Dive test

[Gonzalo]

Page 89 of the October 2004 issue of RC Model World, “optimizing the C of G”, talks about the “dive test” procedure to measure the pitch stability. Basically says:

•Adjust the trim levels so the model flies ‘hand-off’ straight and level.
•Push forward the elevator and place the model on a 40° to 45° dive, then release the elevator.
•If the model recovers gently to straight and level, then it is stable.
•If it recovers quickly, but then continues climbing and ‘zooms’ up into a stall, then it is too stable, and the proposed solution is to remove weight from the nose…

I have not much experience with RC model planes, but it looks to me that this is a mistake, and if the model continues climbing up to a stall, then the rigth solution is to add weight to the nose. Am I wrong?

Regards

[DICKEYBIRD]

Initially it doesn't make sense but it works like this: If your plane is noseheavy, it will take up elevator trim to maintain level flight at one particular speed. When you push over into a dive and release the down elevator control, the airspeed goes up and the up trim causes your plane to pull out of the dive on it's own. If the CG is correct, your plane will continue on the same path in the dive until you decide to pull out.

[Andy W]

.. you can also do this test inverted, as a too-far-forward CG will require lots of trim to maintain inverted flight. Not for the beginner, however.. :)
..a

[Al M]

Should you do this test with the power on or throttled back?

[Bill Glover]

Generally, power off. You are tuning the basic airframe first ... having got that right, if you (for example) still have an unacceptable pitch-up with throttle then you'd need to add downthrust to fix that (not start tinkering with the CG).

[Bill Glover]

.. you can also do this test inverted, as a too-far-forward CG will require lots of trim to maintain inverted flight. Not for the beginner, however.. :)
..a

This only really applies with symmetrical airfoils, so as you say .. not for the beginner.

[Andy W]

Not sure - I saw it on Paul Naton's HLG Pro Clinic - I'd never tried it on anything but 3D and Aerobatic models before, haven't got my Gambler high enough to try it safely either! :)
..a

[Lewist]

i had this problem on my flying wing.. as i sped up it would nose up. when coming out of a dive it would loop all on its own. moved the c of g farther back and re-set the trims and now it will dive and just stay diving until you pul up. it caught me out the first few flights after i changed the c of g becuase i was used to it sorting itself out.. but it does fly MUCH better now. especially in windy/blustery conditions.

[Gonzalo]

Initially it doesn't make sense but it works like this: If your plane is noseheavy, it will take up elevator trim to maintain level flight at one particular speed. When you push over into a dive and release the down elevator control, the airspeed goes up and the up trim causes your plane to pull out of the dive on it's own. If the CG is correct, your plane will continue on the same path in the dive until you decide to pull out.


I have a Spirit sailplane, that mostly flies well and docile, but if for some reason it starts diving (for example if I use much up elevator and make it stall) then it will start waving, recovering by itself and climbing up until stalling again, and the only way to recover it is to as soon as it start recovering and before it start climbing again, push down elevator trying to maintain a level flight until it looses all the energy that got during the dive.

Next time I was thinking to add some additional weight to the nose, but does this means I should remove it?

[Bill Glover]

Yes. But go carefully, if you remove too much weight the plane will become unstable ... if put into a gentle dive it will get steeper and steeper rather than levelling itself out.

[Andy W]

It also becomes very "twitchy" at rearward CG settings. The thing to do is tape some weight to the fuselage immediately behind the wing - a small amount, say 1/2oz of lead to start with - and fly over and over, slowly moving the weight backwards until the model recovers at a nice, slow rate. From unpowered, trimmed - this means stable, hands off - level flight at an altitude of at least 200' or so, push the model into a shallow, power off, dive at about 30-40 degrees. Immediately, you'll get a sense for whether the nose wants to pull up or not. It should take no more than a couple of seconds for you to complete this test - if the model continues the dive, or worse still, tucks under, you'd want to add nose weight (or move the weight on the tail forwards) but, as seems to be the case here, if the model immediately balloons up, move the weight back about an inch, and try again. You'll quickly see a change in recovery, and you'll also notice a huge difference in control response as you approach the correct CG.
..a

[Bill Glover]

and you'll also notice a huge difference in control response as you approach the correct CG.
..a

Yup, hence you might find you need to reduce the elevator throw a little.