All planes dive when inverted

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mkirsch1

New member
A properly trimmed aerobatic plane is supposed to fly as well inverted a upright, right?

Why do all the planes, no matter what type, dive straight for the ground when inverted? I've tried all the Dave Patrick trimming tips, messing with incidences and CG, and all I do is build unwanted tendencies into the plane...

I've tried moving the CG back, and all that did was make the plane uncontrollable.

I've also tried adjusting both wing and tail incidences... now it flies straight inverted but dives for the ground upright.

What would I need to do, for example, to make the Yak54 fly true both upright and inverted?
 
No matter what aircraft you fly it will NEVER fly completely straight.

Calibrate your Interlink and/or reinstall.
 
mkirsch1 said:
A properly trimmed aerobatic plane is supposed to fly as well inverted a upright, right?
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"As well"

Yes, but that doesn't mean that they will fly level, either upright or inverted!

A "properly" trimmed aerobatic plane will be a bit nose heavy and in normal upright flight at mid throttle it will begin a dive torwards the ground.

That same "properly" trimmed aerobatic plane will also have exactly the same rate of change towards a dive when inverted.

mkirsch1 said:
Why do all the planes, no matter what type, dive straight for the ground when inverted? I've tried all the Dave Patrick trimming tips, messing with incidences and CG, and all I do is build unwanted tendencies into the plane...
Click to expand...

Calibrate your controller.

Take any Aerobatic plane, ( I suggest starting with a Yak 54 )... and REMOVE any trims so that the controller inputs are neutral.

Fly the plane and manually hold it level at 70% throttle.

Now let go of the elevator and watch the RATE of CHANGE, e.g. how FAST it begins to drop the nose towards the ground.

Fly the same thing again, but this time turn the plane upside down and fly it level.

Again watch the RATE of CHANGE.

The plane is "properly" trimmed if both the rates are the same.

If it dives more quickly when inverted, then it needs NOSE DOWN trim to make the two equal.

It sounds like you are confusing 3D trimming with Aerobatic trimming.

For 3D planes, the C.G. is brought as far rearward as possible to make the plane neutrally stable both upright and upside down.

You can simulate this too...

AFTER you have performed the above, bring the C.G. BACK in .5 to 1" increments, until the plane seems to fly ALMOST level at 75% throttle ( it should drop the nose SLOWLY ). Again fly it upside down. It should drop the nose at the same much slower rate.

This is the "sweet spot" for 3D flying, which I believe you are looking for.


mkirsch1 said:
I've tried moving the CG back, and all that did was make the plane uncontrollable.
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Yup you went too far, and did not allow for downthrust... always CHECK the pull to belly or canopy by taking the plane HIGH and diving it straight DOWN with the throttle pulled completely off, engine at idle.

The very same proceedure is used to trim a real plane...

If the plane angles towards the belly, you have too much down trim... if towards the canopy, too much up trim.

Once neutral, if you notice that the plane pulls DOWN when you throttle up, you have too much down thrust on the engine, etc...

mkirsch1 said:
I've also tried adjusting both wing and tail incidences... now it flies straight inverted but dives for the ground upright.
Click to expand...

You have to be very careful with this.... and avoid it if possible.

In effect you are compensating in the wrong place...


mkirsch1 said:
What would I need to do, for example, to make the Yak54 fly true both upright and inverted?
Click to expand...

No properly designed plane flies level hands off with out adding some trim.

The plane is designed to enter a gentle dive ( and pick up seed if the engine goes out! ). We tend to trim out our planes so they fly level hands off at normal flying speeds...

Most F3A pattern participants WANT the nose to drop lightly but equally as I indicated above... I've helped a few trim out their real planes....
 
mkirsch1 said:
A properly trimmed aerobatic plane is supposed to fly as well inverted a upright, right?

Why do all the planes, no matter what type, dive straight for the ground when inverted? I've tried all the Dave Patrick trimming tips, messing with incidences and CG, and all I do is build unwanted tendencies into the plane...

I've tried moving the CG back, and all that did was make the plane uncontrollable.

I've also tried adjusting both wing and tail incidences... now it flies straight inverted but dives for the ground upright.

What would I need to do, for example, to make the Yak54 fly true both upright and inverted?
Click to expand...
just constantly use down elevator which now makes your plane go up and straight. No plane goes inverted as well as upright.
 
The basic shape of the wing, regardless of the plane's aerobatic nature, provides more lift in it's natural position versus inverted. No wing is a completely flat surface, otherwise you'd have to increase thrust to weight to get off the ground.

A lot of mistakes made in designing a plane's lifting surface are related to the shape of the wing. The wing itself is but one part of the lift equation - with the exception of rotary wing aircraft, the fixed wings do not on their own generate lift. They require forward momentum to produce that lift.

The amount of lift generated by a lifting surface - blade or wing - is related to it's aerofoil shape. The more curve you have, the more lift you generate. However, there's a point where the curve becomes too great to be overcome by forward momentum, so you have to play around in what's considered the "safe zone" for wing shape.

