after completing the Skyleada Cutlass I told myself (sternly) 'never again', that is, covering an open 'stick 'n tissue' structure with pre-printed tissue. Others, covering WW I biplanes with, say, tissue printed with lozenge decor with Maltese crosses , seem to end up with a scheme Lothar von Richthofen would be proud of. For me, the technique is at the limit of my modest modelling skills. So, I determined, 'Never (ever) again'! But the Cutlass has garnered a little praise, viz: "It will look great at a distance", and the thought I should have another go at the technique, and perhaps do better, became irresistible .
OK, but which model?My profile F-100 Super Sabre looks flies well great in its aerobatic team colour scheme:
and the templates, created by Rob Smith, are really rather good:
So an F-100 it was to be. Skyleada produced a kit for the YF-100 in the mid-fifties:
and the Vintage Model Company (VMC) have a laser cut kit:
So what is there hold back this 'bleak mid winter' project?
Well quite a lot, actually.
Some years ago Mike Stuart built this model for Rapier L-2:
Above: Mike's Skyleada YF-100. Mike did not add nose sheeting or extra stringers; in fact, it is built exactly as the kit plan.
Mike had every reason to think this lightly-loaded model would fly as well as his Mystere from the same stable. But glide tests did not go well: mike had to adjust the tail plane angle to stop it 'diving in' (he did not want to add lead to the tail) and then could get no consistent glide from it. Mike said, It just floundered about like a freshly-landed salmon". I do not know if he ever attempted a powered flight.
One does expect kit designs to fly, and Mike is an accurate builder, so one has to conclude there is a flaw in the design. Is there a clue from the model's side view?
Above: Skyleada's 'YF-100'. I have highlighted the positions of the wing and tail plane. Note the angle of incidence and the model does appear a trifle 'short coupled'
I then compared this with Jetex Tailored F-100, which, Bert Judge assured Andy Blackwell and me in an interview, really did fly (though a colleague later added, "In a straight line!").
There are other F-100 models out there. When I've garnered them in I'll be able to compare them and l see if there are any obvious differences to the Skyleada version that could explain why even an expert modeller like Mike Stuart gave up on it.
More later .... perhaps someone else has a thought or two?
I've been able to revisit my archives, and found some interesting thoughts about the Skyleada Super Sabre from expert modellers Mike Stuart (who built it) and Steve Bage. First, Mike wrote:
"Now I really like this model, which had been on my ‘to do’ list for quite some time. But, built as the plan (Perhaps Pete Smart was correct when he wondered if the later Skyleada models were ever test flown!) it is all but unflyable. I wondered if it suffered from the ‘inertia coupling’ of the full-size prototype, which was cured by increasing the fin area and extending the wing tips".
Steve believed the situation was even worse than this, and wrote:
"The motor is too far forward, leading to big trim change between the initial launch and ‘re-launch’: it needs to move back to straddle the CG and any nose-up pitch is then dealt with by downthrust.
Secondly, the way the model pitches up into a deep stall then sinks to earth, or flips over backward, is without doubt due to tip stalling. Swept wings tend to do most of their lifting at the tips and are especially prone to this phenomenon. Since the tips are aft of the CG, when they do stall, a nose-up pitch results. This was a serious problem on the full size F-86 and FY-100, so the behaviour of Mike’s model, the so-called ‘Sabre Dance’, is actually authentic!
The simplest fix would be to build in a stack of washout, maybe 4 - 5 degrees. I’m pretty confident that these modifications the model would fly in a much more predictable manner".
One thought I had was that Mike's model was very light, the implication being that the design needed the weight of a Jetex 50 under its belly to stabilise it, writing,
"Given the lightness of Mike's model, a little more weight will not go amiss: one school of thought has it that the faster your model flies, the more stable it is. Good news for us 'fettlers', who can't resist adding sheeting, extra stringers and wing spars"!
But the 'bottom line' is that some serious modifications to incidences and thrust lines will have to be made to this model.
There are quite a few designs and kits for the F-100 out there - it appears to have been quite a popular prototype to model. Here, for example is one from Comet:
and one from the French 'CB' brand:
Extracting the side views from these plans(and others) it is possible to compare them and see what changes the designers have made to make their model 'flyable':
Above: montage of some F-100 models. Top; Skyleada , next CB; next my own profile design; bottom: Bob Linn's profile catapult glider (published in the early 1950s)
One can even overlay the various designs, whereupon differences in the models becomes more obvious:
Above: overlay side views. green: Skyleada; blue: my profile F-100; red: Comet. The CB and Jetex 'Tailored' designs are also lurking in there somewhere!
