Running Long on a Must Make Approach
The Situation
You are on approach, and do not have enough charge/fuel to go around. You must land on this approach, but you realise that you are too high. How do you land?
What to Do
Here's how I tackle it.
As soon as you realise there is an issue, but not while the plane is too far away (not over 40 metres for a park flyer for example) close the throttle and dive as sharply as you dare at the ground. Pull up when you are at the lowest possible Angle of Attack from which you think you can land. If you are still looking like running long you can dive all the way and pull out at the flare position.
Let the airspeed wash off the plane and add throttle to reestablish flying speed if necessary.
If you must land and beyond the runway strip is not plane friendly then force the aircraft gently onto the deck and hold her there as best you can.
Why
When we are too high on an approach our problem is how to get rid of potential energy (altitude) without picking up too much speed (kinetic energy). Diving might seem counter intuitive because we pick up a lot of kinetic energy. However, it also increases the frictional force, slowing our plane down, at a rate equal to velocity squared.
Let's say that at 5m/s you get 50 grams of drag slowing your aircraft down. If you dive and accelerate to 15m/s this drag would now be 450 grams. Effectively we have vastly increased the frictional force trying to slow your aircraft down for a short period. And, as the physicists amongst you know, we have gotten rid of more energy as heat (from friction) than we would in a normal approach.
The diagram on the right shows a graphical representation of this. The huge drag spike is the key to making this approach do able.
Risks and Limitations
This technique is more effective for airframes with higher drag. However, even if you have a low drag airframe this is still your best shot if you must land on this pass.
You may need to add throttle to stabilise your speed once you have leveled out from the dive. You don't have long to sort this out, and could easily stall if you aren't careful.
You are quite close to the ground, quite a long way away, going quite quickly. Be aware of glitches.
Alternatives
Another alternative that I have found very trick and risky is "slipping" where you add aileron and opposite rudder. If done correctly you lose altitude quite quickly and pick up very little speed. It is a tricky technique, difficult and easy to lose orientation on in my opinion, but it is an option.
posted by Oz RC Boy @ 11:11 AM
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2 Comments:
Thats a good one. I fly in a place surrounded by ~30m high trees and generally *have* to zoom at the ground and pull up at 50cm to land. Took me my own practice and errors to realize it, but it definately works.
Another one Ive heard of is to do a few fairly sharp turns (about 30degrees) on the approach like a space shuttle - if youre that far out when you realize. It requires some skill, but Im finding it fairly easy to do even as late as a metre or two high with more practise.
If youre 20cm off the ground, power off, waiting for speed to drop enough to do a flare, and quickly running out of runway; just force it into the ground (as gently as you can). Ugly, but by that time you are probably risking hitting a fence or something, so its still a better option.
Being aware of ground effect (see latest/advanced landing story), will prevent this most of the time I think. Its not something that only exists for helicopters, suprisingly to me.
Flaps are a useful and often nessecary feature of a plane that will solve all these problems. Its why 'real' planes *always* have them. Sadly, few models incorporate them. Im sure getting annoyed at having to add them to my models anyway. Grr.
Hi Surge,
Turning a corner shortly before landing is a great technique if you have limited space. Only problem is it takes some confidence with the controls - needless to say an orientation mistake at that point equals crash, and of course glitches typically wait for moments like that to pounce as well.
I've had mixed results with flaps and in the end have decided not to bother. My main issues are how ungainly the aircraft becomes with them, and the other issue being that the aircraft typically goes slightly out of trim when they are engaged.
Just today it was suggested to me by a model flier that flaps are best left til after you have turned onto final. On the other hand an actual aircraft pilot has told me you should have flaps engaged on base.
With my space to land in I'm not finding I really need them so I'm just not bothering.
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