Topic: folland gnat

Cripes!!  Wish I'd seen that before my flight test.

As a matter of interest my Gnat model flew just beautifully which is why I kitted it.  Sold the whole business on after a few years but there are some still out there.  Hope the guys go ahead with the completion, just leave the flaps alone for the first few flights.

Hi John

Pim Smith's Gnat still flies beautifully. 

In Victor Bingham's book "Folland Gnat Sabre Slayer and Red Arrow" there's an excerpt from a technical description which says that the tailplane datum shift mechanism was mechanically linked to the retracts.  Presumably the combined u/c door/airbrake caused a nose-down trim needing the tailplane datum to be shifted negative and in proportion to how far the gear was extended.  In fact all the pics in the book of Gnats stationary on the ground show the tail at maybe 2 or three degrees negative, no doubt because the gear is extended.

There's nothing in there about the tail datum being dependent on flap angle, but the tail datum shift system seems to have been complicated enough that it might well have been.

Anyway, it certainly looks as if this aspect of the Gnat would repay some deep research for anyone making a model equipped with flaps.

Cheers

Gordon

'Pilot's Notes' on the aircraft should supply the answer.  Anyone got a copy?

Now a little quiz for the Gnat 'anoraks' around

1. What are the 'Bungees' on the side of the bang seat for?  To enable short arsed pilots to adjust the seat height.  When you released the adjusting lever the bungees lifted the seat.  To make it lower you had to bounce on it!

2. Where are the airbrakes?  The 3 undercarriage doors (everyone knows that!)

3. Where would you find the main battery? In the nose (as far as I can remember)

4. What is the Hobson Unit used for?  It is the powerful hydraulic motor system for moving the all flying tailplane.

I should say I flew the Gnat T1, and was on the first ever Gnat course at RAF Valley (Feb to Oct 1963), being the second student to go solo.  Fantastic thrill, believe me.  It was such a pleasure to fly. Don't get me started.  BTW I am 6'3" and fitted in very comfortably.  I was however slightly too tall to fly Lightnings, although I did squeeze in and get a flight once in the T4 (1962).  My Gnat instructor at one stage was Al Pollock who in 1968 flew through Tower Bridge in a Hunter FGA9!  I remember on one occasion our tailplane appeared to lock solid half way round a loop, but freed itself.  Al was determined to try and see if it would do it again, but after 6 more loops gave up!  After Valley I was "creamed off" to be a QFI but sadly was posted to the JPs on 7FTS at Church Fenton.  Returning to the JP felt so desperately slow and frustrating after the Gnat.  Later flew Canberra B15s and B16s out of Akrotiri in Cyprus, all over the Middle East.  Flying at 100ft all over Iran was something.  Imagine that now!  Or skip bombing at 10 ft, yes 10 ft, repeat 10ft.  This involved approaching the target at 50 ft and then pushing the nose down just before releasing the bomb.

Somewhere on this thread people have wondered about the trim change when flaps were lowered.  As far as the pilot was concerned any trim change was minimal, so I assume the tailplane datum was shifted in the same way as when the undercarriage was lowered.  As a matter of interest we used to lower flap 10deg when initiating descent from altitude to increase drag, but not until we were below Mach 0.7.  I remember inadvertently lowering the flaps 10deg at Mach 0.8. Stupid mistake as this was above flap limiting speed, but nothing got bent.

A Canberra pilot... superb 

1. What are the 'Bungees' on the side of the bang seat for?

Bungees just about visible (I think!) in the first of my pics below.

3. Where would you find the main battery? In the nose (as far as I can remember)

"That's a big battery," I thought looking at my pic of the Gnat.  Until I realised the large cylindrical object apparently in the nose of the aircraft was actually an oil drum on the fork-lift in the background... 

Returning to the JP felt so desperately slow and frustrating after the Gnat. 

The two pics below of the Gnat were taken at North Weald the day I had a flight in a JP.  Now to me the JP was quite an experience,  what would a flight in the Gnat have been like? 

