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Aerodynamics & the "Dive Test"

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Dave Hopkin04/04/2016 10:07:25
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Re-reading the Jan 2106 issue Dave Burton's series on better flying addresses the stall, in that he talks about CoG and use of the dive test to verify its position

"If the model pulls out of the dive pretty smartly, then the CoG is probably too far forwards, if it tucks in and steepens the dive its probably too far back"

I don't doubt for a second the accuracy of that, but, to me at least, it appears counter-intuitive, I would have (in my blissful ignorance) that a forwards CoG would have tended to steepen the dive and a rearwards CoG pulled the tail down decreasing the dive

Can someone put me out of my misery and explain why?

Bucksboy04/04/2016 10:19:24
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I'm no expert so I'm quite happy to be corrected but this is the way it was explained to me. The vertical dive test shows how much up or down trim is in your elevator. If your plane is nose heavy, you will have introduced some up into the elevator to compensate and keep the plane flying level. Now, without changing any settings get the plane high and dive it vertically down without power. If you've got up trim in the elevator, this will cause the plane to fly 'up' and therefore come out of the dive. If your elevator is at neutral, the plane hasn't required any trim compensation and will continue to fly vertically down.

This made sense to me, I hope it does to you. I've never had one tuck in as I lost one model because of C of G problems and always err on the side of caution.

Dave Hopkin04/04/2016 10:27:56
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Posted by Bucksboy on 04/04/2016 10:19:24:

I'm no expert so I'm quite happy to be corrected but this is the way it was explained to me. The vertical dive test shows how much up or down trim is in your elevator. If your plane is nose heavy, you will have introduced some up into the elevator to compensate and keep the plane flying level. Now, without changing any settings get the plane high and dive it vertically down without power. If you've got up trim in the elevator, this will cause the plane to fly 'up' and therefore come out of the dive. If your elevator is at neutral, the plane hasn't required any trim compensation and will continue to fly vertically down.

This made sense to me, I hope it does to you. I've never had one tuck in as I lost one model because of C of G problems and always err on the side of caution.

That does make sense to me, apart from the article says do the test at 45 degrees
not sure that makes any real difference though

Jon - Laser Engines04/04/2016 10:29:16
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Hi Dave

I had to read this a few times to understand what he was talking about as its easy to get the terminology backwards. I am still not clear on it myself as a model with c/g too far back will be tail heavy, and too far forward nose heavy.

if we take a model that is tail heavy, i can see how the down trim required to make it fly level could also hold it into or make it tuck under in a dive. The opposite would be true for a nose heavy model. this leads me to wonder if there has not been some sort of clanger dropped in editing the article as it makes no sense to me with the current terminology as its currently backwards.

However, this test is somewhat flawed anyway as the continually rising airspeed from the dive will increase lift which will in most cases will cause a pitch up that will overpower the elevator trim anyway. Most of my WWII models pitch up at high speed and need to be held down. Full size aircraft do the same.

Given these issues, and the fact that I dont fancy doing vertical dives in all of my models to check the c/g as many are biplanes and would not appreciate such treatment, I would suggest doing it another way.

 

 

 

Edited By Jon Harper - Laser Engines on 04/04/2016 10:30:37

Dave Hopkin04/04/2016 10:49:56
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I wasnt planning on doing it! Its just the logic of it that defeated me

Denis Watkins04/04/2016 11:04:26
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Dave, you are absolutely right that it defies normal logic, but it is correct.

Do it at good height and at different speeds at 45 degrees

It does indicate C Of G positioning, but is quite unnecessary for light weights such as vintage designs. It is more indicative when the model is for example a low wing speed bird, Spitfire for e.g.

The best start is as you know, 30% chord or on the main spar, and is uncritical for sport flying. IC flyers start the flight with 9 -11 oz of fuel and finish the flight with none usually, he he, and the model is barely upset

Sometimes a model is particularly sensitive, say with a long tail moment arm, or a short nose, as you know

At the build stage, I always use a sliding main servo tray and daddy this up and down while building, to get nearer the c of g using onboard components

With a bought model, this is not at all possible sometimes, but avoid tail weight as you can build in a pendulum/ see-saw effect. Judicious carving is sometimes a better option than added weight

Andy.I04/04/2016 11:21:09
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I don't think the crucial point has been defined succinctly, so here goes:

1. If a nose-heavy model is flying straight and level with the controls at neutral then it has been trimmed with up-elevator to keep the nose up / tail down.

