Ok ... a couple of things here ...

When I make the calculation - I have to use stated numbers ... KV, Volts, Prop size. I derive from this THEORETICAL MAXIMUM. That is my base line.

When I first started collecting numbers - I used 80% of theoretical as a guide, but as you can appreciate - that was way over the reality. As I gathered data from own flights and others - I found that from that base line number, 60 - 65% was a better factor to arrive at speeds actually obtained.

I then started to collect Wattmeter readings and this is where it really started to get weird .......... I was finding that Watts did NOT always equate to speed ... power yes, but speed no.
Second that the lighter the motor the less it was capable of spinning a prop required for our PKJ. That includes even the moderate speed machines.
I threw shafts, burnt motors, all sorts happened when a lighter motor was pressed intio service.
My motor choice was driven by need to have power and strength in the construction to withstand the stresses involved.

I am also able to test static thrust and with my motor - the 3700kv 74gr 450 Heli motor ... 4.75 x 4.75 gave good numbers significantly higher than lower props ... large differences for small increase in fact. The 5x5 gave a marked increase but at a lower rate of increase ... so I know that the 5x5 is in the higher regions of reasonable prop. The 3700kv is actually advertised as capable direct drive from 6" up to 8" ... they don't state pitch though at that ...

As I have said before I would agree that efficiency is better with lower revving motors ... no doubt about that. Trouble is as I have also said before - we are stepping outside efficiency in this and looking for sheer speed regardless of what we need to put in to achieve it.

Here I must declare my knowledge : My work is cargo's and ships ... as well as Petrochemicals etc. The ships work at times includes ship design / construction along with performance evaluations. This brings me into the filed of Ships Propellers .. and they have EXACT same problem ... Pitch / Diameter as we have ... one gives speed, other gives thrust.
Speak to any Naval Arch about commercial ships power levels and then look at lighter but faster Navy vessels - the figures go of the chart.
I applied that knowledge to my hobby here ... and it paid of with immediate results ... the 222kph ... even if you reject that - there are still the 206 ... 203 .... on tired airframe and 5x5, the 183 on 4.75 x 4.75 .... Speeds that are higher than most - in fact with a PKJ very few have surpassed that on here.

Don ... the comment about the 3000 being faster is based on the implied comments referring to the 3000 in various posts.

Suimmary : I can only accept a lower KV motor can be a record maker if :

a) It is able to achieve a higher % of it's base line KV * Volts spec than other similar kv / higher kv motors
AND
b) able to spin at that rev a high pitch prop

The reason as you increase beyond the upper limits of prop .. high pitch / larger prop performs less is because it overloads the motor it's fitted to and it cannot achieve the RPM needed to get the additional benefit of increased prop size. ie a 5 x 4.5 spinning at 32000 will perform better than a 5 x 5 at 30000 rpm..... up till the point that tip speed exceeds design max.

Jpette ... study my calc sheet and you will see that you cannot compare the through calc figs to your method ... as they use diffferent routes to the end result. MY end result is to arrive at as near actual speed as possible to be predicted .... to help others and myself decide a power-train that will give speed and longevity. No way will I push a smaller motor to levels I am now doing. The Heli motors can take the amps and stresses ... the only reason my first 3700kv gave in was a hard crash that displaced the magnets and appears to have damaged the windings.

The Turnigy 4400 Heli motor that's on its way is also used in Skyfun's and other pusher models ... I'm just going to be using it in a smaller model than others !

At moment I'm stuck in South Korea not able to fly out ... so this is driving me nuts wanting to get home and prove my work !!

i just want to compliment the guys here at this thread,you guys are sharing info on calculating speed via charts that i am only minimally grasping the concepts of but really enjoy reading the differing opionons......you guys have kept the topic on target without any stress to the reader and for for that i want to say thanks gang!

we here on the side reading the posts are learning a lot and i hope to be able to apply whats said to increasing the pkj speed.

as asked before regarding the 3000 wicked,am i happy enough with it's speed..answer is yes! get my heart racing to do wot flybys. but the runs are not the only thing Nigel and i and other are after....bottom line,how fast can we get this little airframe before equipment no longer goes faster.) could we have chosen a funjet,or stryker..ect...ect..sure,but then the size factor,cost factor,and sheer fun of just choosing this bird to push is awesome.

i am so glad to get the parts needed for pkj3 back from the lost pkj being found. i was going to use a 60amp esc "mystery"on pkj 3 but the turnigy plush is a better esc. now i feel motivated to start the new pkj this week.

one thing i won't be doing again is fly for speed when another pilot is up flying patterns in my way,corrections are very quick and over correction can result in dangerous situations when you have 15 pilots sitting along the fields pitt area.I've never come close to the Pitt area ,but if i did the 100mph projectile would have serious consequences.

what i would love to see from your calculations is an agreed consensus as to how i should calculate equipment used speed. this way i can focus on one proved and discussed method,even if it's done in 2 steps using to formulas.

keep up the great subject guys,I'm fully into it!