Both nice Teddy Petter efforts 

Both nice Teddy Petter efforts 

Canberra, Lightning and Gnat in that order were all Petter designs!

Now a little quiz for the Gnat 'anoraks' around

1. What are the 'Bungees' on the side of the bang seat for?  To enable short arsed pilots to adjust the seat height.  When you released the adjusting lever the bungees lifted the seat.  To make it lower you had to bounce on it!

Well I learn something new, (yet again ), as we were taught they were part of the canopy jettison system.  In that when you pulled the handle, (seat pan or face blind) the bungees operated the (un)locking arms.  Probably some p1ss taking instructor's idea of a joke though

Oh, you're right about the battery location too   We used to take it out before ground handling, then you could lift the nosewheel off the deck and walk it around by hand


Mark.

Pete, the builder / designer isn't going to do another having moved on to another project but the other two Gnats will at some point be built by the other designer and his brother.

I am now the very happy owner of one of those two remaining kits, to be built after the Lightning, which at current rate of progress will be about 10 years    Really looking forward to it as the Gnat is another of my all time favs.

Been searching for info about the alleged violent nose down pitching when flaps are lowered.  I found an article by an Indian Air Force pilot reminiscing about his time in the single seat fighter the IAF used.  He mentions that after take-off, on raising wheels and flaps, if you were not already pushing forward on the elevator trimmer you would be taken around a loop, which implies that it needed a lot of up trim when flaps and wheels were down.

Would that have been the reason why the other model from the same moulds met its demise last year?

I didn't know it had been built and stuffed in too.  In which case I am the owner of one of the one remaining kits!  Forewarned is forearmed, I will be experimenting very gently with the flaps when mine flies.

I have a jim fox gnat kit, the build will commence soon. I have spoken to a guy who has just test flown one and it goes superb!!
Agreed MPX i will take the flap setup in very small increments!!

An old pal of mine was once the pilot of a gnat (yellow jackets colours) owned by Nick Mason (yes, THAT nick mason). I'll see if I can get in touch for his comments regarding flap trim changes.

PDR

I thought it was Dave Gilmour that bought the Gnat, Mason is more into vintage biplanes and cars

I thought it was Dave Gilmour that bought the Gnat, Mason is more into vintage biplanes and cars

It was Gilmour who had the Gnat, the serial was G-MOUR  

Sheridan

It was a while ago (back in the early 90s) and as I remember nick mason bought it but never got to be comfortable flying it, so he later gave (or sold) it to dave gilmour. I'm fairly sure that at the time my colleague was flyiong it the aeroplane was owned by mason, but I could be wrong - could be my CRS kicking in again!

Anyway, when I get back to work next week I'll try to contact him.

PDR

Hi all a friend of mine has a gnat up and flying,i test flew it 2 weeks ago and it now has 10/12 flights logged,wren 70 for power,it is a copy of the old ducted fan version and no flaps!! It flys very well and really looks the part,the best news is he has the molds to make some more but there is a list of people who want one,we should have it flying at classic jets at abingdon on the 1st oct regards Keith

we should have it flying at classic jets at abingdon on the 1st oct regards Keith

looking forward to that!

but there is a list of people who want one,we should have it flying at classic jets at abingdon on the 1st oct regards Keith

I'll look forward to seeing it at Abingdon.  Will this be a limited production run or can anyone get on the list?

Gordon

When I sold my Gnat kitting stuff, including the moulds, it all ended up with Jim Fox.  I lost track of it after that.  Could it be that the new kits mentioned in a recent post where mention is made of moulds, are the originals resurfacing again?  Would be nice to see them in use again as the original model was a dream to fly (without flaps!).  If it is a copy of my original I hope the structure has been modified to cope with the extra weight and speed that Gas turbine power will impose.  The original was for K&B .45 power and weight target was 111/2 Lb as I recall.

