WesWorld

Krabl Aviation Aircraft:

XF-1 Fighter (as yet unnamed):

Status: Glider Testing (since there's not a large wind tunnel in Thailand) as of May 1949

Design: Single Seat Day Fighter, modified flying wing

Construction: All-Metal

Engines: 2 x Rolls Royce Derwent V Centrifugal Turbojets, 15.57kN static, 17.79kN maximum thrust

Length: 7.4m

Height: 2.8m overall

Wingspan: 16.8m

Empty Weight: 4800kg

Max Takeoff Weight: 7200kg

Fuel Capacity: 1600kg (as planned)

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Performance (Theoretical currently)

Maximum Speed: 1100km/hr (clean), 950km/hr (with external stores)

Cruise Speed: 900km/hr (clean), 800km/hr (with external stores)

Range: 1500km (clean), 1000km (with external bombs/rockets), 1800km (with external fuel tank)

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Armament:

4 x 20mm cannon (220 rpg)

Centreline hardpoint for 500kg fuel tank or rack for 2 x 200kg bombs or 8 x rockets

Thai National Aircraft Corporation XC-9:

Type: Medium Cargo Aircraft

Seating: 16 Passengers or 4000lbs of cargo

Crew: 3 (Pilot, Co-Pilot, Flight Engineer (in military variant, steward in civilian variant))

High wing monoplane, all metal construction

Engines: 3 x De Havilland Turboprop, 575shp apiece. 2 in wing nacelles, 1 in nose with ducting for bypass exhaust.

Doing some back-of-an-envelope calculations on the thrust-weight ratio of the proposed design I have to point out that it is seriously deficient in this department when compared to historical designs for leading-edge fighter aircraft. Using the given engine thrust and mass figures I calculate that is thrust-weight ratio is .22 (using the static value) or .25 (using the overall value). For comparison, the historical F-86 Sabre had a thrust weight ratio of .42, and the contemporary MiG-15 a thrust-weight ratio of .54

This would suggest that the XF-1 would not be a match for its potential opposition.

I also have reservations regarding the weights, as it seems that the difference between the empty and max takeoff loads barely cover the weight of fuel, ordnance, and, of course, the pilot.

I took some historical figures and did some rather simple changes to them. However, I agree with you on most of this, so I'll get back to work on the basic design specs to put it more in line with what's possible.

Actually just looked up some engine weights, which was quite interesting. Also caused me to revise my weight projections significantly as the historical engines were not only less powerful but also 150kg heavier than the Derwent V. Also, keep in mind that there are 2 of those engines, so the thrust-weight ratio is for 2 engines, not one. By my calculations, the T-W is currently .504 at max takeoff weight (which would be with a max loadout of external stores), .541 at a 'standard fighter weight' (no external stores, all internal fuel and cannon ammo) of 6700kg, and .575 at 'light fighter weight' (6300kg, full internal ammunition but only 1200lb of onboard fuel).

Ultimately at higher speeds it's going to have compressibility issues that are going to keep it from using its higher thrust efficiently. So while it might have a higher thrust-mass ratio on paper, it's not going to have a higher top speed, though it might have better straightline acceleration on the flat from low-moderate speeds to higher speeds. The climb rate should also be quite good, same with the length of runway required for takeoff. Downside compared with aircraft like the MiG-15 would be double the maintenance issues with 2 engines, slower turn rate due to lack of a proper rudder, and reduced pilot visibility to a degree. Some of that could be compensated for with tactics that used the higher acceleration and rate of climb to keep a fight in the vertical.

Reasonably I could possibly put that additional thrust to use with having higher internal storage or loadings, but I'm also considering that this is supposed to be a bit of a 'leap' aircraft for the nation, so it's going to have its unrealized potentials and deficiencies. This would be a plane that in its Mk 3 or 4 variant might be noticeably improved.

Quoted

2 x Rolls Royce Derwent V Centrifugal Turbojets, 15.57kN static, 17.79kN maximum thrust

Ah, I see where I misinterpreted the specifications. I took the figures to be the total thrust, rather than thrust for each engine. My apologies. A question though - have the British developed the Derwent to the historical Derwent V at this point in time?

It's possible that it's not. And I'll have to confirm with Hood that it is the Derwent V that is being sold to Thailand. So the figures could drop a bit, but not out of the realm of being at least reasonably competitive with regional rivals. Based on the Derwent V being historically a 1945 engine, and the Derwent 8 being the quintessential engine for the Meteor before the Brits upgraded to better things, I would presume that the Derwent V is indeed in production.

If it does wind up being the Derwent 1, then the overall dry weight will drop by an additional 220kg, and the overall thrust would be 4000lb. Which would give a .259 Thrust-Weight at maximum weight, and a .28 T-W at the heaviest 'clean' weight. Which would probably require some substantial reduction in overall size to drop the weight down to 4000kg to get the T-W to acceptable fighter levels.