As far as low KV verse high KV all that matters is the end result. I know the flying wing guys that go 150-170 mph use lower KV like ~2700 and 6S lipos I think and 6x4 props. So for 3S you would need a KV of ~5400 to get the same result as they get. Would you get the same result? I don't know but it is worth exploring to see if it really equates.

As far as pitot tubes, te only issue I have with pitot tubes is people always tell the peak speed (like a radar gun), well that usually is when they are diving to go level so that is not typically the top level speed. Guys will have 8 passes at 90-95 mph and one pass at 101 and declare the model goes 101 mph. It is human nature and happens all the time.

That is why I like doppler , you have to fly a model for 3-5 seconds level and you get the average speed for that level pass. Not a peak speed that may have only lasted for 0.1 second during a dive. But that is my personal preference and the way I prefer to declare my speeds and I do that because I sell product and if I tell a customer my EDF models goes 95 mph and they only get 85 mph and I only can get 95 mph from a dive or with a 10 mph tailwind then they get upset so I go the opposite way, I tell them 85mph flat and level with no wind then when they go and fly and use a radar gun etc and fly downwind and then dive they get 100 mph and tell me how much faster they went and how happy they are

Originally Posted by The Don

As far as low KV verse high KV all that matters is the end result. I know the flying wing guys that go 150-170 mph use lower KV like ~2700 and 6S lipos I think and 6x4 props. So for 3S you would need a KV of ~5400 to get the same result as they get. Would you get the same result? I don't know but it is worth exploring to see if it really equates.

Problem we have is the PKJ is low aspect ratio and lacks lateral stability to counter large diameter slower revving prop. It may be possible with gyros etc. to assist keeping model under control with a larger prop.

As far as pitot tubes, te only issue I have with pitot tubes is people always tell the peak speed (like a radar gun), well that usually is when they are diving to go level so that is not typically the top level speed. Guys will have 8 passes at 90-95 mph and one pass at 101 and declare the model goes 101 mph. It is human nature and happens all the time.

Absolutely agree and is why very early on I declared that Pitot was for me a development tool that would later once I get to speeds etc. I'm aiming for would be double verfied via radar and doppler.
I don't always have Andrejs with me who's best video handler I know ... so Pitot it is.

That is why I like doppler , you have to fly a model for 3-5 seconds level and you get the average speed for that level pass. Not a peak speed that may have only lasted for 0.1 second during a dive.

I would suggest that the 3 - 5secs is actually short as well ... better if longer.

But that is my personal preference and the way I prefer to declare my speeds and I do that because I sell product and if I tell a customer my EDF models goes 95 mph and they only get 85 mph and I only can get 95 mph from a dive or with a 10 mph tailwind then they get upset so I go the opposite way, I tell them 85mph flat and level with no wind then when they go and fly and use a radar gun etc and fly downwind and then dive they get 100 mph and tell me how much faster they went and how happy they are

Fully understood and I would do exactly same. i have a reading of 222kph confirmed by 5 others who were present at the time ... but I prefer the 206 / 203kph figures as they confirm each other on consecutive flights. But they were with damaged airframes with canopy that wouldn't sit down properly. A 15+kph difference is more than likely.

The whole thing will be sorted once I can get back to a clean airframe and repeat that run ..... I also have a Rx switch that can start / stop the Pitot by flicking a switch on the Tx ... I haven't fitted it so far - but it is part of my arsenal for later speed runs....... even to a second person switching the Pitot on / off ....

😔 zzzzzzz zzzzzzz enough theory already, let's see cold hard data, lol

Everybody should be using the same measuring method,

Originally Posted by aeromaniac

�� zzzzzzz zzzzzzz enough theory already, let's see cold hard data, lol

Everybody should be using the same measuring method,

I'd rather be flying !!