Hi it really depends on how many he can get out of the mold,,but you would have to ask him ,i am sure that he will try to do as many as he can regards keith

the gnat i am taking about has been strenthened and well tested it has been a three year project for him and he is really delighted with the model,weight dry came out at 15 lbs and will fly for 9mins at resonable throttle settings regards Keith

Hi every one first of all i have some bad news,Dennis Wyman the guy behind the rejuvanation of the john carpenter design Gnat has sadly died,his enthusiam for this model was such that he was deturmind to turbine the gnat and fly it which i am glad to say he did before he died.His face after the maiden flight was a real picture,it flew very well with Wren70 for power.
His wife now has ten sets of foam wings,tail planes and fin together with canopys and ABS top deckings all paper patterens of the wooden parts are avalable with pictures.A mold for the fusalarge is also avalable.his wife and i would like to see these parts used to build the Gnats that Dennis would have enjoyed  seeing.
If anybody is interested in building one please get in touch with me and i will give more details,There will be a cost involved but this will only cover he cost of parts,i have almost built mine which is almost at the painting stage,email me on jetjock@talktalk.net regards Keith Whiddett

Sad news, Keith...I had a lovely chat with Dennis at the last CJ and he was so enthusiastic about the Gnat. Pse give my condolences to his wife and family.

Rgds,

Dick

Hi Dick yes very sad it was a great pity that the weather was not very good at the last CJ because he would have enjoyed seeing it fly at at Abingdon regards Keith

Having sold on the gnat kitting stuff many years ago it is great to see it coming to the surface again.  I'm not personally about to get back into jets but hope someone will pick this up and run with it.  The gnat  is a great flyer and i am sure will be even better with a gas turbine installed, although never designed for it.  Watching with interest.

JC

Hi All picture of my wyman /carpenter 1/6th scale GNAT ready for the primer

My oh my what a beauty!  My heart goes out to Dennis’ wife at her loss, and I’m really grateful to her and Keith for having the opportunity to purchase one of Dennis’ kits, which I’ll collect at the Dec 17 CJ.

Mine will be finished in the final No4 FTS colours as worn by G-RORI/XR538 in the pic.  Power will be an MW44 Gold at an estimated AUW of 13lb.  It’ll have nearly 200sq in more wing area than my Foxcomp Hawk, so should land rather more slowly.

I’ll be fitting the flaps.  On previous jet models I’ve successfully negated trim changes when flaps are deployed by using a mixture of aileron and elevator deflections.  Regarding this model’s likely huge nose-down trim with flap, what I propose for my Gnat is to raise the ailerons about 5mm, besides adding up-elevator trim.  The nose-up pitching moment of the ailerons will assist the elevators, reducing the amount of elevator trim required, which might be more than I’d like otherwise.

Gordon

Nose down trim change is GINOMOUS!  Trust me been there and only got away with it because they were tried at height (after slowing down).  Recovery was only possible by retracting them again rather smartly.  Change of underwear also required!

jc

Well I’ve certainly taken your experience on board, John, along with the likelihood that the other model Gnat’s crash was related to flap deployment.

Having been forewarned by your experience, I should be able to avoid a first-flight disaster so it’ll be fun seeing if I can do anything to minimise the effect.  If all else fails, I’m not averse to increasing tail area significantly to cure a sport-scale model’s wayward behaviour – a 20% increase in linear dimensions wouldn’t be noticeable to most, yet would add 44% more area. 

This scheme worked many years ago with a sport-scale CAP-21 aerobat I had designed.  It had full-span strip ailerons operating as flaperons mixed with elevator for aerobatics, or drooped for landing.  As first flown, when the flaperons were linked with elevator the model’s pitch response reduced markedly, and when they were drooped for landing lots of “up” elevator stick was needed to prevent the plane from pitching nose-down dramatically.  In fact lowering the flaperons induced such a great nose-down effect that if I rolled the model inverted and then operated the flaps without touching elevator, it would fly up and over in a figure-9 shaped outside loop. 

After a bit of amateur aerodramatic thought I decided to double the tail area by pinning strips of balsa round the outline, with the result that the model then had a nose-up tendency with drooped flaperon.  By reducing the excess area I eventually achieved an almost zero trim change on lowering flaperon, at which point I replaced the lashed-up tailplane with a nice new one made to the new size.

