Monday, May 31, 2010

Can my ultralight legally cruise faster than 55 knots?

Yes it can.

You may already be saying: "James -- you are just flat wrong. I have Part 103 memorized -- and it says that the cruise speed limit of an ultralight is 55 knots."

Well, I think you are flat wrong. I've been mulling on this a while, and I thought it was worth some discussion.

First of all, let's cite the appropriate text from Part 103:

Sec. 103.1 Applicability.

This part prescribes rules governing the operation of ultralight vehicles
in the United States. For the purposes of this part, an ultralight vehicle
is a vehicle that:
(a) Is used or intended to be used for manned operation in the air by a
single occupant;
(b) Is used or intended to be used for recreation or sport purposes only;
(c) Does not have any U.S. or foreign airworthiness certificate; and
(d) If unpowered, weighs less than 155 pounds; or
(e) If powered:
(1) Weighs less than 254 pounds empty weight, excluding floats and safety
devices which are intended for deployment in a potentially
catastrophic situation;
(2) Has a fuel capacity not exceeding 5 U.S. gallons;
(3) Is not capable of more than 55 knots calibrated airspeed at full
power in level flight; and
(4) Has a power-off stall speed which does not exceed 24 knots calibrated

I think we can all agree that this is indeed the relevant language. Let's take a look at the last bullet point (3) a little more closely:

"(3) Is not capable of more than 55 knots calibrated airspeed at full power in level flight;"

And this is where I make my point: the language in the law describes calibrated airspeed as being the correct determination of the maximum cruising speed of an ultralight. What is calibrated airspeed?

First of all, it's not true airspeed.

Let's take, for example, a typical day in Kansas:

temperature: 88 degrees.
pattern altitude: 2000 feet.
density altitude: 3700 feet.

OK, with these conditions, let's say that I'm cruising along, full throttle, in my Belite Superlite at 55 knots Calibrated Air Speed. How fast am I really going?

The answer is, of course, a good chunk higher than 55 knots. One place that has a True Air Speed calculator online is here. And it shows that my Belite Superlite is actually cruising at 58.5 knots, which is 67.3 mph! Sweet!

You may think I'm splitting hairs. Not really -- this delves fundamentally into the philosophy of Part 103, which was based on the air pressure that the airframe would 'feel', which means Calibrated Air Speed.

The consequences get more beneficial for our friends who are flying their ultralights in very high altitudes. Consider a Kitfox Lite owner I heard from who was operating his aircraft in the mountains of Idaho -- his altitude would regularly be 7000 feet (or perhaps more.)

By calibrating his aircraft to cruise (full throttle) at 55 KCAS, he would actually be cruising at 63 knots True Air Speed. That's 72.5 mph.

Now let's turn this into a real world benefit for our Superlite owners. Our Superlite is easily capable of cruise exceeding the Part 103 speed limit, due to its wonderful wing design and the big Hirth 50HP engine. For legal part 103 operations in Kansas, we have to detune the engine to about 38HP. We do this by installing a throttle stop.

For Superlite owners who want to operate, for instance, in Colorado, it's an easy matter to change the throttle stop so that 55 KCAS is observed at their high altitude -- the Hirth 50HP has an enormous power reserve and can do this. Our customers in mountainous locations can have an airplane which is really cruising at close to 70mph groundspeed, while flying at 55 KCAS.

I rest my case.

I appreciate your comments. If you have an opinion, please leave a comment.

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