A question for Martin about Phoenix aerodynamics

[edwalker]

Hi, Martin -

I'm working with an instructor on my glider commercial rating in the Phoenix right now, and the practical test standards are slowly pushing my abilities closer to the edges of the approved performance envelope. I have to say that it is one of the sweetest and most forgiving aircraft I've ever flown, particularly near the low speed end of the polar where we do so much flying.

I was able to get some spin training earlier in my career, and I have great respect for remaining within the approved flight envelope. Nevertheless, I have always been cautious about the edges of the flight envelope and try to learn as much as I can about the characteristics of the aircraft. Some planes in which I have had spin training seem to snap back quickly given proper procedure, others seem to be a bit more challenging. It would help to know more about why the Phoenix was placarded against intentional spins. There are a variety of reasons why this might be true, including known spin recovery resistance. The AOI describes the traditional spin recovery methods I have learned for an unintentional spin entry, but what is the background of why it was determined that intentional spins are prohibited? How easy is it to break the spin using these methods, and approximately how much altitude would be lost if the recovery were performed correctly?

Thanks for your thoughts.

Ed

 

[Martin Stepanek]

Hello Ed,

the reason for sticker "Intentional spins are prohibited" is very simple - is mandatory in Czech Republic for this category of small airplanes.

We cannot legally make spin testing. Spin tests are normallly requesting a lot of spins in all CG configurations - especially with rear CG - and airplane must be equipped with anti-spin parashute.

6 years ago Evektor "moved" one Eurostar to experimental (or VLA - I don´t remember exactly) category and they did more than 100 of spins...

As far I know, as far as I tested and as far as Jim tested - there is no "known spin recovery resistence".

I agree with you that in case of issue "spins on Phoenix" is possible to write "one of most forgiving aircraft" :-)

In case you enter to a unintentional spin after all Phoenix warnings, than I recommend you to apply the traditional spin recovery methods.

Best regards!

Martin

 

 

 

[edwalker]

Hi, Martin -

While we're on the topic of aerodynamics, I was wondering if you could give some advice on flaps. I've been using 10 degrees of flaps in thermals and negative flaps during higher speed cruise under power. Now that I'm gaining more experience I'm wondering if you have any additional advice on how you intended the flaps to be used. For example, is there a speed at which we should switch to negative flaps while not under power between thermals or in ridge soaring?

Ed

 

[Eric Greenwell]

I am also very interested in the flap settings. I tend to use 0 deg for thermaling and cruising, just because I don't know what the correct setting is (also, my flap lever takes a lot of effort to move it out of the detent ).

Another aerodynamic question: I'm going to add Mylar fairings to the rudder and elevator, as is standard for gliders from Schleicher, etc. Schleicher also puts zig-zag tape ahead of the Mylar fairing on the rudder and fin. Should this also be done on the Phoenix?

And, of course, I still hope you will find that zig-zag tape improves the wing performance.

[Eric Greenwell]

Today, I installed a mylar fairing so it covers the gap (on top) between the stabilizer and the elevator. It looks quite sleek, and now I'm wondering if a mylar fairing on the bottom gap would improve performance any. Any thoughts, Martin?

And while I was thinking about that, I remembered the control surface gap on the bottom of the wing does not have any Mylar, either. Would mylar there help the performance? Would it make the controls harder to move?

[Eric Greenwell]

I have no idea if this change in taping the tail improved the aerodynamics, but it does improve the appearance. The junction of the fin and horizontal stabilizer is difficult to tape smoothly, and a distinct gap forms between the two pieces. The first picture shows the gap filled with modeling clay that smooths the fin and stabilizer junction; the 2nd picture shows the clay covered by tape to protect it.

Ideally, the saddle would have that a permanent fillet that perfectly matches the horizontal, so it could be streamlined and easy to tape, but this will do for now. I'm pretty sure the Mylar fairing I put on the elevator will increase the performance (and handling) much more, and it didn't take very long, either. See http://phoenixairusa.blogspot.com/2014/08/great-ideas-from-phoenix-owners.html

[Jim Lee]

10 degrees of flap is best for thermalling the Phoenix.  5 degrees helps, but 10 degrees is better.  It allows about 4kts slower speed in the thermal which translates into a smaller diameter circle and more time spent in the thermal core. 

10 degrees of flap increases lift but does not increase drag to the point that it assists with landings, which is why we use 0 flap for take off and landing.  10 degrees of flap provides a slower airspeed as mentioned, so one can land in a shorter distance using flaps.  But every landing in a Phoenix is a short field landing compared to most aircraft so there is not much point.  There are also major disadvantages to using flaps for landings.  10 degrees of flap produces higher stick forces.  In gusty or crosswind landing conditions, requiring significant control deflections, the increased amount of adverse yaw produced by the flaperon almost leads to a control reversal, and the ability of the crosswind to get under a big deflected flap can also result in loss of control.  Therefore using flaps for landing the Phoenix should only be practiced with caution in perfect conditions, with an understanding of the drawbacks.  And the landing distance being shortened by only 50 feet or so is not a significant reason to use flaps for landing.  I practice short field landings with flaps just for kicks, but if faced with a dire outlanding situation, I will land with 0 flap because I have thousands more landings in that configuration.

