Discussion The Great Harrier Debate...

[kepople]

Since this is a 3d FLYING forum, I thought we needed another thread on actual flying technique rather than the latest plane to drool over so here goes...

Everytime I see a section in a Manual describing 3d manuvers I read it to see the different techniques in use. I read the 3d Section in my Reactor manual and came across thier description of Harriering...

"the harrier is nothing more than a high angle of attack flying stall.." Good description I thought. It goes on to warn of the need to make constant aileron corrections to correct wing rocking (more on that later). It says to reduce power and pull back slowly, the add power back to maintain airspeed.

now here is where I begin to sense this descritption is talking of high alpha flight: "This is one manuver where less control is needed. Too much elevator and the model goes into an uncontrollable stall." They further suggest using spoiler and moving CG aft.

Okay so the debate here in my mind is this:
For a wing to stall, the airflow over the wing is disrupted which can occur at any attitude or airspeed. So we add power, and maintain airspeed with a slight touch of the elevator, and the wing is flying. Good stable harriers I have seen and occasionally performed, invovled a fully deflected elevator, and just enough power to keep the nose up.
So my question here is. What is the actual difference between high alpha flight and a true harrier where the plane is flying on the prop and no longer the wing?


BTW: the next section is a rolling harrier which consists of about 6 lines of text thats a subject for another thread...

So discuss harriers amongst yourselves, I am feeling veclempt...

Kirby

[The L]

What I do to enter is reduce power to nearly none, gradually pull the nose up, and then add in power at the same time to hold the nose up.

I believe even a stalled wing is deflecting some air downwards. To hang on the prop entirely with the nose up at 30 degrees you'd need a higher thrust to weight ratio than the average 3D plane has...

[800mZero]

[QUOTE=The L]What I do to enter is reduce power to nearly none, gradually pull the nose up, and then add in power at the same time to hold the nose up.
QUOTE]


How i do it as well!

[Safety Hawk]

Originally Posted by kepople

Good stable harriers I have seen and occasionally performed, invovled a fully deflected elevator, and just enough power to keep the nose up.

Usually when I get a good harrier going, it requires a lot of elevator to get it to harrier, but once the planes is harriering it only requires a little to hold it there along with throttle adjustments. I'm sure that the amount of elevator may have something to do with the location of the CG. Anyhow, I think high-alpha is just a high angle of attack, and a harrier is beyond the stall. Not sure though.

[bsc]

on my 50CC yak I have to input full elevator on high rates to get a good stable harrier it will rock until you give it full elevator. w/full elevator and just a couple notches above idle on the throttle it will harrier around w/hardly any aileron input at all.

[sukhoi26mx]

Maybe I'm not understanding the debate, but here goes. Most wings "stall" between 15-19 degrees of AOA (not attitude.) If you're flying at an AOA below the stalling AOA for that particular airfoil, the wing is still "flying" i.e. it is unstalled and there is some base amount of thrust required to maintain level flight. As the AOA increases beyond the stalling AOA, the wing transitions through a seperation point where it can either transition cleanly past the stall, or depending on the yaw on the airplane, quickness of the transition, or basic aerodynamic traits of the planform and airfoil, it can snap roll or spin. If the model transitions cleanly through the stall and has a planform that is relatively "rock-free" simply increase throttle and elevator to hold the new AOA and maintain altitude. The base idea is that it takes more thrust to maintain harrier flight than it takes for "slowflight" at an AOA below critical, which makes sense because of the dramatically increases drag in harrier flight. So is the wing producing lift in harrier flight? - absoutely. Stick your hand out the window of a moving car at a 45 degree angle, and it most assuredly is forcing your hand up and backwards.

Back to rock, a lot of theory is changing on that subject as 3D designs evolve. Most designers are leaning towards designs that utilize far thinner airfoils than in the past coupled with very low wing loadings and sharp leading edges. This may seem counter intuitive at first as the thick "fun-fly" wing was so prevalent early on, but it really works well. The thinner sharper wing section allows sharper cleaner pitch transitions, making maneuvers like the wall and parachute happen without snapping out at all. The sharper leading edge also has far less rock than the bulbous funfly foil which makes it less susceptable to reattachment if flown close to the critical AOA (i.e. wing rock.)

