This Makes Me Happy

It’s about time.

I’ll get on my soapbox later this week to expand on why this is a good thing.

Designing A Dragon: Part 1

Since I’ve been working on a civilian supersonic aircraft since around 1995, I’ve amassed a large amount of data. Notice, when I say “working on”, I mean studying and testing on my free time at little to no cost. The project was originally nicknamed Dragon Wagon and was supposed to be a fighter jet that couldn’t fight. If that doesn’t make any sense, allow me to provide a little backstory.

Back in 1995, I saw the BD-10 fly at the Sussex Airshow in upstate NJ. Many things stood out to me, with the most important being the noise, the excess power and the ownership. The GE J-85 engine made prodigious amounts of noise, enough to wake neighbors three states away. The airplane was able to stand on its tail and climb nearly vertical. And most importantly, it was civilian built and civilian owned (meaning that in an alternate universe even I could own one). Needless to say, the BD-10 suffered developmental problems and never achieved the success that was hoped for. But the concept had set off an unstoppable juggernaut in my mind. Build an inexpensive non-fighting fighter jet for civilian pilots to fly and enjoy.

Owning a private fighter with 4th generation performance was and still is restricted by cost and legal issues. The massive expense of fighter jets is due to the fact that they are no longer just airplanes with guns, but weapons systems with wings. This is why there are so many WWII fighters flying around today and almost no 3rd or 4th generation fighters in private hands, even though Davis-Monthan AFB is littered with them. Remove the need to carry 350lb missiles, a Vulcan cannon, AESA radar, countermeasures and armor and suddenly a fighter jet loses a lot of weight and cost. Plus there would be no problem with demilitarizing systems or the State Department having a fit that a civilian is flying an airplane with hardpoints.

Armed (punintentional) with this knowledge, I set about designing a fighter that couldn’t fight. The first few design sketches looked like fairly conventional trainer jets, the most promising variant resembling the T-45. A low thrust to weight ratio was a goal (at most 1.4 lbs/lb st) with speed expected to be high subsonic and overall performance very respectable by any standards. However it wasn’t really what I wanted to create. All this changed once I happened upon an article in Aircraft Illustrated on the Facetmobile. It was a low aspect ratio lifting body with extremely light loading, didn’t stall, couldn’t spin and had a large AOA range. Plus it looked cool. Starting to research low aspect ratio and vortex lift, I abandoned my old designs and started to work on blended body designs.

The aircraft went from T-45 lookalike to a little arrow shaped airplane. The blended fuselage was designed around the occupants and the engine. The wing planform was at first a 70 degree single delta and later a double delta with a 70 degree leading section and a 50 degree aft section. Yaw control was achieved through a single large vertical stabilizer while pitch and roll is controlled with elevons on the trailing edge of the aft wing. The high sweep gave me the mathematical courage to increase the maximum speed from Mach 0.95 to Mach 1.4. At those sweep angles, the wing would be in subsonic flow throughout the entire speed range. I had no idea how to do a flutter analysis but did know enough that a higher aspect ratio on control surfaces would raise the minimum flutter speed if balanced properly (again, something to worry about in the distant future).

As for a powerplant, turbojet engines have small frontal area but also high fuel consumption (on the order of 1.0 TSFC for most power settings). Additionally, an afterburner was also out of the question. I love the noise but the fuel burn would have been far too high even with intermittent use. Thus a low bypass turbofan of lightweight and hight thrust was the only option. Engine selection should actually be credited to Cessna. I sent off for one of those deluxe information booklets on the Citationjet when it first hit the market (remember, this was before the internet and PDF files so waiting a week to get a big envelope from Wichita was a huge deal for a 14year old). It used a pair of FJ44-1 turbofans, an extremely compact, lightweight and efficient engine. A rough estimate in my mind quite literally went like this: “If I used only one engine, the Citationjet fuel flow numbers would be cut in half and with a smaller frontal area thanks to tandem seating, that half would be closer to 40%.” Later calculations with refined drag data showed that the 40% guess was pretty accurate.

I’ll stop there for now. Next time I’ll include original sketches and discuss cruise performance. In case you need motivation to read it, the initial performance numbers called for a stall at 50 knots, subsonic cruise of Mach 0.98, supersonic cruise limit of Mach 1.4, a ceiling of 50,000 ft and a climb rate of 30,000fpm.

See ya next time!

Light Jet Thrust Formula (#1)

For determining minimum static thrust required for non flat-rated turbofan and turbojet engines.

As temperature and or altitude increases, the amount of thrust produced by an engine will subsequently decrease. On larger turbines, flat rating is available to supply rated thrust above sea level and ISA. For smaller engines that may not have the thermodynamic capacity to flat rate, the designer must take into account the degradation of thrust. Failure to do so will result in unsatisfactory performance, inability to meet expected book values and possible payload restrictions.

The simple fact is as engines get larger, the weight of passengers becomes less of a fraction of the static thrust. Those who wish to build cabin class transportation aircraft are probably going to find their optimum engine (or engines) in the 2000lb thrust class or larger. Use of small jet engines under 500lbs thrust is therefore likely to be associated with very light multi engine craft, experimental/test vehicles and unmanned aircraft. Development of small jet engines however, should not be deterred by market analysis that may not fully understand the different potential uses beyond business and high-end general aviation. Too often advances are cut short by the opposite nature of engineering and profit, which work on completely different time scales.

The urge to use as small an engine as possible should be avoided when building light jet aircraft for the simple fact that passengers cannot be reduced in weight. Smaller engines can operate closer to their optimal TSFC at cruise altitude, but like all of aviation, there has to be a tradeoff for other flight envelopes. Therefore if one wishes to have a 4 place jet with a 200 lb allowance for each passenger, using twin 400 lb thrust engines for a total of 800lbs will provide a 1:1 thrust to payload ratio. This number will gradually change as airframe, fuel, avionics and interior is added. By the time a weight of say 3200lbs is reached (1600lbs empty weight, 800lbs payload and 800lbs fuel…very minimal levels), the total thrust to weight has declined to 4:1. This figure is at sea level on a standard day. Take the same aircraft to Colorado Springs at 65 degrees and the thrust to weight balloons to 5:1. Each pound of thrust has to push over 5lbs of aircraft, thus increasing takeoff distance, time to climb, and fuel consumed over a given stage. Humidity and air pressure may also conspire to rob the engines of performance.

The following formula is very simple as it is meant to be a rough estimate to warn a designer of low thrust levels at a nominal altitude of 6,000 ft MSL and 65 degrees. This altitude also equates to approximately 0.785 with respect to air density at SL ISA. But we’re conservative and/or lazy so the easy to remember 0.75 ratio is the rule of thumb to remember. The thrust lapse rate is based on a low bypass turbofan engine (less than 3:1) and will vary with fan size, nozzle design and turbine temperature. Your engine will vary so consult the appropriate manufacturer tables when advancing beyond the initial planning stage of your design. By using a reduced thrust computation when setting gross weight, unpleasant surprises can be avoided while in the flight test stage.


Thrust to weight ratio 1 = A/C weight divided by thrust at sea level, ISA.

Thrust to weight ratio 2 = A/C weight divided by thrust at roughly 6000 feet MSL, ISA.

If you really want to scare yourself, multiply 0.75 by the static thrust of your selected engines (assuming they are not flat rated) and see what number you get!