captainkttyhwk

recently read a thread on tip stalls that was a little old, guys were saying plane keeps snapping right and didnt sound like they figured the root of the problem. i saw a lot of dicussion on tip stalls. i saw talk of p-factor and the like, but did anyone consider gyroscopic precession?? when the nose is pulled up, particurlly when its pulled hard, and if you have a counter clockwise rotating prop, you have a significant yaw to the right, just like you do on take off when the tail comes up but this way it would be to the left. toque, p-factor (sprialing slipstream into the rudder), and assymetrric propellar loading all take you left on take off.
i was curious if anyone hit on this? it would provide the setup that you discussed. and now tip stalls, the tip is the last part to stall, why do so many talk of the dreaded tip stall? the root typically stalls first.

slagburn

If that was the case I don't think it'd be just Funtana owners *****'n about tip stalling.

seanychen

My Feedback: (4)

My Funtana as well as my 120 size Cap & 120 size Staudacher stalls to the right.

pauldo

My Thinking was along similar lines. At high angles of attack, the prop in a horizontal position will pull the plane to the right. It does this because the prop pitch, relative to the direction of the plane, is higher on the RHS of the plane, than on the left, thus generating more thrust. This will yaw the aircraft to the right.
When at high angles of attack, you also lose some of the effect that the rotating prop air has on the airframe. The built in Right thrust on the motor therefore yaws the aircraft to the right.
Both these could cause the aircraft to QUOT!tip stall!QUOT! to the right, when pushed hard.
My Funtana does it, and so does my Katana. It would be interesting to monitor what it does when inverted. I don't normally pull it as hard inverted!!!!

seanychen

My Feedback: (4)

The other day I tried inverted pull-ups on my Funtana. It drops the left wing, as viewed from me. So I thought the lateral balance is the culprit. So I re-did the lateral balance, and the wing drop reduced: upright pull-up doesn't drop right wing as severe, and inverted pull-up does not drop left wing as severe. But there's still wing rock during upright harrier, and it always starts by rocking to the right.

I favor the gyroscopic precession theory, but haven't yet confirmed it w/ flight. If gyroscopic precession is the main culprit, then changing to a light propeller (like from APC wide prop to a wood) will reduce the gyroscopic precession, because mass (moment of inertia) of the prop is a factor in gyroscopig precession torque. I will test it hopefully this week.

While I'm at it, I will also zero-out the right thrust to see what it does. I have several planes w/ zero right thrusts, like Flip & UCD. They don't have way-off right rudder trims. Personally I think right-thrust is a simple bandaid to counter the conflicting forces in SOME of the maneuvers. My dream airplane is a mid-wing, mid-thrustline, 2 engines back-to-back (1 pointing fwd, 1 pointing aft, both using identical tractor props) canceling out the prop-torque, P-effect, & gyroscopic precession. Oh if I can ever pull this off, it should make most maneuvers much easier, except for torque roll.

Baron Johnson

There's some misconceptions in this thread... I'll try to summarize the prop effects briefly.

P-factor: This is caused, as mentioned before, by the descending blade having higher pitch than the ascending blade when at high angles of attack, which causes a yaw to the LEFT when pitche up.

Spiral Slipstream: This is the rotation of the air around the fuselage as it goes back. It's the same direction as the prop rotation. This spiral, or helix, impacts the tail surfaces, and the assymetric vertical tail area causes there to be a bias in the force exerted on the tail, causing a yaw to the LEFT when the power is on. This is the only effect that you correct with right thrust.

Gyroscopic precession: When a gyro (prop) has a force exerted on it, the reactive force is as if it is 90 degrees later in the rotation. So, a hard pitch UP will cause a yaw ro the RIGHT, and vice versa, or a yaw to the right will cause a pitch down. This effect is the premise of the lomcevak in full scale aerobatic planes. This only affects the plane while you're pitching or yawing. Once established in a harrier, let's say, gyro precession is not a factor. Doing a wall, it is.

There's two other effects, torque and prop normal force, but they haven't been brought up so I'll leave them be

Talking with George Hicks at some event, he told me he goes out and tries to find the perfect pitch rate with his precision airplanes, so that P-factor and gyro precession cancel out. Spiral slipstream is not a factor because proper right thrust corrects it. George is a wealth of information on aerodynamics, and maybe he'll jump in and give a more in-depth examination to the prop effects. I personally don't think the prop is the primary cause for the bad habits on this plane, otherwise we'd see it on a lot of other planes.

