HK EPO Parjet, ARF - Page 34 - RC Groups
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Feb 24, 2012, 02:38 PM
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gchrisgo34's Avatar
Hey solentlife, whats your speed record? hopefully I'll be breaking it soon. turnigy 28-36 3200kv, 6x4 prop, 26c 3 cell batts. i guess you have a 4 cell settup, ive only got 3 cell batts small enough for the parkjet, so not sure if I'll be able to. It'll be fun trying though. ive also got the 28-36 2350kv, if i ever want to try it.
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Feb 24, 2012, 10:49 PM
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clearly you guys are a bunch of speed junkies........possibly in need of help,so here i am to try and help.

to start with an intro:" hi,my names stu...and I'm a speed junkie" where you all say "hi stu,so are we!"

Nigel [solentlife] has been mapping his builds for reaching top speed on a pj now for some time. top speed i believe is 138mph!!! the bar has been set for me to beat. i have a pj with a2200kv on 3cell to warm up the thumbs on as it won't be very fast.this pj originally had the Dons wicked jr.

the 2nd pj has donrc wicked 3000kv using a 4 cell......yet to be maiden-ed

it's the 3rd one not yet built sitting on the table thats waiting for all data from experimenting that will use a 4850kv tp with 3cell thati hope kick a/s on speed runs,i plan on using the hk v-3 speed pitot reader to verify runs without a dive into the run for recording my speed runs. we hope with combined info from everyones effort to max speed runs to save other from spending a fortune and buy the best power source 1st time round. pictures are of my 2 pj's that have gotten beat up but still fly...i used carbon fiber rods on the elverons te,and used thicker rods in the fuse with a few mods to strengthen it for high g pull ups at the end of wot runs.

note:to me ,this is like a dragster running the 1/4 mile,it's wot with power that may blow the motor....but the recorded runs speed will be tough to beat. i plan on burning the 4850kv on a 4cell for a few seconds wotshould be interesting[at least for a short

heres a few picture of my pjs. thanks to all who participated in this thread....but lets start recording our speeds via Doppler,pitot,gps,radar gun...whatever we have ,and let push this bird to the max!!
Feb 24, 2012, 10:54 PM
Registered User
some pictures of these 2 pj's are before those haard landings[otherwise called crashes]
Last edited by stuartb; Feb 24, 2012 at 11:01 PM.
Feb 24, 2012, 11:06 PM
Registered User
used a thumb screw on the canopies and a dowel i the canopy nose since the canopy magnets don't hold well. escs are mounted to the stick to max the cooling effect since these will run hot.
Feb 25, 2012, 02:20 AM
222 km/hr Parkjet flyer
solentlife's Avatar
Originally Posted by gchrisgo34
Hey solentlife, whats your speed record? hopefully I'll be breaking it soon. turnigy 28-36 3200kv, 6x4 prop, 26c 3 cell batts. i guess you have a 4 cell settup, ive only got 3 cell batts small enough for the parkjet, so not sure if I'll be able to. It'll be fun trying though. ive also got the 28-36 2350kv, if i ever want to try it.
Mine was on a Trex 450 Heli motor of 3700kv, ESC 30A with throttle restricted to just under 30A, 1300mAh 3S 25C Turnigy standard LiPo ... with a 5x5 prop as spec'd in advert on Hobby King. So in fact the only thing different from others was the use of a heavy high power Helicopter motor - which some claimed would burn out ! - with EPA on Tx used to cut the amps back from it's WOT draw of 54A.

Power output was estimated at over 320 Watts based on the WOT wattmeter readings before restricting the throttle.
Originally I cut it to 25A / 280 Watts but we found the thrust was not as much as we thought ... so on field I upped the EPA from 30% upper throttle to 60%.

Speed by Eagle Tree on board Pitot tube sensor : 222 kph ( 138 mph ).

As you can see - I have a fair margin left of throttle but this then brings us to ridiculously short run times..... AND need to upgrade ESC / LiPo to take such amps.
I have a 4S 920mAh 25C pack that fits but balance is terrible and I don't think she'll launch ok with it.