OK, so this isn't s rigorous comparison, but I think the conclusions are pretty clear, viz: most designers have brought the wings forward, the tail plane back, and enlarged the fin and moved it back as well. I did all of this with my own profile design and trimming it went OK once I'd found the right motor as it is quite power fussy.
So: modify the Skyleada plan by bringing the wings forward (which will also bring the motor under the CG) and enlarge the fin and move it back.
Mike also noted that the original designers of the Super Sabre extended the wing tips to help prevent 'inertia (or roll) coupling. Perhaps the Skyleada design is a trifle 'under winged?
There was an easy way to test this by looking at the ratio of wing span to fuselage length for various designs:
The above table would seem to indicate that the Skyleada design is based on the original YF-100, whilst the later designs are more similar to the production Super Sabre.
I hope you have enjoyed this tour of my somewhat constipated thought process prior to tackling the Skyleada model. I really didn't want to end up with an unflyable model!
To conclude: I will extend the wings. Happily, this means they will match my templates better, I'll also incorporate 3-4 degrees of washout and bring them forward. I will also move the tail plane and an enlarged fin back. These mods, together with genuine Jetex power will, I hope, be sufficient to make the F-100 as splendid a flier as the Skyleada Mystere.
Other minor modifications, for example extra stringers, nose sheeting, diagonal ribs (to maintain the washout) will be added as I go along.
Thanks for this intriguing and thorough study Roger, it is an example to us all to look before we leap and rush headlong into balsa cutting. Your profile model has clearly shows the way forward and I hope this proves to be the case with the built up model. A pity more designs did not have this level of study before committing to production. The F-100 must be up there on most people list as a favourite particularly if like me you were a teenager at the time of its introduction. Love it.
Note added by Roger: Now I have no excuse to not proceed with this project
As I have intimated, I was pleased with my 'hemi-demi-semi profile Super Sabres, especially the rather more refined one for Rapier L-2:
which really looks the part in the air. It made a great flight at Old Warden in 2018. It does, however, need the 'right' motor and as Rapiers have been more than a little unpredictable of late, I haven't (yet) been able to quite pin down the specification of the 'right' motor. Perhaps I'll convert it to genuine Jetex power. It might go quite well with a more predictable Jetex 50C and the extra weight below the CG . . . .
The following user(s) said Thank You: rogersimmonds
If I can interject here, yes the Mike Stuart F-100 did fly if memory serves correct. From memory, it was totally unpredictable if overpowered, knife edging every which way. However, with a 100/120 mN L-2 it would wallow at waist height but in a straight line.
I remember filming it coming toward me then realised I was in the firing line, the machine hitting me full square in the groin . Enough said.
Note added by Roger:
Many thanks for the correction - I didn't know this .... I wonder where I was when all this was going on, and what year (2007?).
But it would appear the design does need some sorting out.
The following user(s) said Thank You: rogersimmonds
The estimable Howard makes these excellent observations about a 'stick and tissue' F-100:
"The F100 is in indeed an intriguing problem, here are some thoughts on the matter, they may be relevant!!
I have found with our Rapier or Jetex powered models that it is better to put positive incidence on the wing rather than negative incidence on the tailplane. The idea behind this is that if the wing is at 0 degrees and the Rapier parallel to it underneath it then when the model is flying at its normal attitude, the wing will be flying at say 2 degrees positive and 2 degrees upthrust on the Rapier motor, not what we want. However if the wing is set at say 2 degrees positive and the Rapier motor is at zero degrees below then there is no upthrust in flight.
The F100 is extremely close coupled but not insurmountably so as you have proved with your profile models but the full fuselage models seem to be a different ball game and everything changes. The fuselage shape here in my opinion is critical it seems, the flat underside is part of the problem and appears to cause similar problems to the models I have seen of the Blackbird and indeed my L1 profile Hunter when I discovered it had no decalage. In these cases the model will rear up or even bunt down unpredictably, pointing to lack of decalage and or the CG being in the wrong place.
On most aircraft fuselages the top and bottom are fairly equally rounded or flattened so the slip stream is more or less even all round from front to rear. On the F100 the bottom is flat and the top humped and rounded and much like a lifting non symmetrical airfoil. My conclusion is that the fuselage may have to be considered part of the wing area to get the CG in the right place. Furthermore there is possibly downwash from the top the rear sides of the fuselage acting down onto the tailplane further upsetting CG calculations.
Lots to consider!"
I commented: "these are good thoughts; the smaller F-100 for L-1 is a consistent and stable flyer (it coped well at Flying Aces) the larger more rounded one looks fabulous on the ground and in the air, but is very(very) tricky and needs just the right motor. Getting just one (but really great!) flight to 12 plus goes (with all sorts of L-2 motors) one Old Warden in May last year. It was a memorable sortie just as the airfield was clearing, using an old L-2X, and since then I've put it to bed".