Speed sensitive stability?

[Simon Chaddock]

Elsewhere I have described the building and flying of my "Endurance" Wing Dragon that was specifically set up to give aerodynamic efficiency rather than good stability with:

A small tail volume in comparison to the wing area.

No conventional wing incidence - the symetrical tail plane is in line with the flat bottom of the Clark Y.

CofG positioned to give neutral elevator when flying close to the stall.

 

I had noticed during the long flights that when in a much of a dive there was little tendency for it to recover yet it was adequately stable in normal level flight.

To explore this further I replaced the big battery with much lighter one to give a greater margin of structural strength but kept the CofG exactly the same .

 

The result was interesting. Trimmed for slow level flight and then put into a power off vertical dive it continued straight down, hands off, but above a certain speed it nosed over to fly inverted. As the speed fell away it returned to a vertical dive.

Any up elevator resulted in a normal recovery and a return to stable level flight.

 

Somewhat alarming the first time it happened I can only guess that one or more of the above characterisitcs, above a certain speed, results in the centre of pressure moving in such a way as to overcome the "built in" stability.

Power on the result was the same, it just happens faster.

 

So beware rearward CofG and insufficient incidence!

[Former Member]

[This posting has been removed]

[Simon Chaddock]

Agreed Eric but I did use the term "conventional" incidence meaning that which is easy to measure.

Under power with its pylon mounted wing (i.e.fuselage drag low down) and high thrust line the elevator is exactly neutral in level flight so, as you say, there is some positive incidence but at very low power or gliding it needs slight down trim which I suspect all but removes its natural stability.

 

Well that's my reading of the situation.

 

ps I have flown it again and it is a most alarming characterisic.

I will add some nose weight to move the CofG forward just a bit to see if that cures it.

 

[Evan Pimm]

Right answer Simon, this is how the experts find the 'most rearward' balance for their gliders. S&L at a nice cruise speed, then gently ease the stick forward to about 30 deg nose down dive. Without touching the elevator or trim (stick free), watch the model. It will tend do one of three things, either nose up quite positively, continue in a straight line, or continue to increase speed and dive angle. The first indicates that the balance can come back, the second shows the max rearward, and the third shows a balance too far back. Re-balance to obtain a gentle pull-out from the dive. You will have to re-trim the 'cruise setting' a bit as the balance changes, but in essence you are finding the best balance for your particular model set-up.

Evan.

[Simon Chaddock]

Evan thanks for the confirmation.

What surprises me is that although I had in mind the need for aerodynamic efficiency I just built it and flew it. The fact it ended up with the CofG just about as far back as possible yet remain flyable was pure luck. Nothing was altered for the maximum endurance flights. 

[Evan Pimm]

Something for you to try next Simon. You are no doubt aware of the 'min drag/best L/D' condition often bantered about, well now that you can trim your balance to the best place, now you can trim your glider for different conditions too. First, no matter what you do the best glide angle (greatest distance from a given height) is fixed by your design. Second, the best time from a given height (best duration) is also fixed by the design. What you now have to do is find the elevator trim settings that match these conditions. The two do not occur at the same place, you will find, through flight testing, that best L/D will be considerably quicker than best duration (best Cl). A few flights will indicate the idea, and for best efficiency you really should, when flying, be looking for lift at the best distance trim, and once found, be maintaining the lift in the best duration trim. These trims can be seen visually as a change in speed, get used to the 'looking' airspeed and the 'duration' speed. How you know where to look  for the elusive thermal is more dependent on luck, you know, "the more I practise the better luck I seem to have." sort of thing. Gliders can be an all embracing part of aeromodelling, and certainly are among the most efficient airborne shapes.

Evan.

[Simon Chaddock]

Agreed Evan

I think it would be true to say that for most conventional propellor planes the min sink speed is also quite close to the stall.

On the duration flights I flew as slowly as practical but avoiding any sort of actual stall as the recovery would represent power wasted.

I seem to remember this technique was used by the Japanese Zero pilots to achieve the great range for a relatively light shipboard fighter that so surprised the allies.

[Former Member]

[This posting has been removed]