Measuring method ........ part of the dual thread (this and Wattflyer Need for Speed) has an early section about measuring and sorting out the different ways. Not everyone wants to pay out the money for the ET V3 sensor, many like the doppler method, most would not spend out on radar guns ... so how ?

Each has it's faults / limitations ... so as generally seems to be agreed ... any form can be used as long it's backed up by supporting alternative ... my choice is V3 sensor and Video Doppler.
Later I intend to get pals from Traffic Police to radar gun with their gear once I am actually at record making speeds. I may even go as far as GPS recording - that way you can see the changes of speed due to dive / flat / start / etc.

Given the number of readers - I'm surprised at the lack of numbers coming in ... we need not only fast data - but slow ... medium and of course those fast ... all data along with motor used, lipo, ESC, prop is all valuable.

looking forward to sharing my numbers, both slow and fast, i'm waiting on my second pj, as my first will probably be a little too hot to practice on, and I don't wanna bust it trying. Does anyone know what angle the motor should be mounted at? I did some vertical cg calcs/estimates and it seems to need a fairly aggressive angle to put the thrust line through the cg...

rtf weight: 660grams (fully fiber glassed fuse)
static thrust: 1150 grams (4.1x4.1 APC)
4S 1300mah draws 40 amps on 3200kv turnigy for 580 watts (~680 peak)

I just went with the designed thrust line when fitting the beam mount.

Later modification of motor mount being a DIY metal affair allows me to "bend" the mount .....

As regards theory - all well and good about CoG point etc. - but really it depends on neutral of elevons, CoG actually of model, amount of thrust produced by motor / prop combo etc.
My 3700kv high power set-up actually required MORE downthrust to stop the climb ...

HELP .....

I need to know maximum thickness, length and width of the elevons on the Parkjet. I have a friend at home who will go to Hobby Shop to buy balsa sheet for me to replace the flimsy foam ones ... I am working away and no access to my PKJ's to measure.

?????

I agree with thrust angle but reduce the thrust line down in line with the fuzz.

Originally Posted by solentlife

HELP .....

I need to know maximum thickness, length and width of the elevons on the Parkjet. I have a friend at home who will go to Hobby Shop to buy balsa sheet for me to replace the flimsy foam ones ... I am working away and no access to my PKJ's to measure.

?????

I used 5mm balsa on mine 20mm wide, just under 220mm long

Originally Posted by aeromaniac

I used 5mm balsa on mine 20mm wide, just under 220mm long

Tks .... I have him getting 1/16th (1.5mm) sheet ... so I planned to laminate the elevons with epoxy. That will do 2 things ..

a) laminated will give real strength
b) gives an immediate hinge line to fit mylar ...

DHDS racer talked about sloping the Parkjet (the one he had so nicely covered, silver etc.)

Finally got around to slope mine. Left the motor on (it's just the 10g HK motor for parkflying, mine is intended for parkflying in tight spaces - AND sloping, with a lot of weight added).

Worked fine, except that the canopy came off, and that was that. Little damage, but before the next flights I am also going to replace the flimsy ailerons.
Did put Hobbykings 5.7g metal gear servos in the wings, further out than the stock locations, and linkages and horns are on the top of the wing, so they're protected.
With balsa aileron moving the linkages further out for better control wouldn't be necessary, but well, now it's done, and those slim servos fit in quite well. I would definitely recommend mounting horns etc. to the top, can be done easily also using stock servo pockets by making a small slot (and taping the underside so there is no air transferred).

Parkjet HK stormy slope maiden (2 min 32 sec)

cheers
Clemens

Originally Posted by solentlife

Tks .... I have him getting 1/16th (1.5mm) sheet ... so I planned to laminate the elevons with epoxy. That will do 2 things ..

a) laminated will give real strength
b) gives an immediate hinge line to fit mylar ...

Wow heavy lol

2nd attemp to post video of pkj#1....,man i su.k when it comes to computers.




sorry gang ,i give up with rcg posting vids......if you want to see a shaky first 50 seconds and a much better following 2 min flight . go to wattflyer thread "hobbykingparkjet....need for speed"

why can't i grab photos or vids by copy and paste them here at rcg....it's buming me out




Hobbyking parkjet...need for speed!!!!!!!!!!!!! ( 1 2 3 4 5 6 7 8 ... Last Page)
stuart Today 06:09 PM
by stuart........................................

dang ,i can't even post a link,what am i doing wrong?