Dive test - someone explain it to me!

[Stevo]

Yep - I'm aware of the dive test for a fine tweak of CoG. Powered off dive, elevator off... if it carries on all is good. If it climbs, CoG too far forward (add weight to rear) if it increases CoG, too far back add weight to nose.

Here's the thing; from purely a balance POV, adding weight to the nose makes the nose dip, so how does adding weight to the nose actually make it climb?

[Frank Skilbeck]

Basically if it's nose heavy and trimmed for level flight, you will have trimmed in some up elevator, model in a dive will pick up speed and the up trim will increase the elevator down force causing the model to climb out of the dive. If the model continues to dive in a straight line it is perfectly balanced but has minimal stability, so the aim is to get the model to gently pull out of the dive.

[Stevo]

Well why didnt you tell me earlier, Frank

 

Was I over thinking again...

 

Edited By Stevo on 30/08/2017 14:35:45

[ASH.]

It's something I've never fully understood myself. Maybe it works similar to side thrust.. and/or a pendulum effect?!

[Stevo]

Well Frank has it

Sidethrust? easy

[Jon H]

There was a heated debate on this topic some time ago. I have tried it a few times and to be honest think its useless. Certainly in the case of my la7 the disturbed airflow from the idling prop caused such a trim change (i notice on loop downlines too) that the test was utterly useless.

A friend uses it to set up his gliders however and claims its helpful sometimes but usually not :\

[Stevo]

Thanks Jon, I was just curious about how the CoG was moved and what effect it would have against the dive test. I wasn't advocating its usefulness.. perhaps as a guide really. Any tips?

[Simon Chaddock]

The dive test is useful but does have some limitations, particularly if any form of aerodynamic flexing is involved be it the wing, control surfaces or even the whole rear fuselage.

Setting up some foamies with a CofG for minimum natural stability in slow level flight can make a fast dive "interesting".

[Frank Skilbeck]

I must admit that I don't use the dive test on powered models, as John says the engine thrust, prop wash etc can all have an effect. But I do this test on all my gliders, while they fly OK nose heavy, they are more efficient when the c of g is in the optimum place.

[John Stainforth]

I prefer the vertical dive test with the power off. A neutrally stable plane will go straight down, a stable one will gradually pull out, an unstable one will tuck under. Most of my planes pull out by about 30 degrees from the vertical over a 300 foot drop. This amount of stability usually corresponds to about 1/4 forward stick when flying inverted horizontally.

[Peter Jenkins]

I do a lot of trimming of F3A aerobatic models and I never use the dive test as I'd rather be going away from "terror firmer" when checking CG position. Rather just roll inverted and check how much down elevator is needed to maintain level flight or, pull to 45 degrees and roll inverted and check again.

The other issue is to sort the engine/motor thrust line. Trim for cruising power and then apply full power. If the model climbs then add down thrust but if it dives add upthrust.

I won't go into side thrust but that is just as important to get right.

What you are aiming for is as little interaction from the application or reduction of power on the aircraft's flight path. That means you can open or close the throttle and know that the aircraft will continue on its existing flight path until speeding up or slowing down changes the aerodynamic loads on the airframe.

For some, and I stress some, aerobatic aircraft, the other check that the CG is in the right place is if there is no control movement required when in knife edge - but that is for well designed aerobatic aircraft.

The vertical dive in aerobatics is another way of tweaking CG but once the aircraft feels right as regards CG then I usually mix enough down elevator to come in at fully closed throttle to give a vertical down line - in still air! In the real world with wind you usually need to adjust the flight line with a tad of up or down elevator to achieve a vertical downline.

Just my way of doing things.

[Denis Watkins]

The G in the C of G is the key thing

Gravity plays a big part in this

[Geoff S]

Posted by Peter Jenkins on 30/08/2017 19:49:13:

What you are aiming for is as little interaction from the application or reduction of power on the aircraft's flight path. That means you can open or close the throttle and know that the aircraft will continue on its existing flight path until speeding up or slowing down changes the aerodynamic loads on the airframe.

... but surely an increase in engine power will result in increased air speed, increased lift and hence a climb if no other action is taken?

Geoff

[TIM Shaw]

Hi Geoff

The type of aircraft Peter is talking about are designed to be neutrally stable, and when properly trimmed each individual control input should have no side effects at all.

The downthrust angle set means that there is a small downward thrust vector which cancels out any tendency to climb with increased power.Tthey have symmetrical wing sections and fly exactly the same inverted as they do the right way up - except of course that down becomes up and up can get expensive....

I actually always do a dive test, power off at 45 degrees and hope it gently pulls out or maintains the angle, but I don't do it until I'm pretty sure it's somewhere near - tuck under can be a frightening thing.

I think a more precise indication - for the types of model I fly - is the amount of down you need to hold straight and level inverted, I prefer to have none, but thats just me

[The Wright Stuff]

Posted by Geoff Sleath on 30/08/2017 21:23:35:

Posted by Peter Jenkins on 30/08/2017 19:49:13:

What you are aiming for is as little interaction from the application or reduction of power on the aircraft's flight path. That means you can open or close the throttle and know that the aircraft will continue on its existing flight path until speeding up or slowing down changes the aerodynamic loads on the airframe.