Got of on a little bit of a tangent.... I think the base question was what is high alpha versus harrier flight. I consider them the same thing - sustained flight beyond the critical AOA. Slow flight is a better (and more common in the full-scale world at least) term for high"er" attitude but unstalled normal forward flight.

Scott

[sun.flyer]

Maybe I'm not understanding the debate, but here goes. Most wings "stall" between 15-19 degrees of AOA (not attitude.) If you're flying at an AOA below the stalling AOA for that particular airfoil, the wing is still "flying" i.e. it is unstalled and there is some base amount of thrust required to maintain level flight. As the AOA increases beyond the stalling AOA, the wing transitions through a seperation point where it can either transition cleanly past the stall, or depending on the yaw on the airplane, quickness of the transition, or basic aerodynamic traits of the planform and airfoil, it can snap roll or spin. If the model transitions cleanly through the stall and has a planform that is relatively "rock-free" simply increase throttle and elevator to hold the new AOA and maintain altitude. The base idea is that it takes more thrust to maintain harrier flight than it takes for "slow" below the stalled AOA flight, which makes sense because of the dramatically increases drag. So is the wing producing lift? - absoutely. Stick your hand out the window of a moving car at a 45 degree angle, and it most assuredly is forcing your hand up and backwards.

So in reality the term "hanging from the prop" could only be used with such maneuvers as hovering. The wings are not producing lift and all airflow over the control surfaces are being produced by the prop wash. I think there is some misconception out there that if a 3D plane is flying slow with "some" elevator it is automatically considered high alpha (harrier) flying.

Back to rock, a lot of theory is changing on that subject as 3D designs evolve. Most designers are leaning towards designs that utilize far thinner airfoils than in the past coupled with very low wing loadings and sharp leading edges. This may seem counter intuitive at first as the thick "fun-fly" wing was so prevalent early on, but it really works well. The thinner sharper wing section allows sharper cleaner pitch transitions, making maneuvers like the wall and parachute happen without snapping out at all. The sharper leading edge also has far less rock than the bulbous funfly foil which makes it less susceptable to reattachment if flown close to the critical AOA (i.e. wing rock.)

ie.. Great Planes Reactor (small and .40 sized) and similiar planes. I would assume since these planes are using less than the standard 13% airfoil being used on many of the planes today? Cubic loading would also play in here as well correct?

Thanks Scott

Tim

[kepople]

Scott, etal;

Thanks for the response this will be helpful to many.

1) I have read in the past from some pro pilots that in a harrier, the wing is stalled and pitch chnages and wing rock are due to the wing gaining and losing lift. So is the wing partially stalled?

2) regarding wing rock: I have found that I dont catch up if I try to chase the wing rock as described in the manual for the reactor. I found that I cancel the wing low on one oscillation then hold it and the wing rock stops. If I try to compensate on the fly, I make it worse.

Any other techniques for wing rock, or more stable pitch Harriers?

Kirby

[tommy321]

I agree with Scott... that the wing rock is caused by momentary flow reattachment creating lift on one wing.

It would seem that a thinner airfoil with a sharper leading edge would reduce this tendancy. If you can keep both wings deep in the stall, neither will have a tendancy to intermittently produce lift.

The other trick I find is to get through the critical AOA quickly. You have to push through it since it's at the critical AOA that wing drops will occur. I've had aircraft that do beautiful elevators and walls (with sudden, full elevator application), however the same plane, when subjected to a gradual increase in elevator back pressure will snap out.

But I'm also not sure that wing design is the whole story.

Aircraft weight: lighter is better.... My theory is that with a lighter aircraft, it's easier to keep the aircraft aloft with the small amount of lift created by the post-stall wing. The more weight the aircraft has to carry, the more turbulence the post stall wing has to create to keep the plane in the air. More turbulence = more wing rock.