To design a truly aerobatic plane, you'd think that you'd want a flat wing - or maybe not. No wing is flat, as you would not generate lift as the pressure on both sides of the wing would be identical. So you curve the wing. There is such a thing as an "aero-neutral" shape, where both top and bottom of the wing are curved to the same dimensions. However, these are generally tilted from their mounting point up to their wing tip, giving reduced pressure under the wing tip providing slightly more lift. If you mounted "aero-neutral" wings exactly horizontal, or rather directly perpendicular to the fuselage, you could still generate lift with forward motion and using flaps and tail wing, and there are models out there like this, but man oh man are they hard to fly. The tiniest adjustment to the stick and you find yourself rolling over in the blink of an eye.

So, while aerobatic planes do have generally a wider wing surface and pretty close to an aero-neutral shape, to give the advantage of upward lift and allow for an easier take-off you have to adjust the wing. Then when you're inverted, you're going to see the effects of that. It's darned difficult to trim for this, it's almost easier to adjust flaps ever so slightly, and trim the nose down ever so slightly, so that you almost neutralize the over the wing/under the wing pressure equation.

Anyway, sorry for the lecture, I've spent a couple of years with a couple of friends building experimentals and half our time is spent discussing wings...
 
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Any time I find one of my planes, ( or several, for that matter ) doing unpredictable things, when they had been flying just fine the other day, I go to the controller calibration screen and perform the operation. It only takes a couple of seconds, and in almost all cases, straightens out the squirrelly behavior!

Aerobatic planes, such as the Extras, Sukois, etc. should be capable of flying inverted with very little correction needed. Otherwise they would not be so graceful in the air as they appear to be!

I have even re-designed simple trainer type models to be capable of inverted flying with very little tendency to "drop straight toward the earth" ! True, they do not fly as well upside down as they do rightside up, but the DO a fairly admirable job of doing "something you can't do with a trainer" !

In conclusion, if your airplanes are "dropping straight to the ground", I would say your controller is out of calibration.

Good luck, Jim
 
symmetrical airfoil

As good (or bad) a time as any to make my first post here :rolleyes:

Some good points brought out already...

Not sure if you said all YOUR models or ALL models in RF..


Are you talking about high wing as well as low wings?
Trainer types as well as sport? ETC.

I've not run into anything that didn't feel pretty typical for the model to me, but that's only me and what I've used in the sim to date.

CG should not bother you with this if it's correct normally, but note that center of balance and center of gravity are two different things also.

How far did you go with your dihedral changes? Taking an aircraft inverted that has a good amount of dihedral it will tend to dive more when inverted and depending on what airfoil it uses could do so at a pretty good rate... if you were to draw two straight lines from the upper wing surfaces from each wingtip and keep them 90 degrees to the spar, at a point the lines would cross and that is where the wings are trying to lift the aircraft to, dihedral working to make it more stable by lifting towards a certain point over the aircraft space etc. Always going to need more down or "bottom" elevator added to stay level.

Horizontal stab use any - incidence? Inverted it will now be + incidence to the direction of travel through the air.

Now...One basic design item that will give you changes in upright/inverted is the airfoil shape.

Flat bottom like the Clark Y is high lift and stable, used on many basic trainer types, but does not have much upward lift inverted.. for example, they might use this type mounted upside down on a race car to push it harder to the ground etc.

To get the aircraft to not care if it's going straight up, straight down, upright or inverted (and any angle between) with respect to the lift, you need as close to a symmetrical airfoil shape as possible. This producing as close to as much lift UP as DOWN when the datum line is horizontal.

Then there is the thickness of that airfoil no matter which style it is,

Just saying that even it looks like a aerobatic type on the screen, make sure the specs show proper airfoil shape and in the percentages needed to do what you want to do.


It's all a compromise for what you want it to do best, can't have it all without being able to modify the airfoil with controls from the cockpit, or TX Or like a bird does :cool:

Anyway, SO many variables involved I won't go on and on, just try this... start with one of your good flying models that doesn't have a lot of dihedral and adjust the airfoil to be more symmetrical in it's lift. Don't get the shape too thick doing this, you want to keep thickness to around 18% of the cord.

And don't forget one very good rule... "only make one change at a time and test it" :)

Good luck, and remember , it's all fun... even running down problems like this.
 
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These are bone stock models that come with the software. Controller's calibrated just fine.

Yak54 for example... I've seen guys wringing this one out at the hobby shop's old G3.5 kiosk, doing torque rolls and harriers and all kinds of low and slow 3D stuff. They're flying the bone stock plane near as I can tell.

When I fly the same plane, bone stock, at home, it seems drastically nose heavy. Always wants to fall out of a hover. Heads straight for the ground when I go inverted (centered trim). It's like trying to fly 3D with a trainer that's got a cinder block strapped to the nose.