... but surely an increase in engine power will result in increased air speed, increased lift and hence a climb if no other action is taken?

Geoff

Good valid questions, Geoff. To slightly rephrase Tim's answer, I think of it in terms of making sure that the downthrust largely cancels the effect of the increased air speed, increased lift and climb. The issue is that the explanation is much more complicated than the standard 'four forces' description of flight allows, because one has to balance moments as well as linear forces.

In principle, though, one could say that as you throttle up to increase speed from straight and level flight, the downthrust is calibrated to rotate the aircraft slightly nose down by exactly the right amount to decrease the AoA of the wing, enough to exactly cancel the increased lift that results from higher speeds.

[PatMc]

How can a model "fly exactly the same inverted as they do the right way up" when what was downthrust is now upthrust ?

[PatMc]

AFAICS the dive test serves no useful purpose on a power model. It's used on gliders to simulate increasing power when the model is trimmed for level flight then to observe the effect this has. Opening the throttle on a power model that's trimmed for level flight achieves exactly the same.

The test is very useful for gliders but it must be carried out correctly to get meaningful results. Also the model's wing/tail area proportions together with the distance between wing & tail will determine the rearward limit of the cg. This limit will effect how close to neutral stability is safely achievable with the model.

[Martin McIntosh]

On my classic aerobatic models I set the cg so that they come out of a spin exactly one half turn after neutralising the controls. Stop spinning too soon, cg forward and vice versa. Always worked for me.

[Dave Bran]

Take the model to 100 feet, go full throttle and hold down elevator until its diving at 60 degrees. At 30 feet pull full up.

If it survives, it's OK.

[Dave Bran]

Posted by PatMc on 01/09/2017 21:07:00:

How can a model "fly exactly the same inverted as they do the right way up" when what was downthrust is now upthrust ?

How does the model know its upside down?

These and other questions....................................................

[The Wright Stuff]

Posted by Dave Bran on 02/09/2017 22:16:01:

Posted by PatMc on 01/09/2017 21:07:00:

How can a model "fly exactly the same inverted as they do the right way up" when what was downthrust is now upthrust ?

How does the model know its upside down?

These and other questions....................................................

This is the problem with oversimplified explanations - they are not detailed enough to explain every aspect of the observed behaviour. There is not a simple explanation - the best thing I ever did was read books on model aircraft aeronautics. For example, this.

The apparent paradox lies in the vertical difference in position between the thrustline and the centre of drag. If the aircraft is inverted, then the respective positions are also inverted, and downthrust becomes upthrust, which cancels the resulting pitch moment.

[Martyn K]

Caveat, I am no expert, these are just my personal ramblings..

I use the dive test for trimming. Basically climb to about 300 feet and shut the throttle. Slow the model right up and just before it stalls push the nose down to about 60 degrees. Hands off the sticks and watch what happens.

The type of response that the model will perform depends on the type of model.

A WW1 type biplane should initially start to tuck under then as the speed increases will fairly quickly level off. A trainer type or Warbird with semi symmetrical wing section and a bit of decalage will do the same but take longer to pull up. An aerobatic model with a 0/0/0 setup should basically track straight and simply build up speed until you chicken out and grab the transmitter.

By observing the dive from different angles you can check:

1. It doesn't yaw. Banana Fus

2. It doesn't roll. Twisted wings or lateral imbalance.

3. The decalage is about right for the type of model and the response that you see.

You can also do an inverted dive test which will sort out lateral balance problems. A twisted wing will always roll in the same direction, a model with poor lateral balance will roll with heavy wing going down.

Obviously, you need to be confident that the servos do centralise consistently and correctly. No backlash or slop or hysteresis.

You shouldn't have to tweak anything to get it to dive in a straight manner. If you end up adding a click or two of aileron then all you are doing is altering the wing profile which will have different effects at different velocities.

It doesn't help counter the effect of propwash or engine torque. You can mix in different corrections for different throttle settings, but I don't think Elevator should be touched when it is set up.

I don't think it helps set the CG. It just proves that model is built accurately for its intended purpose.

Positioning the CG become more important when you want to set the sensitivity of the controls, it needs to be far enough back so the model will stall correctly (for a spin) and not too far forward so that elevator sensitivity becomes affected at slower speeds.

I now prefer my aerobatic models to be neutral. i.e I need to hold a little up to keep it level when the canopy is up and a similar amount of down when inverted.

So, that's why I think the dive test is useful..

 

Martyn

Edited By Martyn K on 05/09/2017 15:45:40

[Martin McIntosh]

Never tried this dive test thingy. Must disagree with holding up for level flight also. The model should fly exactly level when upright with no elevator input, otherwise you will be constantly fighting it. Only person I know who flew like that is D.H. His models flew inverted with no down input. Difficult to get used to. I know because I once tried to fly one of his.

As you know, I do have more than a little experience in this.