Category Archives: planes

One of the Greatest Fighters that Never Was- F-20 Tigershark

Originally concieved as an improved F-5 Tiger, the F-20 Tigershark was a Northrup product intended for sales to overseas customers. Replacing the twin J85 turbojets of the F-5 with the F404 turbofan engine, the F-20 was a real hotrod.

Unfortunately, it was also about the same time that our NATO allies jumped whole hog into production of the F-16, coupled with sales of French and other fighters. The market timing was off, and the Tigershark never went into serial production.

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The Enforcer

I can’t even count the numbers of times I’ve written about OV-10/A-29/AT-6/various other light COIN/LAARA/LARA/you name it type low cost light attack aircraft.

And virtually ever time I do, either a comment or an email shows up asking “why not the PA-48 Enforcer?”

In the late 1950s, a very small company that eventually became Cavalier Aviation had the bright idea to use surplus P-51 Mustangs as high speed executive transports. Remember, this was well before the idea of a business jet was conceived.  A nice interior and a second seat in a ‘stang seemed like just the  thing a company president would need to travel in style for business. And it wasn’t that wild of an idea. Several other companies were converting light bombers like the A-26 into transports as well.

In the end, only a small number of these Cavalier Mustangs* were converted. As a way of keeping the company busy, Cavalier also refurbished some P-51s in use in South American air forces. Along the way, someone at Cavalier got the bright idea of replacing the Packard built Merlin engine with a Rolls Royce Dart turboprop engine.

The Dart powered conversion, known as  the Enforcer, was quite the performer. And Cavalier wanted to get some USAF contracts. But it really had no chance. Seeking a better suited industrial partner, Cavalier sold the Enforcer design to Piper Aircraft in 1970.

Piper eventually convinced the Air Force to evaluate the renamed PA-48 Enforcer. By this time, the Enforcer was about 90% a new design, with only the slimmest heritage shared with the original P-51. In 1983 and 1984, two Enforcers were evaluated by the Air Force. They weren’t flown by the Air Force. They weren’t bought by the Air Force.  The Air Force just watched Piper put them through their paces, said “that’s pretty cool” and when asked if they wanted to buy some, said “thanks, but no thanks.”

It’s not that the Enforcer was a bad airplane. But in 1984, the Air Force still had in its inventory several hundred OV-10 and A/OA-37 planes. They couldn’t see the point of adding yet another airframe for essentially the same mission.

Four Enforcers were built over the years. And two still exist. One is in the National Museum of the Air Force. The other has just undergone an extensive restoration at the Air Force Flight Test Museum at Edwards Air Force Base.


*Many of which have been restored to their original configuration and are now seen at airshows.


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The X-31

Spill hit an estate sale this week, and came away with a nice little gem.

This coin contains metal from the X-31 aircraft.

How did they get the metal from the aircraft? Glad you asked.

The use of digital fly-by-wire controls in high performance aircraft, covered by Spill here, 2, 3, 4, 5, meant that unconventional flight controls could be used on planes to maneuver in ways not previously possible. In particular, thrust vectoring could be used to control aircraft at very high angles of attack.

A joint US and German test program conceived and built the Rockwell/MBB X-31 research plane to explore this use of DFBW control in conjunction with high angles of attack and thrust vectoring.

File:Rockwell-MBB X-31 vectorpaddles.jpg

X-31 in flight. Notice the three “paddles” used to vector the thrust.

Two were built, and a highly successful test program showed the X-31 was capable of maneuvers that were then astonishing. Since it was purely a research aircraft, it was quite small, had a very small fuel load (typically, only 4100 pounds) at take off had a thrust-to-weight ratio of 1:1, which meant it could accelerate vertically right off the runway.

After several years and hundreds of test flights, one of the two X-31s had its pitot tube replaced by a Kiel tube. The pitot tube us the pointy stick pointing out of the nose of the jet. It measures the dynamic pressure of the air. Between it and a static air pressure sensor, the pitot system provided air data to the flight control computer to determine speed and altitude.

