Design Challenge: Loyal Wingman / Advanced Trainer / Light Combat Aircraft

[Scott Kenny]

@red admiral , @Arjen - Thank you. I will now go and look up this trickery known as "Reynold" number.

The basic idea is that as you get a smaller flying object, the air acts more viscous. So at people-to-plane size you're in air, at bird size you're in something closer to water, and at insect size you're in oil or even honey.

It actually kinda kicked the Wright Brothers in the jimmies. You see, they had copied their wing shape off of large sea birds, so they had a deeply cambered shape that was relatively thin overall, shaped kinda like a parenthesis (( but laid on its side. That was too thin a wing for an object the size of their Flyer. WW1 planes had a much thicker wing, shaped more like this: (|

Then as planes got faster the wings got thinner again, but with much less camber.

And of course, we see the highly cambered idea come back with the flaps and slats on modern commercial planes.

 

[red admiral]

The Saab study was for their GFF stealth fighter (pic below), so this is relevant for this thread as I could use a 66%-75% scale factor for my notional loyal wingman.

The cuboid law is a reasonable start if there's nohting better, but I'd reccommend using mass estimation methods from textbooks like Raymer or Nicolai. At this scale of aircraft it should be well within the dataset range.

 

[H_K]

I think it would be worth drawing out some cross sections between the end of the weapon bay and the engine face in order to understand the cross sectional area required.

I doubt the current duct outline will work. Unfortunately the tail fins on the missiles mean that the rear of the weapon bay is widest so you can't clip through it as shown here. There's then also a need for some wing carry-through structure, and some nice beefy structure to mount the main landing gear to.

Per your suggestion, here is a refined view of the duct shape and some preliminary cross-sections.

My duct is based on the MRF 2001 studies, scaled down ~50% from an airflow of ~280lbs/sec to ~145lbs/sec (Snecma M88 / GE F404), and extending the midsection upwards to get over the "hump" of the rear weapons bay.

Next up I need to add some more cross-sections (with some CFT-like bulges to accommodate the middle duct sections as they curve up through the wing), add the main landing gear (located in the wing root), check for area ruling, and do a top view... so more work required to make sure it all comes together.

My main question mark right now is where the main bulkheads should be to carry through structural loads... possibly a rear bulkhead right behind the weapons bay, and a forward bulkhead near the middle of the weapons bay?



Link to the MRF duct studies:
Performance Study for Inlet Installations - McDonnell Douglas (1992)

https://www.secretprojects.co.uk/threads/mcdd-northrop-bae-astovl-mrf-jast-jsf-studies.2392/post-49723

 

[red admiral]

@H_K

That's looking more realistic to me. I'd probably have the fore/aft duct sections with a bit less area/angle change.

For the weapon bay then remember that you need to fit a launcher above the missile and be able to attach this to some structure. The door also has non zero thickness to cope with aerodynamic loads. Both will probably increase bay depth.

For major fuselage frames then I'd say front/aft of the bay, with another above the intake joining the front wing spar which should give a nice direct load path through the fuselage. Further aft the load paths will have to go around the duct and engine bay.

 

[Scott Kenny]

Flight loads mean major bulkheads lining up with the wing spars. Probably 3 spars per wing (minimum): 1 spar at the 1/3 chord line for subsonic center of lift, second spar at the 1/2 chord line for supersonic center of lift, and a rear spar that all the control surface hinges attach to.

 

[H_K]

Quick update on my Loyal Wingman / advanced trainer design.. still working on the top view to show off the triple delta wing with Levcons, but this will do for now:

• Added a more realistic front view... "lifting body", increased wing thickness (~6%), more detail around the air intakes, added landing gear etc.
• Larger "humpback" to increase the internal fuel volume (similar to CFTs)
• "Pelikan" tail like on the X-32 (3-axis flight control)

 

[H_K]

To give a sense of sizing, here is the single seater advanced trainer / light combat aircraft compared to a T-38 Talon and Etendard VI.

I chose these as they are representative of lightweight transonic / low supersonic designs from the 1950s... both about 3.7t empty and basically as small as you can get, with similar engine weights and internal fuel capacities to what I'm looking for.



As one can see, the relative size is similar, but obviously body cross section will be wider due to the need to fit the internal weapons bay (+ ~1,250 liters of internal volume) and inclined sides to reduce RCS. My goal would be to try to keep empty weight growth to ~10% (i.e. 4.1-4.2 tonnes) relative to these designs, leveraging modern composite materials and the structural efficiency of a delta wing + Pelikan tail, plus modern aerodynamics to improve lift & transonic drag (lifting body, triple delta wing etc).

Max speed target would be Mach 1.25 - 1.30 with a non-afterburning M88 engine of 50-55kN thrust (11,000 - 12,000lbf). This assumes drag increase can be kept to ~10% relative to the T-38 (based on performance charts, the T-38 needs 4,000 - 4,250lbf of net thrust at 36,000 ft / M1.25-1.30 which translates to 10,200 - 10,700lbf of static dry sea level thrust).

 

[Anderman]

A non-afterburning M88 with 75-82kN thrust ?

 

[H_K]

A non-afterburning M88 with 75-82kN thrust ?

Sorry I meant 50-55kN (i.e. current M88 mil thrust or +10% growth).