Wednesday, October 17, 2007

Cirrus Jet Fat-Wing Design Revisited

Yesterday I heard from Mike Van Staagen, VP of Advanced Development at Cirrus, who saw my AOPA blog of a week or so ago, in which I made light of the Cirrus Jet's short, stumpy wings. He gave me some insight into why the wings are that way. "We're limiting the span to 38.5 feet," he pointed out, "so that the current Cirrus-owner population, which is approaching 4000, can use their existing hangars." This has been a common request, apparently. And it makes good business sense to listen to your customers, so I can see his point there.

He also noted that a thick airfoil cross-section allows for the "lightest possible structure," which I suppose might be true. I don't know. I'm not a structural engineer.

Cirrus is also trying to meet a 61-knot stall speed requirement. This can be approached in different ways, obviously, but starting with a wing that has inherently high induced drag probably doesn't hurt.

Cirrus has prioritized things in such a way as to put aerodynamic efficiency well down the list (which they've admitted all along). Van Staagen notes that even with a short/fat wing, the Cirrus Jet has sufficient power to reach its 25K-foot cruising altitude quickly, certainly quicker than most piston pilots are used to.

But there's a lot of competition in the nano-jet arena (see Philip Greenspun's excellent overview), and not all customers will be basing their decisions on purchase price alone. Some will want a practical long-range cross-country machine that can be justified on the numbers. It remains to be seen whether the Cirrus design will stand up to the competition in a total-ROI sense. Let's face it: if all you want is a snazzy pocket rocket that can get to FL 250 quickly and fly 400 miles before refueling (at a cost well under $1 million), there are any number of L-39s (and other jet trainers) out there right now that can fill the need for $400K. They won't carry 7 people, but the snazz factor is there and you're cruising quite a bit faster than the Cirrus Jet will.

Personally, I think Cirrus is falling into a pattern often repeated in aviation, of designers designing a fine airplane that later needs longer wings. Ted Smith went down this road with the Aero Commander. He did it again with the Aerostar. The original Cessna 421 had short wings; the 421B got longer ones. The Citation I had short wings; they were lengthened in the Citation II. In the Piper PA-28 series, Piper went from a short/fat wing to long/tapered. Peter Garrison eventually lengthened the wings of his Melmoth homebuilt. (The list is a long one if you count military aircraft, airliners, and helicopters that got longer blades.)

I can think of no successful production aircraft that later underwent wingspan reduction.

I'm no aeronautical engineer, but I think history is clear on the fact that adding a bit of wingspan is one of the cheapest ways to increase a plane's utility and efficiency. Payload, range, rate of climb, all increase. Stall speed goes down. Cruise speed, practically unaffected. In a jet, overall fuel economy improves dramatically because you get to high altitude faster.

But if you've got to fit a jet into a piston-aircraft hangar, all bets are off.

I wish Mike and the Cirrus folks well. I can't wait to see their little jet fly. It may be the Grumman Yankee of jets, but you know what? I liked the Yankee.

1 comment:

Gordon R. Vaughan said...

Interesting analysis, capped off with this, "But if you've got to fit a jet into a piston-aircraft hangar, all bets are off."

Considering all the trouble involved in modifying/optimizing jet aircraft and the many $$/pound it costs to build them, you'd think the least of our problems would be addding a few feet to a simple hangar.

That it may not be says a lot about what's wrong with the economics of GA these days.

Considering how important longer wings (or winglets) are to fuel economy, maybe airports should get carbon credits for stretching their small hangars!

Also, thanks for pointing me to Philip Greenspun's excellent site.