F7U-3 Cutlass conversion

[Paper Kosmonaut]

I'd keep it clean.

Same here. Keep it clean. It has the quality to become a very pretty swan.
The curves are beautiful already!

[Old Wombat]

She certainly looks good without canards but my argument for them is mostly aerodynamic; the aircraft has a long section of fuselage forward of the wing & the engines are very nearly on the CoL, which means she's going to be nose-heavy, with the CoG well forward of the CoL.

There are two ways to deal with this;
1) Use the tail trim tabs to hold her nose up; or
2) Add (canard) trim vanes forward of the wing to take some of the load.

The problem with (1) is that this leads to greater forces on the tail structure & a slightly greater angle of attack when in level flight, whereas (2) allows a small amount of lift to be applied forward of the wing to hold the nose up, while the trim tabs do the fine work they were designed for & allows the aircraft to maintain a lower angle of attack in the level flight profile.


(Note: CoL = Centre of Lift; CoG = Centre of Gravity)

[Scotaidh]

One problem with adding canards is placement of said winglets so as not to interfere with the engine intakes or the main wings.  Induced turbulence can be a nasty thing in the wrong places - compressor stalls and loss of lift being two of  immediately concern.


F7U-3 Cutlass - 039
Rather than try to get putty in the gaps, I used thin card-stock - cheap "For Sale" signs - which will then be sanded and puttied as necessary.


F7U-3 Cutlass - 040
The sanding went well.


F7U-3 Cutlass - 041
Some re-profiling will have to be done. This will cover over the cannon ports, but that's not actually a problem - their location certainly caused the induction of gun gas into the intakes. Which, as I'm sure you know, is a huge problem for turbine engines, causing compressor stalls and other evils. Given that a lot of this aircraft's problems were caused by the engines - inadequate to the task at best - why shoot yourself in the foot by adding other ills? "I see you have a bad case of arthritis, Mr. Jones, so we're going to amputate your left foot and right lung to see if that will help." <eye-roll>

[Old Wombat]

I was thinking high (about 45^o elevation) on the front area of the intakes, perpendicular to the curve (so interference should be minimal at worst), & as more of a trim device than an active manoeuvring element given the era of the aircraft (well, the Cutlass, anyway).

[Scotaidh]

<snip>

The J79 is only 10 inches longer than the J46.  If you look at the installation in the A/C the afterburner is mounted in the airframe.  The model has the afterburners mounted well past the rear fuselage.  So I think there is enough room to adjust the fit.  The main problem is the weight, the J79 is about 1500 lbs heavier than the J46.  You should look at the engineering of the period for a solution, not the desire for slapping on some canards.
<snip>

Well, salt6, I could have built the thing OOB and just said the J79s were in there - but what fun would that have been?  I'm not sure what you mean by "engineering of the period" - canards have been in use since the Wright brothers.  OK, not so much in this country, sure - but given that the design engineer at Vought - Mr. Rex Beisel, the designer of the Corsair - was clearly not concerned with current aviation design norms when he designed this aircraft, I think if he'd thought canards were needed he'd have put them on. 

I put the turkey feathers outside the fuselage mostly to have a clear visual difference between this aircraft and the real thing.  But also, I used  the burner cans from a Phantom model, mounted on mocked-up engine shapes of the requisite length and diameter.  I had to drastically cut out the exhaust apertures to get the fit I have now - had I tried to put the burner cans inside the fuselage I'd have had to cut out quite a bit more - and the plastic was getting dangerously thin as it was.

{Personal Opinion Warning}
There's an RAF pilot who has stated that most aircraft are "under-winged" - the only one that he felt wasn't was the Vulcan.  I have heard that the pilots who flew the Lightning loved that it was "ridiculously over-powered" - if only pilots designed aircraft!  With all this in mind, plus my own feeling most aircraft are too short for true proportional, aesthetic beauty, I think aircraft need to have these three things:
1) longer fuselage;
2) larger wing area;
3) more powerful engines.
I'm building this aircraft to reflect two of the three.
{End of Personal Opinion Warning}

I agree that beefier undercarriage would be needed, but as I'm building this one "in flight" ...  Yes, I'm being cowardly.    Also, though, sticking with my own traditions, since most of my aircraft are built that way. 

Without being snarky or with any agenda, I would be most interested to see how you would accomplish this conversion.  I'm sure I would learn something. 

