A perennial question that comes up in various discussion groups, or in "hangar flying" for pilots of nanolight aircraft  is: "How much thrust do I need to take off?"

Well, put simply,  the amount of thrust required to "launch" an aircraft just has to be greater than the amount of drag the aircraft generates at best glide speed.  Of course, if you actually want to *climb*, you'll need more than that!!!!

The surprising thing is that a 220 sq ft floater does not actually necessarily climb better than a smaller double surface wing. This seems counter to intuition, but it is true.  What the 220 sq footer does is launch and land at a lower speed, making takeoffs far less "interesting" (in the vein of the ancient Chinese curse.)

Pretty much all aircraft have their maximum *climb rate* at their best glide speed, and their best *climb angle* at their minimum sink speed.

Imagine your aircraft flying straight and level (not climbing or decending). In this situation, the lift equals the weight, and the drag equals the thrust(*see note 1).  If your 220 sq ft wing (30kg) with a Minimum (20kg) and  you on board (85kg) weighed a total of 135kg.  With the engine off, you estimate a glide of 7:1.  Then the drag at best glide is 135/7, or approximately 20kg (44lb).   This is the thrust necessary to maintain level flight, and any more than this will provide climb.  In fact, you could probably actually take off with this amount of thrust, after a very long ground run, but would be unable to get out of ground effect.  There are a lot of very funny stories (*see Note 2) about some aircraft with thrust in this range!

As a rule of thumb only, let's assume that with the engine off, the machine will probably sink at about 230ft/min at best glide, so to acheive 230ft/min *climb*, we'll need another 20kg of thrust for a total of 40kg (88lb).  These numbers seem to gel pretty well with what we see in the real world.

Now let's look at our hypthetical blade wing.  We'll assume a takeoff weight of 140kg, and a glide of 9:1. This gives us a best glide speed drag of  140/9, or about 16kg (36lb).  We'll also assume that the sink rate at best glide is a bit worse, say 240fpm.  So we'll get a climb of 240fpm for only 32kg (72lb) of thrust!

You can use this principle to give you an estimate of the viability of most wing/motor combinations.

Bear in mind that static thrust is just that, thrust at zero forward speed.   As you speed up, the thrust generally reduces, so your measured thrust is not what you'll acheive in the real world.  And because the blade wing flies faster, there will be slight reduction in the available thrust.  However, a lot depends on the propeller chosen.

So, you ask, what criteria should I use in choosing a wing for nanolight flight?

Well, obviously the wing should be certified to take the weight of both you and the trike. (If not, fly it carefully only ion the still air of early morning!) The  Sting 175 by Airborne (certified to 135kg!) is a great compromise between blade and floater and is very popular with the powered harness set.  Other gliders certified to 120kg are fine for lighter pilots.

If you're not using the machine for soaring, and you actually want to go anywhere under power, a blade wing is recommended.

If you want ultimate soarability, try a rigid wing. (Exxtacy/ATOS etc...)

If you want simplicity and low hassle takeoffs and landings, go for a floater. And they thermal well, too ... but are a bit poor between thermals...

Note 1: This is a simplification, as it depends a bit on the angle of the thrust line. It is strictly true only if the thrust of the engine acts along the direction of travel at this speed.  Often you will find that the thrust is angled upward somewhat, which is a bit wasteful, as you're using part of the thrust to reduce weight. [back]

Note 2: Once in about 1980 I was at an air show at Perris in California where someone had a pulse jet powered, foot launched Icarus5 rigid wing.  After two flameouts, he finally, with the jet roaring, ran like stink down the runway taking longer and longer moon-walk steps, eventually achieving full flight at the grand altitude of 2ft, all the way into the fence at the end of the runway. He was OK, as was the wing. But we nearly died laughing. [back]