If I make a copy and move the CG back, just 1/2". No change. 1/2" more. No change. 1/2" more. No change. 1/2" more. Uncontrollable, unstable, snapping fool. Still won't hold a hover for love or money.

Now, the 1/3 scale Matt Chapman SPLAT 580 on G4 at the new hobby shop kiosk... Makes me look like Quique freakin' Somenzini. Hovers effortlessly. Tail touches. Torque rolls. I'm not that good, or the planes I've been flying are that bad...
 
opjose said:
A "properly" trimmed aerobatic plane will be a bit nose heavy and in normal upright flight at mid throttle it will begin a dive torwards the ground.

That same "properly" trimmed aerobatic plane will also have exactly the same rate of change towards a dive when inverted.
Click to expand...

I would have to say that I disagree. I fly IMAC, in the Advanced class. For trimming, make the first flight on a new plane with the CG as it states in the manual, or whatever. Then, adjust the CG once it is trimmed. CG is a preference thing, and for me, as well as a lot of the fellow IMAC flyers that I fly with at IMAC competitions, like the CG to be set to where the airplane slightly drops when inverted. It shouldnt drop quickly, but should slowly begin to turn towards the ground. And in upright flight, it stays level. Adjust the CG further back if it drops very quickly. Re-trim after that, then roll it inverted and try it again. Repeat this over and over until the CG is where you like it. Like I stated above, this is how I trim my 35% and 40% IMAC airplanes, so that they barely drop when inverted. After that, then I play with coupling and such.

On this sim, you cant get the airplanes to be exactly like they are in real life. It just isnt going to happen. You can get them close, but not perfect. This is one thing that I have found is very different, and almost impossible to get the airplane to do the same as the real plane would.

One tip that I have found that tends to help on MOST airplanes on the sim (G3/G4) is to move the wing either further forward, or further backward to change the CG. In other words, keep the CG where it shows it is in the edit menu, but just move the wing further forward to make the CG further back, or move the wing further back to move the CG further forward.

Hope this makes sense, let me know if it doesnt!

Seth
 
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I see your point :cool:

Chromenut said:
Uh...what he said.. yes, exactly, that's my story and I'm stickin with it...
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LOL, sorry about that ChromeNut, you know how using text only can be when you are typing off the cuff.
Just trying to point out that in a case like this, it's best to start with good lift from the upward facing surface of the wing when inverted not only to combat the pull of gravity, but also the lift of the top of the wing in the other direction, no matter what direction that might be:D

Oh no, think I just did it again..... I'll go back to lurking mode now :cool:
 
3D-Seth said:
It shouldnt drop quickly, but should slowly begin to turn towards the ground. And in upright flight, it stays level.
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Re: Inverted

Yes.

Re: Upright

Often this is not done by those I've talked to, because it causes rolls to be non axial and downlines less linear, since the plane has a slight up pitch.

While a good pilot can compensate for this why introduce unnecessary workload?

Most pattern planes are similiarly trimmed.
 
This is scale aerobatics, not pattern, so I would assume its slightly different. For IMAC (Scale aerobatics) The airplanes are not effected by the plane staying level while upright, and slowly dropping while inverted. It may be noticeable on pattern airplanes that are made to fly straight and true, that I do not know, but from my experience with giant scale aerobatics (IMAC) the airplane is not affected by flying level when upright.

If you have a bunch of up elevator trim to fly level, when you roll inverted, the airplane will want to drop sharply, indicating that the CG is too far forward (for most peoples likings). You move the CG back some and re-trim (which would be less up trim) for level upright flight. You roll it inverted, and the airplane drops slightly (where most people like it). At that point, you have a very slight bit of up elevator trim not enough to make a matter while doing rolls and such. In fact, on Quique's scale aerobatic airplanes, he actually has a decent amount of up elevator trim in it to fly level if you look at it while its on the ground (and the radio is on).

If you are wanting it to fly neutral like you are saying, then why wouldnt you just adjust the CG so that the airplane would fly level inverted without any input and the same as upright? If it is constantly diving when its level, and you have to constantly hold up, I dont see how that is reducing the workload.

Just my .02, but again, I fly IMAC not pattern....

Seth Arnold
 
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everything opjose said above is correct, keep note that different airplanes have different airfoils, different airfoil profiles will have a big effect on your airplane, e.g if its a 3D airplane, your airfoil will be symmetrical, which in theory should grant you a straight flight right side up and inverted. ​

 
littlephoenix said:

everything opjose said above is correct, keep note that different airplanes have different airfoils, different airfoil profiles will have a big effect on your airplane, e.g if its a 3D airplane, your airfoil will be symmetrical, which in theory should grant you a straight flight right side up and inverted. ​

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I really miss opjose in this forum, he contributed so much.
That said...you probably didn't notice that this thread stopped being active 14 years ago...so the Original Poster may have solved his issue.....
 
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