Remember, in a DFBW system, the pilot doesn’t control the airplane directly. He uses the flight controls to tell the computer what he wants the plane to do. The computer uses those imputs, along with air data from the pitot system, and attitude data, to determine which controls should be deflected, and how much.

Obviously, if the air data was corrupted, the computer would provide corrupted control deflections.

One of the most common failure modes for pitot tubes is icing. Moisture from clouds or humidity freezes on the pitot tube, constricting the flow of air through the tube, which makes the computer think it is going faster than it is. To combat this, most pitot tubes have an electrical heater, just like the rear defroster on your car. The normal pitot tube on the X-31 was replaced by Kiel tube, which gave more accurate air data at high angles of attack. But it didn’t have a heater. Given that the flight test rules for the program prohibited flying the X-31 in known icing conditions, this wasn’t thought to be a significant hazard.

Of course, Murphy gets a vote. The engineers knew there was no pitot heat. The pilot didn’t.  And of course, the X-31 encountered pitot icing. Not immediate, but gradual accumulation of ice led to a steady degradation of airflow, and hence data. And that led to instability, as the flight computer tried to make the plane do things that it didn’t want to do.  The X-31 exectuted an uncontrolled pitch-up, and as soon as the pilot realized he had no control, he safely ejected.

The loss of the X-31 is unusual in that it was very carefully documented. It took place almost directly over the airfield, and was being tracked by powerful cameras on the ground.

Here’s the short version:

If you’re interested, a 40 minute video investigates the chain of errors that led to the mishap. It can be found here.

And here’s a brief history of the program.

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Who wants to fly to Hawaii?

I thought I knew the story of just about every 707 accident, but I’ve never heard of this one. It’s incredible.  And it was caught on film.


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Sunset for the Phrog

Originally designed for the Army, and first flown in 1962, over 500 would be built in the next 11 years. Think of that. The last Phrog rolled off the line 43 years ago. That’s some loyal service.

File:CH-46 Sea Knight Helicopter.jpg

It would serve with the US Marines, the US Navy, Canada, Sweden, Japan, and Saudi Arabia, in addition to several civil operators.

On a personal note, the only aircraft I actually remember seeing my dad fly was a Sea Knight.

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We’ve mentioned operating helicopters from smaller ships. In the US Navy, this mostly means destroyers and frigates. Which, at anywhere from 3000 tons to 9000 tons, that’s a goodly sized ship.

Other navies, like the Royal Danish Navy, often operate helicopters from much smaller ships, such as this Offshore Patrol Vessel. And in heavy seas, it can get downright sporty.

Notice immediately after touchdown, a probe extends from the belly of the Lynx. It engages a grate on the landing deck, to keep the helicopter from sliding off the deck, in spite of the pitching and rolling.

The US Navy uses a somewhat different system, RAST, developed from the Canadian Beartrap device.

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The Hazards of Going on Autopilot – The New Yorker

A 1994 N.T.S.B. review of thirty-seven major accidents between 1978 and 1990 that involved airline crews found that in thirty-one cases faulty or inadequate monitoring were partly to blame. Nothing had failed; the crew had just neglected to properly monitor the controls.The period studied coincided with an era of increased cockpit automation, which was designed to save lives by eliminating the dangers related to human error. The supporting logic was the same in aviation as it was in other fields: humans are highly fallible; systems, much less so. Automation would prevent mistakes caused by inattention, fatigue, and other human shortcomings, and free people to think about big-picture issues and, therefore, make better strategic decisions. Yet, as automation has increased, human error has not gone away: it remains the leading cause of aviation accidents.

via The Hazards of Going on Autopilot – The New Yorker.

I don’t look to The New Yorker for aerospace articles, but I thought this one was interesting enough to share. I welcome comments from the pilots here on whether automation hurts or helps.


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