Jet Provost 1.5metre Andy Blackburn PSS Plan

[Jez Billington]

It’s true, the model nearly ended up in Davey Jones’ locker, but not giving up when all seemed lost saved it, Hitting good slope lift with an unstable plane does seem to add some pitch stability for some reason. My model is quite a bit heavier than most (around 7lbs) which would not have helped the situation. That’s the result of adding flaps, nav lights and accidentally buying balsa thicker than required with the wing sheeting!
I will check the decalage before the next flight and add some more weight to the nose. This will hopefully tame the model. Will try again at the Llyn in August

[Andy Blackburn]

> I will check the decalage before the next flight and add some more weight to the nose.

Hi Jez - all that would be really helpful; I did move the c.g. forward from the original plan (partly because I have my c.g. on the smaller one fairly far back anyway), but obviously not enough.

A.

[Peter Garsden]

Posted by Andy Blackburn on 17/07/2020 20:29:29:

Well, this is - frankly - shocking. One just can't get the staff.

My apologies to Jez for the unnecessary excitement. I'll make the appropriate changes to the CAD files and re-issue the plan to Phil. It'll take me a week or so because I have my hands full at the moment...

A.

P.S.

Pete - you built the first JP, did yours need the c.g. moving forwards as well?...

With hindsight I did add a couple of penny lead weights to the nose after my first maiden flight in a 60mph wind at the Orme when I was holding in LOTS of down elevator. This then tamed it in the same way as Steve. Unlike Jez I maidened in a lot of lift which saved me but challenged me in equal measure (what do you expect on 2nd January!)

I missed a rudder so added it after that flight. Inevitably this added some weight to the tail. I added more weight which brought it back to the plan c of g. I am now going to add more weight to get it to the new c of g before the next flight, which may be at the Lleyn.

So the short answer Andy is yes but I was not as accurate or scientific as Stevie has been. It was only when he What's App'd me that I have committed more detail to this thread which most people seem to be following.

[Andy Blackburn]

OK then, that all sounds very reasonable; I have my hands full for the first part of this week but will try and get some updated files to Phil before next weekend.

I take it that we are all certain that 7 mm (85 mm - 78 mm) is the correct amount to move the c.g. forward? If you do a dive test (useful as a general reference even if you don't really believe in it) then there should be a slight pull towards the canopy...

A.

P.S. - hoping to get up to the Orme before the end of the year, Covid and accomodation permitting...

[Jez Billington]

Andy.
I’ve always relied on finger balancIng to determine CG position but after talking to Pete he put me on to the string and plumb bob method that should prove far more accurate.

From my side at least, I will only be able to confirm after flight #2 which will be hopefully far less exciting than the first.

p.s. not sure what you mean by ‘slight pull towards the canopy’ - I would say a successful CG dive test is one which has a gradual unassisted pull out

Edited By Jez Billington on 20/07/2020 12:51:46

[Jez Billington]

I’ve just had to add over 6oz nose weight to the JP to get it within the range of the new CG estimate. Really don’t know how that happened - I was clearly far too hasty to get it finished. The jig certainly makes things more scientific, but I’m now not fully convinced that even the cradle measuring device is giving a perfect reading. Need to build one of those see-saw devices to convince myself that it works 100%

Edited By Jez Billington on 20/07/2020 16:54:17

[Jez Billington]

[Andy Blackburn]

> not sure what you mean by ‘slight pull towards the canopy’ - I would say a successful CG dive test is one which has a gradual unassisted pull out

Yes, same thing - been round power models too long...

What I do for balancing is to get hold of a couple of used matchsticks and (for a low-winger) tape them to the top of the wing, aligned spanwise, at the required c.g. position. Then I turn it over and balance it on fingertips - it's not going to be more than a millimetre out either way.

What I'll do (later in the week) is to move the c.g. on the plan to the 78 mm position with a note to say that it should probably be optimised using the dive test.

 

Edited By Andy Blackburn on 21/07/2020 08:51:45

[Peter Garsden]

Posted by Jez Billington on 20/07/2020 16:58:16:

Jez, the string needs wrapping round the dowels about 5 times this won't be accurate I agree - see this pdf file which explains more - **LINK**

[Peter Garsden]

The new C of G is the result of Stevie Kemp's successful flight in about 22 to 25mph wind at the Orme. He has checked it after going home and that is the measurement - yes 78mm. He initially balanced it on the correct C of G then added 40 grams for the flight which put it at 78mm from the leading edge. And yes that gave him a perfect flight. I don't know if he did a dive test to be honest. I arrived at the same result when I flew in January ie I added 2 penny weights to bring the C of G forwards from the plan level. Jez flew his with the C of G on plan and it was too far back. So that is as much science as there is.

[Andy Blackburn]

It's done. My conscience (such as it is) is once more as clear as can reasonably be expected.

A.

[Phil Cooke]

I think everyone who bought and is building (or yet to build) from the previous issue of the plan is aware that the CoG needs to be moved forward now to this 78mm position. Good coverage on here and on the PSSA WhatsApp group.

Going forward, if anyone orders the download file through the PSSA website they will receive the updated plan with the additonal notes and revised CoG position.

Many thanks all. I look forward to seeing more of these JP 150s on the slope! Superb stuff!!

[Phil Cooke]

Just thought I'd add a few flying photos of JP150s to the blog, quite a few of them in regular operation now and all flying brilliantly on that revised balcanec point!!

 

 

[Phil Cooke]

 

A gaggle of JP150s at the Orme a little earlier this season... 🙂

[Phil Cooke]

Another JP150 has reached the finish line (after a long start/stop/start build) and is ready for its maiden flight.

 

Built to plan and finished in FighterAces glass cloth and resin with LifeColor paints, which has added a little weight resulting in an AUW ready to fly at 6.5lbs.

 

Canopy from Steve Davis @ Vortex Vacforms UK, pilot and ejector seats by Andrew Meade @The Printed Parts Factory.  Model is finished as a 79 Squadron RAF Tactical Weapons Unit trainer based at RAF Brawdy in the early '80s - a scheme we've not seen modelled before either at this or the smaller original plan scale!

 

 

 

[stu knowles]

Good looking model and vefry well finished.