[Scotaidh]

I actually would do it the same as you.  I was only addressing the engines affecting the balance of the aircraft.  If my other post was not clear, I apologize.  I believe that by you placing the engines farther to the rear, as evidenced by the AB outside of the fuselage, that you had attempted to address this issue.

On a personal note, I try to use the techniques of the time period.  That's why in the other post I mention the navy was leaning toward missiles rather than guns.

All in all, I believe this to be a good build and an avenue that this a/c could have taken.

Ah!  No, sorry - I'm not an engineer, so thinks like CG et al I know about but not enough to really let it concern me.  I like Whifs for the same reason I like to scratch-build Sci-Fi - I can do what I think looks good, and no-one can really criticize.   

[Scotaidh]

I have also thought about adding LERX - perhaps from an F-18 - instead of canards.

Thoughts, opinions?

[63cpe]

I say YES to a LERX. The one from f-18 might fit but take the inboard piece off the slat.
Made one slightly larger thand the f-18 one from styrenesheet for a project still not finished 

Read once the lift generated by the LERX was big enough to lift the F-18 at some high angels of attack.

David

[Scotaidh]

I say YES to a LERX. The one from f-18 might fit but take the inboard piece off the slat.
Made one slightly larger thand the f-18 one from styrenesheet for a project still not finished 

Read once the lift generated by the LERX was big enough to lift the F-18 at some high angels of attack.

David

Yes - I'd lose some of the inboard leading edge slat, but I think the trade-off would be worth it.  A Google Image search of "F-18 LERX" turned up this image:



with its interesting claim of 50% increase in max lift.  (This was 'lifted' from a Digital Combat Simulator page - https://forums.eagle.ru/showthread.php?p=2953434). 

The same forum discussion has an image of the Super Hornet showing that the LERX are solid - no gap.  While that would be easier to fabricate, I'm not so sure of the air-flow being useful.  I mean, the Hornet's vertical stabilizers are on the aft fuselage and canted outwards, so have their own air, so to speak; the Cutlass' stabilizers are almost sure to be blanked by the wing at any decent AOA, so they'd probably benefit from a bit of extra air-flow/vortex off the wing-root ... wouldn't they?

So now I need to buy a Hornet in 1/72 ...

[Old Wombat]

Airflow - yes.

Vortex - no.

Vortex would create flutter in the vertical tail-plane & cause control & fatigue issues.

What might work (I'm no aerodynamics engineer) is a vented LERX, with some air flowing up through the LERX onto the upper surface in front of the vertical tail-plane.

[Scotaidh]

Well, I've got a Hornet on order from Rare Plane Detective.  I only hope it has a pilot figure - the ones in my stash are not really suitable.  I guess I could always crack open a few new boxes and see what I've got - problem there is that I like to pose my aircraft as  flying, so they'll need their pilots.

I don't suppose any of you lot have any surplus pilots (1/72) that I could purchase?  Any era will do - I learned long ago how to make flight helmets from putty. 

[PR19_Kit]

I don't suppose any of you lot have any surplus pilots (1/72) that I could purchase?  Any era will do - I learned long ago how to make flight helmets from putty. 

As I never install the crew in my models I must have HUNDREDS of the little guys, but quite where they are I've no idea, yet. You'd be more than welcome to any that I find. Where are you in the world?

[Scotaidh]

I don't suppose any of you lot have any surplus pilots (1/72) that I could purchase?  Any era will do - I learned long ago how to make flight helmets from putty. 

As I never install the crew in my models I must have HUNDREDS of the little guys, but quite where they are I've no idea, yet. You'd be more than welcome to any that I find. Where are you in the world?

Thanks!    PM sent

[Scotaidh]

The F-18 arrived from Rare Planes Detective on Friday - in for surgery today!    Just gotta get through some of my Honey-Do list first ...


LERX donor arrived in good shape! 


LERX separated from the Hornet model. I basically just cut them both off to the wing root, using a fine-tooth razor saw. Turned out to be quite a bit longer than actually needed, so they were bisected again.


After beveling the trailing edge to fit the wings' leading edges, the LERX have been glued in place. I may have to detach them - I'm a bit disappointed that the inboard slots aren't a bit larger.


Some PSR will be needed, of course - you can see the leading edge of the LERX doesn't really align with the Cutlass' intakes.


I think they look OK ...

[TheChronicOne]