Tail design: I've noticed that nearly every plane harriers more smoothly inverted than upright. I think the general consensus on this is that the vertical stabilizer is extending below the aircraft in undisturbed flow. (if you want to feel what propwash is like, try getting too low on tow in a full scale glider ) So the turbulence felt by the tail in an upright harrier can cause wing rock. I've also noticed that some tails are better than others in upright harrier. I think that my Stevens Aero Groove does such nice harriers because the tail is similar to a CAP. I.e. the rudder is aft of the elevator ensuring that it's in freestream airflow in both upright and inverted harrier.

My tips for learing harrier:

-Adjust your elevator travel so full deflection is roughly the amount of elevator needed to harrier. This will allow you to peg the elevator and leave it there. (one less thing to worry about).

-Get rid of the ratchet "clicks" on your throttle. On some aircraft I've found that one click is the difference between nosing up into a hover, and falling back into the critical AOA stalled regime.

-Think of the throttle as doing double duty. With your elevator pegged, your throttle has to be used to control both altitude and attitude. This sometimes necesitates bursts of throttle (to pick up the nose briefly while maintaining altitude). Removing the throttle ratcheting also helps here.

-Some planes have a narrow pitch attitude "sweet spot" where they harrier rock free. Experiment with different attitudes to try and find where your aircraft is happiest.

-Pick up the wings with rudder and aileron. You may find that rudder is more responsive to picking up a low wing and it doesn't result in as much overcontrolling like aileron often does.

My $0.02.

Thanks Kirby this is a great idea... I was also getting sick and tired of a 3D forum full of aircraft reviews

Tom

[sukhoi26mx]

Originally Posted by kepople

Scott, etal;

Thanks for the response this will be helpful to many.

1) I have read in the past from some pro pilots that in a harrier, the wing is stalled and pitch chnages and wing rock are due to the wing gaining and losing lift. So is the wing partially stalled?

2) regarding wing rock: I have found that I dont catch up if I try to chase the wing rock as described in the manual for the reactor. I found that I cancel the wing low on one oscillation then hold it and the wing rock stops. If I try to compensate on the fly, I make it worse.

Any other techniques for wing rock, or more stable pitch Harriers?

Kirby

Kirby,

1. If you're in a fully established harrier, which I consider to be at least 30 degrees of pitch, you're well beyond the stalling AOA and the wing isn't generally trying to reattach. In that case, any rock noticed is from the airframe design, not the wings airflow detaching unevenly. FWIW, that is the benefit of thinner airfoils, as they detach more cleanly than the thicker funfly airfoils, and tend to resist re-attaching airflow at intermediate AOAs.

2. It is really, really easy to get behind countering rock and cause a PIO that increases the "appearance" of rock, when in fact if you'd just do less or make the corrections on cycle, the results would be better.

Couple of things to keep in mind flying harriers...

First - you will nearly always have an aileron input to hold the wings level. Even if only correcting bank for torque, there will generally be an aileron input going on, even a small one. The wings aren't going to stay level on their own.

Second - bank angle changes associated with rudder usage is NOT rock. All models will couple slightly in harrier flight with rudder usage. If you aren't correcting for wings level during heading changes (i.e. rudder) then the bank will change. Typically, the model will roll with the rudder in harrier flight, so you should anticipate with an opposite aileron input.

Third - When it starts to go poorly and seems to be rocking, do less with the ailerons, as you're likely causing more problems than you're solving.

Finally, fly your harriers at more than 30 degrees of pitch if possible. 40+ works even better to minimize rock.

Scott

[sukhoi26mx]

Great post Tom! Agreed on all counts.

Scott

[kepople]

So we have
-reduce the elevator throw for this manuver to allow for 40 degrees on pitch in flight to help minimize wing rock and reduce work load
-to reduce wing rock, do less not more. As I always tell people, dont fly the plane by constantly moving sticks, rather, put in a control input, and see what it does before you react. Makes for smoother flying.
-try using rudder to minimize wing rock
-You will always have aileron input in this manuver
-fly with elevator pegged our and use throttle to maintain airspeed and attitude

Anything else?