1. If a tail-heavy model is flying straight and level with the controls at neutral then it has been trimmed with down-elevator to keep the nose down / tail up.

When the model is put into a 45 degree dive with the controls at neutral then the out of balance moment will be reduced by 1/(SQRT 2), with the result that the nose heavy model's up-elevator will pull it out of the dive, and the tail heavy model's down elevator will tuck it further into the dive.

Edited By Andy.I on 04/04/2016 11:31:11

Jon Laughton04/04/2016 11:22:45
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"I wasnt planning on doing it!" - Thank the heavens!!devil

Bob Cotsford04/04/2016 11:45:29
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Perhaps it would have been better if David had talked about STABILITY and the dive test? In my opinion CofG position is set dependent on how stable you want the model, the dive test reflects the models self-leveling ability, ie is a measure of stability. Most sport models, floaters, warbirds etc are set up with positive stability, a forward CofG, and will climb with increased speed (ie pull out of a dive) whereas aerobatic models may be trimmed for neutral stability, a more rearward CofG and will maintain a steady path regardless of airspeed. Looney 3D fliers may go with an even more rearward CofG that makes the model unstable, tending to tuck under as speed increases but increasing control sensitivity.

Me - I use the inverted 45 degree climb test.

Biggles' Elder Brother - Moderator04/04/2016 12:19:51
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Posted by Jon Harper - Laser Engines on 04/04/2016 10:29:16:

However, this test is somewhat flawed anyway as the continually rising airspeed from the dive will increase lift which will in most cases will cause a pitch up that will overpower the elevator trim anyway. Most of my WWII models pitch up at high speed and need to be held down. Full size aircraft do the same.

In practice this isn't strictly speaking true! In the dive test the model will quite quickly reach its terminal velocity - the point at which the acceleration force due to gravity balances the aerodynamic drag. The aircraft will not accelerate beyond this point. This is the main reason for doing the test at 45 degrees and not vertically. At 45 degrees the graviational acceleration is less and so terminal velocity is reached sooner.

BEB

Kevin Wilson04/04/2016 12:45:58
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The dive test made no sense to me either until I twigged it was the additional airspeed amplifying the effect of the level cruise trim.

Especially confusing to me was the inverted dive test, where a nose heavy plane (one with a forward CoG) will tuck under (increase dive angle) on an inverted 45 degree dive.

I have always understood the dive test to be more applicable to symmetrical airfoils.
However is the speed/lift component the reason for the inverted dive test? A question for BEB perhaps?

It always surprises me how few people consider the additional lift from increasing airspeed. Is this because most people fly symmetrical or semi-symmetrical sections; or perhaps because I learnt to fly on 2ch where throttle = Up.

Steve Houghton 104/04/2016 12:49:00
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For some gliders, a correct CofG is vital to good performance. Take a plank for instance, which is something I built in January. The designer will have offered you a CofG position as a starting point to work from, but then I mounted a 5 gram weight on the fuselage on the CofG point, launched, flew around for a minute, landed and moved the weight ¼" rearward, and kept doing this until there was a noticeable difference where the model was becoming more difficult to control, so I moved the weight forward again until I found that "sweet spot", where the model was flying at its optimum. And this makes all the difference to the models performance, in speed, penetration and turning.

Thermal soaring gliders do the same thing too, where it can be advantages to have a slightly rearward CofG, which can give a better indication of having found a thermal and rising in that thermal.

I use the dive test all the time to give me an idea of whether the CofG is about right or not, but it is certainly worth while playing around with it as you can see a noticeable difference in performance.

Steve

A470soaring

Jon - Laser Engines04/04/2016 14:02:46
5621 forum posts
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Posted by Biggles' Elder Brother - Moderator on 04/04/2016 12:19:51:
Posted by Jon Harper - Laser Engines on 04/04/2016 10:29:16:

However, this test is somewhat flawed anyway as the continually rising airspeed from the dive will increase lift which will in most cases will cause a pitch up that will overpower the elevator trim anyway. Most of my WWII models pitch up at high speed and need to be held down. Full size aircraft do the same.

In practice this isn't strictly speaking true! In the dive test the model will quite quickly reach its terminal velocity - the point at which the acceleration force due to gravity balances the aerodynamic drag. The aircraft will not accelerate beyond this point. This is the main reason for doing the test at 45 degrees and not vertically. At 45 degrees the graviational acceleration is less and so terminal velocity is reached sooner.

