cutarug

I have just test flown a 1/4 scale Nieuport 28 and it flicks upright when I do a loop.
It is not overpowered for sure and flies scale like but it has this violent flick at the top of the loop. I don't know if it is just a speed problem where it stalls or some aerodynamic problem.
I have a 1/4 scale D7 which flew on the same engine and weighed 2 kg heavier but it never did this in a loop.
It lands ok and flairs so COG seems right, I suspect that half chord lower wing has something to do with it any thoughts

?

speedracerntrixie

My Feedback: (29)

Most times when I have seen this happen it's a combination of forward CG and too much elevator. A viable test is to build up a bit more speed and try a larger loop with less elevator input.

RCFlyerDan

My Feedback: (54)

Before flying it, The plane talks to you on the ground, if you listen. If the plane is trimmed for level flight, then the elevator will tell you if the plane is nose heavy. Turn transmitter and airplane ON. Look at the elevator in relationship to the stab. If difficult to see, use a straight edge. If there is up elevator, then the plane is nose heavy.

speedracerntrixie

My Feedback: (29)

Dan, in most cases I would agree with you on the relationship between CG and elevator trim. I'm finding biplanes are a bit more complex though. On Saturday I test flew my new biplane designed for pattern competition. The first few flights confirmed that the CG was a bit forward and the required up trim reflected that. Being electric powered I moved the battery tray back 1.75". Doesn't sound like much but the battery tray also serves as the ESC mount. Combined weight is a tad over 2.5 lbs moved back 1.75". That solved a couple slight handling issues but much to my surprise the elevator trim did not change.

cutarug

I had trouble in early flights with it climbing and reduced the incidence from +2 top wing 0 bottom wing to 0 both top and bottom wings and it still flies with a little down trim.
The nose drops when power is cut and I needed more elevator to flair on landing, I would like to take some lead out for sure if I thought it was nose heavy.

perttime

A flick roll is a stalled maneuver. Do you reduce elevator at the top of a loop?

cutarug

I do not reduce elevator as it struggles to do the loop anyway. The way it snaps suggested to me that it is tail heavy, but most think it is nose heavy.

cutarug

I do not reduce elevator as it struggles to do the loop anyway. The way it snaps suggested to me that it is tail heavy, but most think it is nose heavy.

speedracerntrixie

My Feedback: (29)

Dropping the nose when power is pulled off is a sign of being nose heavy.

RCFlyerDan

My Feedback: (54)

I wasn’t sure what a “flick” was at the top of the loop and thought that it was a snap. I’ve never heard that in fifty years of flying. Must be that English translation issue again.
In fifty years of flying rc, I’ve had my share of biplanes. They are difficult to balance, because you can’t balance using the top wing with the plane hanging as a pendulum. When the top wing is mounted in front of the lower wing, it is also difficult to balance inverted. The balance point is normally on the leading edge of the bottom wing. Using the leading edge lower wing inverted, there is still too much of a pendulum affect.
If the plane is built as per plans. Up elevator trim is nose heavy. Snapping at the top of a loop is too tight of a loop and over controlling the elevator. And, possibly over controlling the rudder. As previously said, a wing has to stall to snap. Most use way too much elevator with scale planes. A scale plane elevator is set up properly when you can hold full up elevator through several loops and the plane doesn’t snap. If it cannot continue the loops, then there is a power issue for pulling it through the loops. If it snaps on the first, then again, too tight of a loop and over controlling.
But, hey, I don’t know what I am doing after fifty years of flying them and a retired jet Captain that understands aerodynamics.

jester_s1

How much airspeed are you losing at the top of the loop? Do you see the plane drop at all before flipping upright? If it's a flick, you should see the plane rise a bit as it flicks/snaps and rolls. But if it's just falling out of the loop due to a loss of airspeed, it will drop first.

Lownverted

My Feedback: (4)

If its struggling to pull the nose up for a loop, tell tale sign of nose heavy, not tail heavy.

cutarug

There is a lot to consider there unfortunately it happens so quickly it is hard to see what happens.
I will try removing a bit of nose weight a little at a time to be safe thanks for all the replies and information.
.

cutarug

I used a cog formula to check position of balance and I am 20mm forward of point where it should be.
I will make a jig to check the cog properly as it seems most think the Nieuport is nose heavy .

