25 - Flying Very Light Aeroplanes

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Hello folks.

I haven't sent out much in the way of tips recently, because nothing much of interest has come my way. However I just came across some correspondence on PPRuNe about ultralight approach speeds that might be of interest to anybody flying a low weight / low inertia aeroplane. This includes anything with a max weight below about 700kg (and it might be of interest to everybody).

There were several replies, so I have combined and edited them for clarity and length.

Question:

I have a problem with flying my new Jabiru. I am applying normal light aircraft handling techniques in it, but they are not working. For my approach I've tried 1.3 x Vs or 57 knots IAS as per the Flight Manual (FM) for my approach speed, and it seems too slow and the controls are rather spongy, requiring some larger control deflections, which don't seem to do much.

Jabiru's FM details the "Landing Approach speed" as 57 knots, which I have been using. It also lists the stall speed at maximum weight, power off, landing flaps (Vs) as 40 knots IAS. I've already tried flying an approach at 60 knots (1.5 x Vs) into a variable headwind of 15 gusting 18 knots. I completed the flare, took the power off promptly and held off, and the controls still felt spongy and unresponsive; to be topped off with a very "positive" touchdown.

I note also from the FM that the max. crosswind for takeoff or landing is 14 Kts. Given what I've experienced so far, I reckon I'd tell my students not to fly in ANY kind of wind greater than 10kts!

Some pilots advocate flying the approach at a higher speed, or leaving a "trickle" of power from hold-off until touchdown, especially in gusty conditions. Now if I were to combine that "trickle" of power maintaining say, 1.7 X Vs or even 1.5 X Vs; would that not introduce the danger of "ballooning" from a sudden gust at the flare/hold-off? My point is that by using 1.7 X Vs I may not need that "trickle" of power?

Answer:

The problem with any ultralight is that it has so little inertia. Also, to some extent (although less so with a slippery beast like the Jabiru) it has fairly high profile drag, as well as a fair bit of induced drag at low speeds.

All this means that if you want to shorten the life of your undercarriage, a 1.3Vs approach is the best way to do it. You need to fly the approach at a higher speed. With a draggier aeroplane like the Thruster, I'd fly about 1.7Vs in still air conditions, rising to about 2.0Vs in very turbulent approach conditions. Now the Jabiru is not that draggy, so I suspect that you may find that around 1.5Vs is a better speed.

Another aspect of this low inertia and high drag is that you don't want to be starting your roundout as high as you would in a Cessna 172. In any larger microlight such as the Jabiru I'd normally be starting my roundout at about 10-15ft, which feels unnaturally low until you get used to it - particularly combined with the relatively steep approach that comes with the high approach speeds you want to be using. But, trust me, it does work.

My suggestion would be to find a nice long runway somewhere, start at around 1.7Vs, and fly some circuits, bringing the speed down a couple of knots each time on finals until it starts to feel a little too slow, then increase your approach speed a little and stick with that.

If you are flying a higher-than-recommended approach speed, do be sure to keep holding off and raising your nose after your initial flare to reduce speed until your nose-wheel is definitely off the ground at touchdown.

Another thing to be aware of is that ASIs in general, and ultralight ASIs in particular, tend to under read quite a lot at low speed, so by flying 1.3Vs (indicated) you are probably actually flying slower than that. The info I've got available to me suggests that the Jabiru is actually stalling at around 35-37 knots CAS with full flaps.

The FAA AC 90-89A (Flight Test handbook - Amateur/Ultralight Aircraft) hits the point well. I quote:- "Ultralights by their very nature are highly susceptible to winds above 15 mph (13 kts)".

"Even more so than America's top fighter pilots, ultralight pilots MUST MANAGE AIRSPEED ACCURATELY .......the single most important concern of the ultralight pilot."

My comment?... Basically, I agree, for ultralights. Our Fly Baby's indicated stall speed is 27 knots. I generally approach it at 60 knots, and NEVER below 55. I do the same in a Turbulent. The Fournier's recommended approach speed is 55 knots (it stalls at 41). It is often so gusty at Serpentine that I usually approach at 70, reducing to the touchdown speed of 45 during the hold off over the runway. In all these types, there is no shortage of drag for slowing in the hold-off.

However... carrying excess speed only applies to lightweight, low inertia aeroplanes, and then only if you have plenty of runway length. Whatever approach speed you use, it is vital to fly this absolutely accurately right down to the flare, and then to hold off until you are in the nose-high, three-point attitude (whether you are flying a tailwheeler or a nose-dragger).

If anyone in the club has any comments (and I am sure some of you do) please read this through a second time before replying, and then post your comment to SABC Grapevine, so we can all benefit.

Thanks. Yours,

Bob Grimstead
SABC Safety Committee