BEB

You think? i wouldnt have thought so. I would expect most models to come apart before they reach their maximum speed. Although this is complicated by the question of power off vs power on and clearly the decent angle. if it was gravity only i would not expect to see them accelerate for too long, but with power on its a whole other story.

I really am dubious about this test. Everything i was taught when doing my Aero Engineering degree suggests it wont show anything. I might strap some lead to my escapade and try it out next time im up as i really cant see the value in it for powered aircraft.

Martyn K04/04/2016 14:24:36
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A power off dive. What happens when you do a stall turn?

If the model cant cope with this loading then it is clearly the wrong type of test, but for setting up an aerobatic model its a very good test for neutral stability. I have used 45 degree up and down lines - I prefer the down line method because you can discount the effect of the torque and spiralling airflow from the power plant. You can also check aileron and rudder alignment at the same time.

If you do the 45 degree down line test with the model flying towards you, you can see the yaw effect quite clearly as the model picks up speed - also the effect of a twisted wing or incorrectly set ailerons. The model start to roll then the roll rate will increase. Start from about 500 feet - you will have plenty of time to observe. It wont get that fast either..

Remember - you need to take your hands off the sticks or you will subconciously start flying again

Martyn

Andy.I04/04/2016 14:32:55
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Posted by Kevin Wilson on 04/04/2016 12:45:58:

The dive test made no sense to me either until I twigged it was the additional airspeed amplifying the effect of the level cruise trim. [snip]

KW: I would tend to differ here, and I would argue that it's the attitude of the model in the dive which reduces the out-of-balance gravitational moment while the lift moment of the tailplane/elevator is maintained, that leads to the nose dropping or lifting.

SH1: a dive test will allow you to reach the point where there is no need for elevator trim as a point from which to start. After this you can adjust the CoG to whatever you prefer, and don't forget you can always alter your longitudinal dihedral as well.

Edited By Andy.I on 04/04/2016 14:53:07

Biggles' Elder Brother - Moderator04/04/2016 14:37:04
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Posted by Jon Harper - Laser Engines on 04/04/2016 14:02:46:
Everything i was taught when doing my Aero Engineering degree suggests it wont show anything.

Mmm, but everything I teach on engineering degrees tells me differently!!

Also - if you actually read what I wrote you will see that I explicitly stated that the dive test should be done power-off!

BEB

Jon - Laser Engines04/04/2016 16:43:34
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In practice this isn't strictly speaking true! In the dive test the model will quite quickly reach its terminal velocity - the point at which the acceleration force due to gravity balances the aerodynamic drag. The aircraft will not accelerate beyond this point. This is the main reason for doing the test at 45 degrees and not vertically. At 45 degrees the graviational acceleration is less and so terminal velocity is reached sooner.

BEB

unless im going really blind i cant see any mention of power off in your post

But anyway, the dive angle will reduce the moments fore/aft of the cg in proportion to each other, so they cancel, lift/weight ratio will change as the lift vector is offset vs weight at whatever the dive angle is, as would the lift from the tail and weight will be the same. So the model looses lift and decends, which obviously its doing anyway as its in a dive, speed remains constant and everything else balances. i still cant see how the trim would impact anything unless i have totally forgotten my aerodynamics classes and there is some glaring oversight.

Picture me confused

Biggles' Elder Brother - Moderator04/04/2016 17:16:08
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Posted by Jon Harper - Laser Engines on 04/04/2016 16:43:34:

unless im going really blind i cant see any mention of power off in your post

The OP is refering to an article I wrote in the magazine. In the article I make it clear that the the dive is power off. Perhaps you would be less confused if you read what the thread was actually about? wink 2

BEB

Jon - Laser Engines04/04/2016 18:22:28
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I am working on the information in the initial post that started the thread. Noone had mentioned power settings when I wrote my post and even after I replied to yours no one had mentioned it so your 'I told you so' was rather wide of the mark.

So anyway mr smarty pants, why does it work? Because other than the relative movement of the CP vs the C/G I cannot think of anything that would result in an pitching forces. I am still racking my brain to see if I forgot something, but so far I am coming up blank.

Biggles' Elder Brother - Moderator04/04/2016 18:28:13
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Stay blank - its easier

BEB

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