V tail question

[Tim Kearsley]

A question for those with a better grasp of aerodynamics than me (almost everyone!):

I have a Durafly Excalibur, which is a foamie "warmliner" with a V tail. It flies beautifully and is trimmed to fly straight and level at around half-throttle. I like to fly a manoeuvre which includes a vertical dive (unpowered) from a good height. When I pull out of the dive to level flight I notice quite a pronounced pull to the right. My question is, is this likely to be due to the two halves of the V tail not "tracking" properly as up elevator is applied to pull out of the dive? I must say that by simply looking at the elevator deflection both sides of the V tail seem to move equally, but that may not be meaningful, as the total deflection is fairly small. My theory is that a small rudder effect is being introduced with the elevator movement. Is that feasible?

Thanks,

Tim.

[Peter Miller]

I would suspect that under the higher speed of the airflow the right elevator is getting some blow back.

[Tim Kearsley]

Good point Peter, thank you. Any way to prove or disprove it? One other thing to add - I don't loop the model very often, but now I come to think about it, I think it similarly pulls right entering a loop. I wonder if I'm unconsciously pulling the stick slightly to the right as well as down and putting in some aileron input? I hope so, because I can easily cure that!

Tim.

[Peter Miller]

That is possible.

I might try applying a measured load to each elevator. Perhaps hanging a known weight to each elevator in turn nd measuring how much it is deflected

[MattyB]

It’s a slight warp to one wing (can’t remember which though, sorry); mine has done it from new too. Every now and then I bend it a bit using books overnight, but unusually for a foamie it has a very stiff wing and relaxes back to it’s slightly warped state after a few days!

Edited By MattyB on 30/07/2020 21:47:56

[Simon Chaddock]

I would have to agree with MattyB.

It is unlikely that a small differential between elevator halves would have a significant effect however even a tiny wing warp or even a small difference in their deflection characteristics certainly will, particularly at times of high g load.

I have flown a V tail with one elevator half accidentally disconnected without difficulty.

[Tim Kearsley]

OK chaps, thanks for the opinions. I hadn't considered a warp in a wing. This is the third Excalibur I've had and I didn't see this pulling to the right characteristic in the previous two versions. Perhaps I was just lucky!

Tim.

[Peter Miller]

While I would agree that a slight varation betwen the elevators would not normally have any noticeable effect Tim Said that it was while pulling out from a vertical dive where the stresses would rather higher than in normal flight!

[Pete Collins]

The other thing that would give this sort of behaviour is a heavy wing. Admittedly this is unlikely with a foamy, but it's easy to check the lateral balance so it would be worth a try.

[David Hall 9]

I've found that it's quite easy to inadvertently use rudder trim to correct (in part) an aileron imbalance for level flight. This is more so in my gliders that have a small dihedral and will turn on rudder, such as my thermal type gliders. Models that are insensitive to rudder might allow some imbalance to go fairly unnoticed.

The test that I perform is to pull a loop from a gentle unpowered dive, with enough energy to complete the loop, and to correct any tendency to screw out of this by rudder trim. This assumes that it is trimmed for level flight to start with. I understood that this is quite a standard test.

[ron evans]

Just a thought, but wouldn't a warped wing in a vertical dive from a good height, start to roll before before pulling out.

Could it be that when pulling out of the dive, the extra load is twisting one wing half more than the other.

I had this with a Phoenix when one wing half was less dense than the other.

As previously mentioned, if you fly mode 2, it is easy to inadvertently nudge a bit of roll in the heat of high speed pull out.

Ron

[Mike T]

Posted by ron evans on 31/07/2020 11:25:07:

Just a thought, but wouldn't a warped wing in a vertical dive from a good height, start to roll before before pulling out.

Could it be that when pulling out of the dive, the extra load is twisting one wing half more than the other.

It's a possibility, but if the screwing out only happens when up elevator is pulled, then a mis-match in the control surface throws is the culprit.

