Shihtzutan

I've got a PT-19 that is an awesome flyer. It's a 56-inch wingspan Pilot kit (an oldie but a goodie). It’s got a Saito .56 and an APC 10x9 prop, which usually tacks a high 8 or low 9k. I've had it for about six years and have logged over 100 hours of flying (my computer radio keeps track of the time logged). Needless to say, I've had my share of repairs, but I'm very careful about building it back to showroom condition, give or take some hanger rash. Anyway, over the last few months I've noticed that I've developed a snapping out problem with full up elevator. I'm not new to this, I know the phenomenon well, and it’s usually simply caused by too much elevator. So my JR8103 and I tweaked it out (less elevator). My dilemma is that over the years, it didn't used to snap out on tight loops! I've checked everything with an incidence meter and I'm right on the money! The only thing that seems to make sense it that maybe my engine is getting tired and I don't quite have the pull-through I used to; does that make sense?

Do we have any aerospace engineers out there? Is there something else that can cause a plane to snap-out with full elevator?

Toro1

My Feedback: (2)

Well, to me it sounds like a stall at the top of the loop. An airplane can stall at slow speed, or high speed. It may be that the CG has changed with the repairs that have been made over the years. What your describing is a classic problem in full size where there is a high speed stall at the top of the loop, and the next thing you know your right side up again. It may be that the engine is tired, and that the same airspeed that used to be there isnt anymore. But some how it is stalling at the top causing a temporary spin at the top of the loop. You may only get 1/2 turn but I bet if you held full up it would continue to spin.

Just a theory.

JB

southern_touch99

In full scales arnt high speed stalls caused by an abrupt change in the attitude of the plane? (i.e. Snap rolls continuing to snap). Also there is some theory I read somewhere talking about if the AOA changes above a certain amount of degrees in a given time the plane can root stall at any speed.

LouW

Both responses are correct in identifying a stall as the initial culprit. The wing will stall at a particular angle of attack regardless of speed, or attitude. During my initial full scale aerobatic training, I would stall either at the top or on the back side of a loop when being anxious, I would pull too hard on the stick. However unless you are also getting a yaw at the same time, the nose will simply pitch forward. To get a snap requires one wing to stall before the other and assuming no warp in the wing must be due to rudder input or thrust offset (torque).

Toro1

My Feedback: (2)

16 degrees is typically the point of seperation this is the point in regards to the relative wind. IE if you exceed 16 degrees to the actual direction of flight you will have a stall on your hands. I remember doing loops in an easy flyer ( full scale ) and an anxious pull out would cause a roll out at the top. remember that models power to weight is very different from full scale, and the engine up front produces enough torque in relation to weight to make it do very strange things at low airspeed with full power.

Shihtzutan

Keep in mind I used to do tight loops, and square loops with full deflection. In short, the plane isn't what it used to be, for no apparent reason. I've checked the obvious, and triple checked the incidence; this one's got me baffled. Cancel any wing warp, thrust offset, or abrupt inputs, however I just thought of something . . . I installed some scale-style stall strips awhile back to tame the landings. It worked, but I wonder if I stole from Peter to pay Paul, sort to speak? Hmmmmm an update is coming. Thanks for the effort Gents.

JohnMcGowan

My Feedback: (15)

I have a World Models Zero 60 size w/ a saito 91 4 stroke. If I ever do a pretty tight loop , my plane will snap out when it is at about 11 oclock high in the loop. Now... if I keep the loop pretty large it will do a very nice loop. Just the way it is

Montague

I'd say your guesses about the engine getting tired or the stall strips are right on the money. A slight increase in airframe weight leading to a higher wingloading would also fit right in, as mentioned above.

Espeially the stall strips. After all, a stall strip is there to induce a stall at a lower AOA than it would normally occur, the idea being to induce it at the root of the wing instead of the tip. Maybe you should shorten you stall strips so they don't extend quite so far out along the LE of the wing?

The engine being tired would do it because at lower airspeeds, it's easier to rotate the nose though the critical angle of attack, so the same elevator deflection at lower speeds will cause a loop, and enough power can over come this. As an example, many combat planes are usually set up to loop forever at full throttle and full elevator deflection. However, I personally set mine up with a tad more elevator deflection, and my airfoil can bleed speed faster, so as I hold full elevator though multiple loops, my plane slowly looses airspeed and eventually will snap out of one of the loops. Some of my engines pull a little harder than others, and the stonger engines can maintain the looping longer, as there is more thrust available to counter the high drag induced at high AOAs, do the airspeed drops slower.

It's also possible that you changed prop size or brand? That would change your engine thrust, and could lead to this kind of thing. Also, odd as it may seem, not all props are exactly the same from the same company, so you may loose a few % of thrust from one prop to another, and that could make a difference. (I have personally seen as much as a 500rpm difference between identical appearing Master Airscrew props. Other guys I know have reported even higer variances). When you start to push the edge of the non-stalled, high speed envelope, small things start to matter a whole lot.

EscapeFlyer

My Feedback: (1)

Realising I don't know where all of your repairs were made, is it possible that you could've changed the incedence in the wing just slightly enough to where you don't notice or see the change visually, but enough to stall the wing out sooner? That would explain why it snaps out with the same throw settings you had to begin with, where it wouldn't before. It doesn't take much to change the stall characteristics of your wing.
You could cut back the elevator throw just slightly-not more than just a hair-and I wonder if it would fly like you remember.

BigSTIK40flier

your stalling at the top of the loop, now thats not the first problem, the biggest one i think is you elevator throw. my experince with split elevators tells me , whats happing is the stall is caused by abrupt AOA change now, that is caused by too much elevator throw, the plane isnt going to loop as well anyways becuase it has dehidral. but im pretty sure the elevator throw is the main cause, so you should try to get the settings to where you do as tight as loop as you can with out it snapping out, thats where the throw should be. also check that one side of your elevator is flexing down.

OscarPilot

My Feedback: (7)

Based on your input, I would say WEIGHT INCREASE. It doen't take much to make it happen. Does it do it with a low fuel load?

Stall strips are the other likely contributor.

Let us know what you find out.

Shihtzutan

Thanks for all the help gentleman! To date I have not 'dicked' with it for awhile, but I will pull the stall strips off and see what happens the next time I decide to fly it. I think it might get worse; the reason I put the stall strips on in the first place was to tame it for landings. Winter's comming; I may strip the PT-19 and go through it 100% then rebuild my Satio. I have well over a 100 hours on the airframe and the Satio has given me eight very reliable years in 4 airframes before finding it's current home in the PT-19!