Baron

GRH

Hi Guys,
Baron did a great job of explaining the propeller effects. Gyroscopic precession is not the cause of tip stall...neither is P-factor. The pitching or yawing moment that arises from the propeller's precession is typically very small. If interested you can estimate this value by the following equation:

M = Ip*RPM*d(theta)/dt

Where:
M = moment (pitching or yawing depending on the prop's rotation direction and the sign of the angular rate)
Ip= moment of inertia of the propeller about the axis of rotation
RPM = propeller/engine/motor revolutions per minute (change to radians per second for calc)
d(theta)/dt = time rate of change of pitch rate...could be yaw rate or a combination of the two it doesn't really matter

Be sure to keep the units the same...once you figure out the moment generated you can then estimate the amount of rudder or elevator deflection required to compensate but that's another story. You'll find that it's typically pretty small for normal aerobatic flying conditions...even 3-D. I have seen precession rear its ugly head on the smaller electric 3-D airplanes. If you pitch up so violently that it yaws to the right you've most likely seen gyroscopic precession but you have to be at full power (high RPM), rapid elevator deflection (to get the pitch rate) and it helps to run a larger prop (high Ip)

Another thing to point out is that this is the maximum moment of inertia for the prop and the you only see this value twice per revolution. The actual precession moment has a sinusoidal time history throughout the maneuver but because the RPM is typically much much higher than the pitch or yaw rate you can consider it a constant and model it by taking a RMS value of the overall response.

Since precession only causes a yawing or pitching moment it is highly unlikely that it could be the rapid rolling moment that we typically refer to as tip stall. I'd look for a L/R wing misalignment. You can typically see this by any amount of aileron deflection required to trim the airplane. Wings requiring different aileron deflections to trim will almost always be asymmetric at CLmax.

Thanks Baron...it was great to see you and your folks at SEFF. BTW, How does that Crazy Chicken fly?

George Hicks

trailingedge

My Feedback: (1)

I may be totally wrong as I'm not an expert, but it seems that all of the guys "in the know" here refer to gyroscopic precession though it is an on and off kind of thing. Gyroscopic precession may very well be the case here since any plane with a prop deals with this and normally overcomes it with lift from the wings (it's always there, but there's normally more than enough lift to counter this). The airfoil and wing shape will have something to do with it also. My thought is as follows.

The plane is put into a position that it is at or near stall with the small wing tips producing very little lift while the massive center of the wing is producing most of the lift. Now that the plane is flying like a pendelum, it doesn't take much to tip it over and that's where the gyroscopic precession comes in. Like I said the airfoil has something to do with it here too. Look at the wing tips of the funtana 40. They're not that far away from perfectly round. Someone with a few different wood and composite props in different pitches could probably prove this true or false.

I'm not an engineer, but does this sound plausible???

Shahid

No matter what shape a wing is lift distribution is always elliptical

Thats why a spitfires wing is sooo efficient, the wing is elleiptical so there aint much part of the wing doing nothing.

This also explains tipstalls.

But I realised forget how the funtana tip stalls get a better plane.

Thouigh someone did actually totally reinforce the rear end of the fuz with carbon fibre, made it much stronger and the tipstalls disappaeard, I think.

GRH

Trailingedge,
Good to hear from someone in my home state.

Gyroscopic precession is an "on or off" situation in several ways. The main one being if there is no pitch or yaw rate then there is no precession. You can prove this to yourself with a gyroscope...until you generate some type of rotational rate that's not in line with the primary axis of rotation (which is the centerline of the crankshaft in our case) there will be no precessional moment. In the case of the gyroscope the mass moment of inertia is symmetric about the axis of rotation but in the case of propeller it's not so... the actual precession moment is a function of the relative orientation of the prop to the axis of rotation. This is why I mentioned the sinusoidal variation of the precessional moment in the previous post. You would have to strain gage the motor mounts to see this because the prop spins orders of magnitude faster than the pitch or yaw rate causing the precession...that's why an observer would sense the effect as a Root Mean Squared (RMS) value of the maximum value. In other words the precessional moment seen at the prop disk will be less than what would be calculated by the equation I posted previously. Looking back I should have left this out of the explaination but there's always someone who likes to be cute and point this out as an oversight.

Another thing to consider is that a tip-stall, snap roll situation generates a large rolling moment but gyro precession, p-factor, spiral slipstream and prop normal force all cause left/right or up or down forces...the engine's torque is the only propeller effect that causes a rolling moment...yet another reason to disregard gyroscopic precession as the instigator of tip-stall. It sounds to me like you're confusing precession with one of the other effects.

Almost 100% of the time any large rolling moment (snap roll, tip-stall) comes from asymmetric lift on the wings. Fix this asymmetry and the tip stall problem will vanish. I recommend checking the twist, adding stall strips to the inboard of the wing or using zig-zag trip tape or vg's to make sure the flow is turbulent out near the 60-80% of the semi-span. It's funny but people always look at things that don't have enough moment arm to generate any appreciable rolling moment when trying to figure out stall characteristic problems. Look for the obvious first.

