Airframe Scaling

[apophenia]

A question for engineers and other knowledgeable folk: How predictable is the chance of aerodynamic success when re-scaling an airframe?

I'm aware that many phenomena do not scale. But I'm wonder about the effects of scaling an airframe by 50%. My example airframe is the Boeing MQ-28A Ghost Bat - shown here is sideviews representing the fullscale UCAV and essentially the same airframe scaled-down to 50% size as a cruise missile.

Were this scaling both feasible and risk-reducing for development, my object would be to re-use aerodynamic work already invested in the MQ-28A to create a rival to the German Taurus KEPD 350 cruise missile.

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FWIW: The fullscale MQ-28A measures 11.70 m (38.38 ft) in length. So, the 50% scaled Ghost Bat would be 5.85 m (19.19 feet) long. The existing Taurus KEPD 350 missile has a length of ~5.0 m (16.40 ft).

[Old Wombat]

Depends on what you want to put inside the airframe, halving the dimensions quarters the volume.

The thickness of the frames & skin will, probably, be unable to be reduced by much; which will further reduce internal volume & ensure that the wing-loading will go up.

A lot of other parts will, likewise be thicker & heavier than a simple scale-down will allow, or will already be miniaturised as far as they can go. So, they will take up a larger percentage of the remaining volume & add more mass.

Also, engines; will there be one a quarter the volume of the original or will a larger engine be required to maintain performance?

Lots of questions, most of which chew up your internal volume (reducing fuel/payload capacity) & add mass (reducing performance).

Which is why things don't scale down as well as they scale up.

[apophenia]

Excellent stuff Guy 

Your comments on internal volume and wing-loading are sobering. To answer your question about what would need to go inside ... in this specific example - beyond fuel, engine, controls, guidance, etc. - it needs to accommodate a BLU-116 AUP warhead (or non-US equivalent).

Weight can be reduced - dumping the undercarriage; restressing for a one-flight lifespan; simplified controls; etc. But quartering the available volume is the BIG reality check here.

For powerplant, the MQ-28A engine is (AFAIK) a Williams FJ44 ... no idea on the exact sub-type. So, we have a thrust anywhere between 1,900 and 3,000 lbf; a diameter of  20.7 to 23.0 inches; and a dry weight of 460 to 535 lbs. My 'close to 50%' choice would have been the PW610F producing 950 lbf; a diameter of 14 inches; and weighing about 260 lbs. But in length (46 inches), the PW601F isn't much different from the Ghost Bat's FJ44. Needing to drop down to a quarter of the volume is a whole 'nuther deal!

The Williams WJ38-7 (F415-WR-400 for the Tomahawk Block IV TLAM-E) might be small enough but I can't find any published dimensions. I suspect that F415 details are still classified ... and, therefore, possibly unavailable for any non-US programmes.

All this has me wondering if turbofans mightn't be the best approach. They give you much better sfc and are quieter but you need so much more space to shoe-horn one in. That probably explains why SCALP/Storm Shadow are using the punchy-but-thirsty Microturbo TRI-60-30.

Another 'old school' turbojet option would be the Teledyne J402-CA-400 turbojet - diameter 12.5 inches; dry weight 101.5 lbs; length 29 inches. That's gets us down to 'quarter scale' but the Teledyne only produces 660 lbf with twice the sfc 

Anyone have any other ideas?

[Kerick]

Just throwing this out there. I know that to double the speed of a ship you have to quadruple the power. So if you scale down by half this would reduce the power needed substantially. Would an aircraft be anywhere near the formula for a ship? Probably not. To get to the point, if an aircraft is half the size is the power requirement reduced by half or more? Once you get down to a certain size would it change that much at all?

[apophenia]

Very interesting observations Ken 

My choice of subjects here may be a problem - little in the way of specs have been released on the MQ-28A, AFAIK.

Hmmm, ... maybe some of the old Scaled Composites project may elucidate. I'll have to investigate that.

[elmayerle]

For your cruise missile, consider the engine in the JSSAM-ER, which I am given to uderstand is the same as was fitted to the AGM-137A/BGM-137B.  That should give you a suitable and proven engine.

[apophenia]

For your cruise missile, consider the engine in the JSSAM-ER, which I am given to uderstand is the same as was fitted to the AGM-137A/BGM-137B.  That should give you a suitable and proven engine.

Thanks Evan. I had considered the Teledyne J402-CA-400 which seemed to be a perfect size match.

That said, two elements make the J402 series less than ideal. The first is having a much higher sfc than (the admittedly bulkier) modern turbofans. The second is a turbojet's give-away screech at low altitude. (Mind you, the latter issue doesn't seem to stop the Storm Shadows from getting through.)

[elmayerle]

Turbofans can be beneficial in reducing noise and, with a mixer, thermal signature.  TSSAM did a good job of mixing the core and fan flows for reduced thermal signature and the mixer had an LO coating for all aspects "Stealth".