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Jet Provost 1.5metre Andy Blackburn PSS Plan


Peter Garsden
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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

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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.

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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...

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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

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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

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> 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...

smiley

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

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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.

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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!!

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