Now properly chastened, I will turn my attention to the XC-9, and ask if the details on this de Havilland turboprop engine are posted somewhere. I do not recall DH being involved in turboprops, but WW might well be different. Details would be most welcome.

Yes the Derwent V exists, has done so since 1945. This is the export version for Thailand.
There are higher-powered further developments from 1946 and 1948 but these have not yet reached production but have been certified.

The de Havilland Globe is a real OTL project by Halford that was bench tested. It was intended as a postwar Gipsy replacement but it never came to fruition. In WW I had thoughts of using it but never found a suitable set of light aircraft. So it was certified n 1945 and DH tinkered with it but no production, hence their delight and gettibg some return now from Thai orders and who knows perhaps brining turbine power to the private owner after all?

My mega thread "1949 Aircraft Specficatios" is the best place to find the data.

Ahh wonderful. Thanks Hood for clearing that up. Krabl is also working on a bomber aircraft prototype using the same powerplant arrangement and general airframe concept, just scaled up to 12000kg, which would put it in the same weight class as the DH Mosquito, roughly. Any larger than 12000kg and it'd need to go up in engine size or number to compensate since ~12000kg with 2 engines gives a .3 T-W. Past that, you'd need to double the engines (or go with bigger ones, that's not an option for Thailand right now)....and at that point you'd be best to go up to 18000kg since that'd give you a .4 T-W which is high performance for even a fast bomber. Or go up to 24000kg, which gives a .3 T-W and makes it still a usefully quick bomber, only now it's not going to be accelerating away from anything, ever. But still a functional bomber by all regards. And I think those are just too big for Thailand to build yet.

Quoted from "Hood"

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The de Havilland Globe is a real OTL project by Halford that was bench tested. It was intended as a postwar Gipsy replacement but it never came to fruition. In WW I had thoughts of using it but never found a suitable set of light aircraft. So it was certified n 1945 and DH tinkered with it but no production, hence their delight and gettibg some return now from Thai orders and who knows perhaps brining turbine power to the private owner after all?

My mega thread "1949 Aircraft Specficatios" is the best place to find the data.

My thanks as well for the clarification. I shall have to delve into those specifications and update my 'intelligence files'. 8)

I have some more detailed comments now I have time.

XF-1: Is this a tailless flying wing you are planning or something more like the historical DH.108? Your comments on rudders suggest a tailless type or perhaps just endplate fins/rudders? I am envisioning the XF-1 to be something like the Horten/Gotha Go.229 in layout?

Developing a high-speed jet-powered flying wing fighter without proper wind tunnel testing sounds a bad move all round. I've seen your latest IC news and note that the glider has crashed already. A glider can only tell you so much, at best the low speed handling and likely stall characteristics. You really well need wind tunnel data for the higher-speed performance. Smaller scale tunnel tests on models of course are probably possible in Thailand. But if you want high-speed larger scale model wind tunnel tests the best bet would be to buy time in one of the well known research establishments in the world. It would be well worth the cost. Of course the Royal Aircraft Establishment would be happy to help or you could approach other players here or even use any number of NPC places like NACA in the States. If you are a planning a Horten clone I would say it took the brothers years of experiements to get to the stage of the Go.229 and that aircraft of course never flew so we have no idea what its handling problems might of been. A very ambitious project for Thailand.

XC-9: kind of reminds me of the Australian DHA-3 Drover in layout but with larger capability. 1,725shp in total though sounds a little puny if you are planning to lug around 4,000lb of cargo. Most transports in this bracket are probably operating with twice the horsepower from two piston radials. Good to see more tri-motors around though.

Future bomber: yes for a flying wing bomber development you could either double up the Derwents or use a couple of Nenes, the result would be something like the historical AW.52.

Think flying wing with a rudder. The rudder can significantly assist with lateral stability as well as it provides an anchor to sideslip problems endemic to flying wings.

And yes, there's smaller scale wind-tunnel testing, though as you noted Thailand would have to outsource prototypes to large nations that have the proper full-scale wind tunnels if they wanted to do full testing. Also, the chief engineer of Krabl Aeronautics was formerly from Lockheed, so he has more than a passing familiarity with much of the issues in play and likely brought with him a lot of his calculations and information he's picked up over his experience with designing aircraft.

And yes, we are talking about nationalistic Thailand.....they're willing to accept a bit more risk in the pursuit of their own home-grown military aviation programs than would necessarily be accepted in Britain, Germany, or the US.

As for the bomber as compared to the fighter, it's really just scaling up the size of the aircraft itself by about 60% or so. Takes it from a over .5 T-W to in the mid .3s T-W. And gives a roughly equivalent ordnance bay to a Mosquito. So a light-medium bomber......certainly Thailand is not up to the task of building its own Heavy Bomber yet.

About the wind tunnel tests

China invites Thailand to use the wind tunnel in the CAICO airmanufacturer complex at Loiwing.