As for the lift distribution....very rarely, if ever, is a lift distribution elliptical...even with an elliptical planform. With a perfect inviscid fluid this is the case but that's not real world. Having said that an airplane with a simple linear taper ratio of about 0.5 is very close to elliptical. Typically a wing planform with a true elliptical planform with encounter low Reynolds (Re) number problems near the wing tips because of viscosity. The low Re at the wing tips often destroy the minimum induced drag (constant sectional lift coefficient) you were after. There are some good AIAA, SAE and JOA papers out there on these subjects. I think the consensus was that an elliptical planform with a finite tip chord can come very close to a spanwise efficiency of 1.

Can you believe they teach all this right up the road at MSU

George Hicks

captainkttyhwk

i wasnt saying that presession was the rolling movement but might be causeing a yawing movement right near the stall hence aggravating one wing to stall prematuerly, since everybody seems consumed with "tip stall". Is it possible that the intense yank upon the nose near the stall can produce enough yaw to stall one wing hence make it "snap"? i just dont think we'er seeing tip stall as much as they are experinceing one wing stall. as far as I gathered, the root will stall first, and that possibly the tip isnt stalling at all. I can stall planes with the ailerons working throughout, how is this possible if the tips are stalled? (non strip ailerons). i argued with a couple local boys about a ground speed vs airspeed issue. they believed that when making a downwind turn their planes were "tip" stalling because they would snap while in the turn, they further believe that there plane is going too fast downwind. So they would "slow it down", reduceing flying speed. Are they maybe focing an acellerated stall while trying to hold altitude in the tight turn? since most guys dont cooridinate their turns, its not as easy to when your not in the plane, whammo, stall/snap/spin. I took a plane out yesterday in a 30 knot wind, as long as i left the airspeed alone it flew fine through the turns, if I tried to fly using ground speed, it fell outta almost every turn, and no, landings werent beautiful, but survivable (one the plane was going backwards at touchdown). How bout a gyro on your rudder servo? seems to work good.

daveopam

My Feedback: (9)

A lot of what you guys are talking about, is over my head. But let me throw this out. After reading this and other post(one of which I made this same stament on) I tried a few things. Mostly different power and spoileron settings while entering and doing an elevator. Here's what I found. My Funtana wont do an elevator ,upright or inverted without spoilerons. The wing rock "upright" is the same at idle,1/8 power and deadstick. Inverted there is no wing rock at any setting.

My point, while I'm sure the things your talking about effect the plane all the time. Wing rock (in this case) has to be strictly AERO related.

I hope I made my point and this is on topic. In closing. What dumb a** thought ground speed had anything to do with airspeed?
Not that I am the sharpest tool in the shed but come on.


David

captainkttyhwk

well, i must say, that i cant confrim nor deny the the fact that person or persons who think their airplanes will fall from the sky when turning downwind are truly dumb asses, their just not edge-i-micated quite like the rest of us. now im talkin, of course, if you are doing coordinated turns.
you are moving with the wind my friends. part of the air mass and are flying relative to its movement. heres a good one, the day he just had to prove that taking off with the wind should be better, thats right, into the weeds! elevator just woulndt do a thang. lately ive benn teaching a couple guys how to taxi in the wind, (IE control positions) because we get so much of it (and we're flyin off pavement). man, talk about gettin right with using both sticks. on the ground and commin down final approach and through the flair. these guys are greasin the landings, and it has really improved my landings as well.

daveopam

My Feedback: (9)

I guess I should have left that one sentence out and maybe have been a little more direct.
Will hold my tounge.



David

talk the torque

I still like the idea that right thrust could be the player here... With all planes I've done upright and inverted harriers you always have to apply some left rudder. If it was P/Factor or Gyroscopic then you would have to cross over to opposite rudder for inverted. I think because of the high angle of attack that the helical airstream misses the rudder for what the right thrust was designed to counter.

As far as other planes not tip stalling during a wall when the Funtana does.....I don't own a Funtana but maybe because of the 20% wingtip airfoil and very rounded leading edge the Funtana reaches its critical angle only when performing things like walls. Most other planes are ok if you pull gently into the hover or if you do a wall but somewhere in between those 2 points it will bite.

Jason

_WIT_

I think it's important to this discussion to remember that not all Funtanas have 'tip stall' tendencies. I've been flying mine since last year, and so far haven't seen any tendency whatsoever to drop the right wing. I'm running a Saito 91, so mine may be a bit heavier than some, lighter than others. I did take the time to ensure that the both left and right wing were adjusted to the same angle of attack, and there was indeed a bit of twist that I had to correct. Also, I did not try to get 80 or 90 degrees of control surface throw, especially on the elevator - you really don't need it.

I think these two things are the reason my Funtana does not try to tip stall.