of these models, as in its second iteration (rebuild after motor mount failed years ago, thread and comment buried in Flying Diary for 2016, do a search on "Rasp" to find), I could not be bothered with the air brakes.  These were flat plates which were servo operated out from either side of the Fus just behind the CG. When rebuilt I did nothing to smooth the airflow , the twin fins for example are 8mm EPP and square edged, so much more draggy!  (Stop wincing at the back!).

Anyway, attached is a small clip from yesterday, one pass, due file size EMail limit. This was with a two year old much used 2200mAh 3S. Bear in mind the conditions yesterday were erratic yet the Rasp cut through it all smoothly.  Probably a longer clip of the flight on EBay later if Ivan can find the time. (half second roll rate, etc).

Count the time boundary to boundary......................

 

 

 

Dave B Speed clip.mp4