I have 2 motors on the bench for this baby to better my own record ... a 3800kv light motor that should draw less amps and be able to still work with a high pitch prop. The other is my secret weapon ! More on that IF it gets used ... at moment we are unsure whether it will turn a high pitch prop at the rpm we need.

My OWN THEORY ... not based on others ..... is that prop diameter is a killer with these PKJ's ... they only increase the torque problems at launch. The real speed comes NOT from watts, but from prop pitch ....

ie : My 3700kv with 5" pitch prop, 30A restriction has a theoretical speed (rpm x pitch calc) of 266 kph ( 165 mph ) ... with losses my spreadsheet predicts a flight speed of about 213 kph ( 133 mph ). If take away the restriction and give her full amps - theoretical speed is then 376 kph ( 233 mph ) predicted 300 kph ( 187 mph ) !!!

Your 3200kv, with 4" prop gives .. theoretical 217 kph, 135 mph and predicted speed of 173 kph / 108 mph.
I would expect ypu to have launch problems because of the 6" diameter and torque effects resulting ...

My "arsenal" contains various motors / props all from completely different set-ups and I am not afraid to experiment.

One experimental idea I have is : How to effect a take off from ground without having wheels and the associated drag / weight items ? AND still keep prop clear of ground. Reason for the idea ? My theory is that most of the launch problems are not because of motor mounted above or below wing ( yes of course it affects and adds to any other factors ... ) but actually because we are installing such power that the poor model cannot handle it till the wing actually flies. The sort of power I'm using - it took 5 modifications and attempts to get her to launch and even then on the successful one - she still showed her 'teeth' and want to torque into ground. So the RoG idea is try and get in control in those first few secs till the wing takes over and she flies. How to do it though ?

Feb 25, 2012, 07:30 AM
222 km/hr Parkjet flyer
solentlife's Avatar
As Stu says ... lets push her to the max ... and in so doing we can list out the necessaries that others can use to get the best out of her.

There are some fundamentals that i would like to post (if anyione finds different - then great - lets read about it ....not every PKJ is the same)...

CoG position : That is at mid position of the finger-holes underneath. The PKJ is extremely sensitive to CoG and placement there is in my opinion the most important part.
My way of accurately setting this is to insert two small tubes into wing leading edge at 90 degrees to the fuselage. the position is directly in line with advised CoG. Two hooks suspended from celing then can be inserted and the model hang from this for you to move battery around until it hangs level.

Tubes in LE ..

Here without battery in and tail heavy :


Here with battery in and canopy laid on top ...

Engine mount This comes in for a lot of discussion but in my opinion is already sorted by design. The fuselage and wing form of this model indicate a pitch down moment in flight. The underside is fairly level but top is curved significantly. the long nose section would in my opinion cause the model to pitch nose down by air force acting on it. Therefopre the motor needs to be angled as per the design to push the tail down. The wood beam that fits the sloty right at the back is a tight fit and must be pressed into place firmly to bottom out and get the right angle. I also advise not to use the supplied glue here as the power levels we are using - I saw mine flex seriously and had to rip it out to epoxy it more securely. I found when I ripped it out, the supplied glue did not remove any foam - it was as if it had never been there !
I use the Hobby King alloy beam mount and not the thin mount supplied. Purely again because of the power levels we are putting through this model.

Here you also see the heavy high power heli motor ! This motor needs a heavy LiPo to balance and the 4S 920mAh 25C pack I wanted to use is not enough because it's long and I cannot move it more forward.

Inside canopy I believe that design is for canopy and a piece of velcro to hold LiPo, but this is only good enough for 'general sport use' as this model experiences serious G forces and body stress. I had my LiPo move twice before I came upon the solution ...
I now have a strap Velcro to really hold that LiPo down and it does not use the canopy in any way ..

Fuselage flexing Now this i regard as more significant than wing flexing. The battery pack is sitting at a weak part of the model and as such causes the fuselage to flex in flight ...

Here are photos of my PKJ after the 222 kph run ..