So I might have to see if the Gnat needs this cure as well.

Gordon

I never got round to trying ele/flap mixing as the one event where the aircraft was well on its way to a full bunt was quite enough for me.  However, i suspect that the full size did have some form of interconnect because an ex Gnat pilot pal of mine says that he never noticed a big trim change.  With the all moving tailplane an enlarged area may not be needed.  I suggest you try it out (cautiously) before deciding.  I will be fascinated to hear what you find Gordon!

The nose down pitch of the Gnat when flap is lowered is most puzzling.

The affect on the wing of lowering flap is to cause it to pitch nose down due to the increase in camber.  Tail-less aircraft use this effect for their elevator control  If nothing else were involved, applying flap will always cause a nose down pitch, which is why flaps coupled to elevator to make sharp changes of direction often makes pitching worse, not better, as Gordon found out with his CAP.  The elevator is trying to pitch the plane nose up while the downgoing flap is trying to make it pitch nose down and the two fight one another.

However it is not always the case that nothing else is involved.  The lowering of flap increases the angle of the downwash behind the wing.  If the tailplane is sitting in the airflow affected by the flow, it experiences a large change in angle of attack and produces a lot more downforce.  Since the tailplane has a lot of lever arm compared to the location of the flaps, it can often more than compensate for the nose down pitch caused by the flap camber, and a nose up pitch is the result.  That is why some planes pitch nose down, and some pitch nose up when flap is applied.  A high wing, low tail puts the tailplane deep in the downwash so it generally pitches nose up, a low wing and high tail keeps the tailplane out of the downwash so it generally pitches nose down.  The CAP is a low wing with a high tail, so lowering the flap will generally cause a nose down pitch.  The Gnat is a high wing with low tail so you would normally expect a strong nose up pitch when flap is applied.

In addition to that there is another effect with a swept wing.  The flaps are on the inboard section and as well as causing an increase in nose down pitch, they will cause more lift.  Because they are inboard they are forward and the increase in lift is therefore biased towards the front rather than the aerodynamic centre of the wing, which could put it ahead of the CG and therefore cause a nose up force.

What the plane does depends on the winning balance between those forces.

The Gnat, being high wing and low tail, and being swept wing with inboard flaps, has the ingredients for a strong nose up, not nose down, pitch.  For it to pitch strongly nose down means that some interesting airflows and forces are taking place.  I note it has a large chord with moderate sweep so it could be that the change in lift force caused by the flap is still behind the CG and has quite a lever arm, as will the change in pitching force, enough perhaps to overcome the strong extra downforce from the tail.

  However, i suspect that the full size did have some form of interconnect because an ex Gnat pilot pal of mine says that he never noticed a big trim change. 

Probably right.  All the pitcures of Gnats in landing configuration show a massive amount of up elevator trim.  The one description I could find of flying the Midge talked about its enormous trim change when flaps were raised, and since the Gnat was a pilot trainer version of the Midge we can imagine that an automatic trimmer was specified so that trainees were not subjected to that problem.

harry - why not contact these guys and ask one of their pilots?

http://www.gnatdisplayteam.com/

nice one tony, didn't know about them, email sent.

http://www.gnatdisplayteam.com/

Thanks Tony.  What an excellent website.  

I've just browsed the whole lot of it.  There are some splendid photos to download and one of them is my new desktop.  

Gordon

Hi all can any body tell me what the incidence was on the main wing of the original carpenter/fox Gnat kit,i would like to check mine for peace of mind.
got the primer on today and should have top coat on monday regards Keith

The moulding of the fuselage wing seat set the incidence angle for you.... nothing to check if the wing fits properly.  Cannot remember the original design incidence moulded in (well it was over 20 years ago!).  Set the neutral tailplane at -1 degree to the wing as a start point.

JC

Hi thanks for reply,i wanted to check incidence in case any thing had been altered in the mold copy and it would have been usefull to know the original incidence perhaps some one has a fuz that they can check regards Keith