K

[kepople]

Now a couple points of my own:
I agree inverted is easier than right side up for the harrier, (dont forget the backwards rudder input all), also different planes have much different amounts of elevator required to attain the flying on the prop attitude. (I assume this due to wing cord and tail moment as well as power setting)

So should your entry to a proper harrier be at near idle?

This comes up because seems that my Elevators which I enter from a slight dive with a parachute, are a lot more stable (less rock, almost none) with the same plane. I am sure PIO is to blame partially, but also the entry has to bare some blame as well right? Something to do with power setting as well which is idle in an elevator...

BTW: I agree completley with the thinner airfoil thing. This has been trending this direction for some time now, but the designs released towards the end of last year are really slimming down. The small reactor for instance, super thin airfoil which by the way does the smoothest harrier I have seen in ANY of my other planes. It will fly at full elevator deflection stable, if I dont chase it. Its actually too easy, then I go back to a much shorter coupled plane and find it harder to maintain a constant pitch.

THanks for the positive comment Tom. I was not sure if this thread would see a lot of activity, but this forum is been foamies and "this plane flies great" for far too long, I felt we should utilize each others knowledge to improve. Heck since we cant fly together daily, we might as well hangar fly daily...

Kirb

[hercdriver7777]

IMO, making the entry to harrier look good, i.e. no funky chicken tip stalls in the transition, is best done by starting with a high alpha maneuver to begin with. Like to from a high alpha rolling maneuver or roll to it from an high alpha KE. Coming at it from the elevator can look really cool but elevators are kind of flat and it is possible to hit that transition stall as you add power to get in a harrier. One thing I have found that makes the elevator to harrier look better is to spiral down in the elevator with a little rudder (like a really big flat spin) and then on the last 90 degrees of turn start to add power to come into the harrier while still in the turn. Having that little bit of bank in as you transition really cuts down on the chicken dance.

[tommy321]

Originally Posted by kepople

-fly with elevator pegged our and use throttle to maintain airspeed and attitude

I like to think of throttle as attitude and altitude (not airspeed). The amount of airspeed you have in this maneuver is inconsequential (it's really just a direct result of the attitude).

A quick burst will affect attitude.... the long term setting will affect altitude.

So lets say a model harriers along at constant altitude at 30% throttle.

If my pitch attitude is too low, I can pick it up by a quick burst of throttle. Returning to 30% throttle before the aircraft starts to climb.

If I want to decrease altitude, I'll reduce power to 20-25% throttle and wait for the aircraft to settle into a descent. This usually takes a second or two before the descent is established.

In reality it's not this cut and dry... you'll have to adjust the elevator setting to maintain different attitudes smoothly, but I think that this is a good way to think about it when you're just starting out. (I like to remove as many variables as possible when learing... taking the elevator control out of the picture by pegging it is the one I chose.)

You can use this effect to drag the tail along the ground. Get the plane low enough that the tail will hit the ground if the plane pitches up. Add a burst of power... if all goes well, the tail will be pushed down to the ground. It will take long enough for the power to affect altitude that you'll generally perform a 3-5ft tail drag before you climb away... (i'm imagining this with my Groove or Edge). Now you can revell for a couple seconds in the ooo's and aaahs from the peanut gallery.

While I'm rambling here, here's my tip for learning inverted harriers and getting the reversed rudder thumb going:

-Fixate on the nearer wingtip when inverted harriering. Imagine that there is a rubber band attaching this wing tip to your rudder stick. Whatever direction you move your rudder stick, the nearer wingtip will go in that direction.

So, imagining an inverted harrier from left to right, if you want to turn away from yourself, you'd want to make the nearer wing tip advance faster than the rest of the plane... which is done by tugging the wing to the right (as viewed by you) and therefore moving the rudder stick right is the necessary control input.

Can you tell I like these kind of discussions?

Tom