So a stiffening needs to be made without incurring weight. Many will have CF rods or similar to use. I didn't so improvised with scrap 1/4 x 1/4 balsa stock. Slots created by use of soldering iron, ( I do not cut away any foam as that weakens it, I heat it so it contracts and creates a strong skin ), and then epoxy the spars in. A smooth coat of epoxy to finish of as a hard skin.

Elevons These are extremely flexible and in normal use would be OK ... but again the stresses we are p[utting through this model - they could not survive. So the trick is to insert stiffening ...
Some people like Stu use the CF rods supplied in the kit and substitute other in the wings.
Not me !! I had light weight METAL control rods spare from an aileron conversion I had done to my Cessna. These were thinner and also allow me to bend out any moulding defect in the control surface. Mine are inserted into slots just immediately aft of the control horn. i wanted to pass it through the horn but the designed horn is a flat plate idea through the foam.

I hope the above helps others as these can be a right pain to get balanced and right ... the number of us who have had the trouble to get launched !
Feb 25, 2012, 10:18 AM
Registered User
top of the morning nigel,another mod if you don't cut the original elervon hing is to add strips of the fiber tape in end and center locations. also laminate the elervons with a light ply to make them very strong and ridged. i have sheet of base wood thats are strong ,but thinner than any other stock sheeting in my shop. works perfectly.
Feb 25, 2012, 01:31 PM
Registered User

I know this has been discussed a lot but after 20 successful handlaunch with 390 W of power, I ca
Feb 25, 2012, 01:45 PM
Registered User

I know this has been discussed a lot but after 20 successful handlaunches which were absolut non events despite 390 W of power in , based on thoses facts , it is obvious and proven that power has nothing to do with your pitch down problems and only difference comes from the low position of my motor .

I am surprised you got 220 kph with only 320 W .
Mine has 390 W with a 4.75x4.75 on a 2700 kV which acts more like a 2200 kV motor on 4 cells .

My theoritical pitch speed calculated with a 2200 kV on 14.5V is of 230 km/h.

I don't really know about the speed but I estimate it far under 200 km/h .

Maybe shall I try a 4,75x5.5 to test your theory with a theorical pitch speed of 267 km/h, if you were right , I should see a noticeable difference .
Feb 25, 2012, 03:58 PM
Registered User
hey gang,does rcg send emails to suscribed threads,it works that way at wattflyer where "the need for speed" thread is located and i would love to quick click onto this thread when new post are written.thanks for help,stu
Feb 25, 2012, 06:22 PM
Registered User
Is anyone on 5-6s, if so what motor and amp draw?
Feb 25, 2012, 06:46 PM
Registered User
kushal_22's Avatar
Originally Posted by Jpette
Thanks to Angry Birdie for the design idea .

Mine didn't come as well as yours but hopefully it will be more visible .

With almost 400W, I really need some help when this crazy thing makes three rolls a seconde !!
thats all? i think you think seconds are shorter than they are...but i hear what your saying Quick and FUN FUN FUN
Feb 25, 2012, 07:50 PM
222 km/hr Parkjet flyer
solentlife's Avatar
Jp watts are good of course but if the prop has not enough pitch /rpm - you are not going to break any records. I already posted the youtube of my pitot display. I admitted then we were all surprised by the results. But remember I was using a high kv and high pitch prop. The watts I quote are estimated as I adjusted the throttle epa up on field. When I get back home again I will wattmeter it to get real figs.
3700kv + 5x5 prop is a big difference to yours....

I really want someone to beat my speed, to me it is a starting benchmark to aim at !
Feb 26, 2012, 02:16 AM
Registered User
I was amazed yesterday to see how my new paint job helped to follow the plane .
For the first time I could relax a bit and be sure of what was happening !!

This plane is a real blast !! I never go back to the real Stryker that feels like a pig compared to this crazy one .

My 4S-1000-30C batteries are limiting me by know and if I want to get more pitch , I"ll need to get some better ones first .

Anyway, I love it so much like that that I"ll wait for thoses batteries to dy first and then Solentlife better you watch for your six cause I'm coming and fast on you !!
Feb 26, 2012, 09:40 AM
Registered User
What is the official mph to beat? 135?? waiting on a (hk) wattmeter to run this home-put-together 5s

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