If it's possible, I'd reduce the travel slightly on the affected side. Alternatively, just dial down the overall throw until a full up pull on the stick results in a smooth pull out from the dive. Put this setting on the elev rate switch and keep the full throw rate (if needed) for landing.

[MattyB]

Posted by ron evans on 31/07/2020 11:25:07:

Just a thought, but wouldn't a warped wing in a vertical dive from a good height, start to roll before before pulling out.

Yes it would, and that's how I know mine is definitely a warp - it starts to roll (very slowly) in the dive, and the faster you go the more it increases .Having looked along the wing it is very small (almost too small to be visible), so I've just learned to accept that I have to trim it out under speed and left it at that. You are probably right that it isn't certain Tim's has the same problem, but it would seem fairly likely.

[ron evans]

Well gentlemen, how about pulling out of the dive inverted.

If it's a miss match or a one sided weakness on the elevators, it should roll in the opposite direction...maybe.

Ron

[Martin Harris - Moderator]

Another possibility is that you might be introducing a slight bias when pulling up elevator. Unless I've missed it, there's been no mention of which mode you use but at those high speeds, any small movement will give far more noticeable results.

How hard/fast are you pulling with this particular manoeuvre ? You might be causing a high speed stall if there's a hard pull out. At this point, the heavier wing would drop but having rotated towards level flight, the action of reducing elevator would tend to un-stall the wing...remember that stalls are caused by an excessive angle of attack and given a powerful enough elevator, at any airspeed up to the point of structural failure.

[Tim Kearsley]

Well thanks to everyone who's contributed to this. Next time I fly the Excalibur I'm going to pay much closer attention to exactly what happens to try and get to the bottom of this. As I fly mode 2, it's quite possible that I'm slightly pulling the stick across as I pull down. However, I don't notice that effect in other models I fly, nor did I see it in the two previous Excalibur models I've owned.

Martin, it isn't a particularly vigorous manoeuvre I fly. I climb to height, roll inverted and then power off and put the model into a vertical dive, slowly pulling out as terra firma approaches! What is very noticeable is that, without intervention, by the time the model is in level flight it is now flying some 45 degrees to the original line, to the right.

Tim.

[ron evans]

Hi Tim, it would seem that rules out pilot induced roll, and if it rolls in a dive without elevator input, asymmetric elevator response as well.

Assuming tailplane halves are in the same plane, a small warp, or less likely, aerodynamic distortion, is the probable culprit.

Ron

[Tim Kearsley]

I don't think I explained myself very well, looking back at what I wrote Ron! As far as I can tell, the Excalibur maintains a straight line in the dive. It's when I apply elevator to pull out that it turns to the right. This is why I originally thought that there was some rudder input occurring through the two elevator halves not moving equally. But I can't see anything noticeably unequal in their movement back on the ground.

At the end of the day, I can live with it by applying corrective input, but I like things to be right!

Tim.

[Piers Bowlan]

Just a couple of wild theories to throw into the mix...

First I wondered if the fuselage near the tail could be twisting slightly when pulling out of a dive, although I understand that, like the wings, it too is reinforced with carbon fibre so probably unlikely. I had a Radian Pro XL that exhibited squirllly behaviour sometimes before I reinforced the aft fuselage with carbon. Obviously a totally different model to the excalibur I hasten to add.

Another possibility is that as the model has forward sweep this will have a tendency to wash-in the wing as it flexes when pulling out of a dive. If the amount of wash-in is only marginally asymmetric this will cause a roll away from the more washed-in wing. The wings are heavily reinforced with carbon probably to make them as rigid as possible. The full sized X-29 incidentally was built with very stiff wings because of it's extreme forward sweep.

It is only a theory - discuss. (I have my tin hat on!)

[Piers Bowlan]

Grumman X-29 incidentally, for anyone interested. Another one for Tony Nijhuis to model 

Edited By Piers Bowlan on 01/08/2020 17:37:53