UStik

Things are not always as they always are. Or, as my driving instructor told me 50 years ago: you can't even think as silly as it comes. cutarug, I mean what you told here might all contribute to a stall on top of a loop and a following flick - just an inverted stall.

Let me assume a few things: The airplane is balanced OK, it's not exactly overpowered, and elevator is rather too little than too much. Then a loop will not be tight and full elevator is needed just to keep it round. Coming to the top, the airplane has lost a lot of speed and needs lift to stay aloft - now inverted lift. Lift at slow speed means a decent angle-of-attack, and now look at that wing airfoil. It seems to be flat-bottom with a sharp leading edge, what is prone to inverted stall. So it doesn't need much to provoke such an inverted stall, which won't be symmetrical so the airplane will yaw. That in turn leads to a flick, and if it's not just randomly asymmetrical stall then there is still propeller torque which will make for it and which is big in that situation. I think half a flick roll to upright is simply because the airplane (the flat-bottom airfoil) is again perfectly happy because now there is plenty of lift at big AoA and low speed.

So I would try to make out if the flick is to the left. Then I would try a (really) high-speed entry to the loop, maybe by a prior dive or split-S, to see if the flick still occurs. And finally I would try what a bit more elevator would do, I mean if it could make the loop a bit tighter and speed on top higher. Checking the airplane's balance with the usual metheds wouldn't hurt either, I admit, but I don't see a problem here.

RCFlyerDan

My Feedback: (54)

20mm is a lot on a short nose plane.

RCFlyerDan

My Feedback: (54)

First thing correct.

RCFlyerDan

My Feedback: (54)

A loop with a stall is an accelerated stall.

This guy explains it better. In the video, during the stall, he doesn’t allow it to snap out.

https://www.google.com/amp/s/bruceai...-airspeed/amp/

jester_s1

Bickering removed. It's just model airplanes, people. There's no need to showboat about it.

To the OP- does the above description of an accelerated stall look like what's happening to your plane? It definitely makes sense of what you've told us.

JacoS

Agreed.

And if the plane is like the OP said "not overpowered" then it leaves you with a bit of a dilemma - you run out of airspeed at the top and you are already at full power, the wing is loaded and causing a lot of induced drag so it does not want to accelerate. To gain speed you need to point the nose down (or level off inverted which will ruin the loop) , but the wing is already very loaded and close to a stall, so pulling harder will just accelerate the onset of the stall.

The size and shape of the loop (in a plane with lower thrust to weight ratio) is very much dependent on energy management. Pull too hard at the start and you lose too much energy due to g-loading and drag. Pull too little and you trade too much kinetic energy for height.

Most full scale aerobatic planes which are on the lower end of the power spectrum have to enter a loop with extra energy gained from a preceding dive followed by a firm pull.

jester_s1

You're forgetting that we're talking about an inside loop where the problem is at the top. The plane is likely still positively loaded due to the stick being pulled back, but not much. It's probably at less than 1 G on a slower flying plane like this. The phenomenon you're describing JacoS would explain a snap that happens at the bottom of a loop or when pulling up into one without enough airspeed.

JacoS

Not forgetting, just working with the problem.

Once again you have to consider whether the plane is a high powered Extra that has ample power to pull it through the top or low powered WWI fighter.

Flown correctly the, at the top of the loop, backpressure should be released and the nose will drop below the horizon almost by itself, but watching from the ground the RC pilot has got no feedback (buffet or g-force) and cannot really judge what the plane wants to do next, thus he just keeps pulling back and that is when the stall occurs. The stall speed will be much lower than normal, but if you just continuously pull back it will still stall.

UStik

Jaco, that's what I thought at first, but wouldn't you end up in a spin then? (Hence my crazy idea with inverted stall, assuming the loop is so big that there is not even positive G and the airplane is creeping up to the top.)

cutarug

Diffidently not overpowered in fact it needs a dive to pick up speed to get it at a speed it will have any chance of doing a loop.
I have a 1/3 scale pietenpole that is very scale in power and virtually stationary at the top but it does not flick like the Nieuport.

UStik

Do you think the engine could stomach a prop with more pitch? Could be even less diameter, just to get more speed. Maybe that could pull the airplane through the loop no matter what kind of stall is happening now.