As far as negative flaps are concerned, at any speed above 65kts the Phoenix wing is more efficient at negative 4 degrees of flap.  So on climb out from the airport, when 65kts is exceeded I go to negative 4 for the remainder of the climb (usually at 80kts) and then the level cruise and descent, going back to 0 flap in the pattern.  When gliding engine off, the same thing applies.  At any speed above 65kts, a negative setting is best.  However, 65kts is about the highest speed we ever use for gliding in Florida, so rarely do we go to negative flap while gliding.  Out west in strong conditions, negative flap gliding is common.

These points are covered in Phoenix transition training.

Additionally, the Phoenix with minimum fuel on board is nose heavy.  Two people on board make it more so.  A rather large elevator deflection is required to fly slow in the thermal.  Full aft trim is the norm.  The turbulence from the deflected elevator can be felt at slow thermal speeds and higher bank angles while using 10 degrees of flap.  This situation can be improved by putting about 30 pounds of weight in the aft baggage compartment.  For those of us who spend considerable time soaring, this speaks towards a large battery in the aft baggage compartment, to trim the plane aft and provide for more engine off electrical power.  Those pilots flying with an alternate  "paxbatt" in the right seat might consider a secure mounting in the aft baggage compartment instead.

Also many gliders perform better in a thermal turn with a slight slip (outside rudder during the turn).  The amount of slip in the thermal turn is quite apparent when watching top competition pilots.  The Phoenix thermals much better when using this slip technique.  The reasons for this are rather long winded, but suffice to say that the deflected rudder acts partially as an elevator and assists in holding the nose up and creates less drag than the deflected flaperon when the stick is held to the outside of the turn to counter the overbanking tendency.

[Eric Greenwell]

Great information on the flaps, Jim. Apparently, I've been flying way to fast while thermalling. The  manual should have this info in it, as not of all of us will remember everything from our transition training, especially if we don't immediately apply it to soaring ("airplane flying", Skyview, and landings occupied me for many months after I left Florida).

What do you think of using flap -4 for landing during windy/gusty conditions? I use a negative flap setting in my ASH 26 E in those conditions as I can touch down in a two-point landing attitude at a higher speed, the higher speed gives me better control on short final, and the glider isn't affected by the gusts as much, once it's rolling on the ground ("sticks" better).

I'm trying to find a safe way to temporarily stow the 22 lb paxbatt just behind the seats for when I have a passenger. I'll now look at putting it back with the other batteries.

Is there an optimum performance position for the propeller while soaring? I think I feel some turbulence from it in the horizontal position, which I prefer visually.

[Jim Lee]

Eric, I haven't noticed any advantage to landing with negative flaps.  And when I allow Phoenix drivers to land with negative flap "by accident" they never even realize that they did it.

But you have a good point about landing in gusty conditions.  If the wind alignment is good, a wheel landing is desirable to keep the angle of attack lower, speed higher, and maintain better control than the high attitude slow speed 3 point landing.  It is possible that negative flaps will also help in this area.  I will experiment with this.

The trick to the wheel landing is to decrease the spoiler setting in ground effect, much like wheel landing a taildragger airplane with a little power added.  Slowly approach the runway with minimum vertical descent.  Don't rush it and don't force it or a bounce will occur.  When the wheels kiss the pavement move the stick forward 1 inch.  Don't go too far or a prop strike will result.  A wheel landing cannot be made in any significant cross wind or loss of control will occur.  That is why the 3 pointer is like money in the bank.  In any crosswind, the 3 pointer must be used.

[Martin Stepanek]

Hello!

I am glad that Jim answered the "flaps" questions.

Honestly - I am still "glider pilot student" :-)

My experience is that I don´t use 10° flaps except landing (and except crosswind or gust wind landings). During thermal flights I keep flaps on 0°, when thermals are "narrow" I use 5°, but that is not very often.

I also agree with Jim about landings - we have here mostly very bad quality grass runways, but when I have chance to land on nice concrete RWY I like to land on main wheels.

But in LKUO (Usti nad Orlici) is practically impossible to do that :-) So I enjoy landings with engine stopped and on 3 points (with 3 points I mean main wheels and tail wheel - not wheel, wing tip and prop) :-)

Best regards!

Martin

P.S.

And something for all of you as motivation. Text of song in English was really inpiring me :-)

 

 

 

 

 

[Dave]

On the subject of aerodynamics and technique....I have recently operated in significant gusty and cross-wind landing situations with my Phoenix. I always use 0 flaps and try for a 3 point touchdown. When flying my old Ventus B-16.6 I managed x-wind landings mostly with a slip to landing and a last moment adjustment to wings level at touchdown. With the Phoenix I find myself mostly crabbing in and kicking out the crab at the last moment.  

What is the preferred method?

Your advice and experiences in this matter are welcomed.

Dave

[Jim Lee]

Dave, you are doing it right.  Crab to land and kick straight just before a 3 point touchdown is the only way to land in a strong or gusty crosswind.  That way the tailwheel is on the ground, the stick stays in the touchdown position, and the 40 pounds of downforce on the tailwheel gives you directional control.  There have been two wing strikes in Phoenix aircraft, so don't use a wing low slip to land.

When it gets really hairy, I land with 1/4 spoiler, 3 point, and then go to full spoiler on touch down to keep the plane planted on the ground.  But if someone thinks they should try this, they should go to another airport and land into the wind instead.

[Eric Greenwell]

23 hours ago, Jim Lee said:

There have been two wing strikes in Phoenix aircraft, so don't use a wing low slip to land.

I might be remembering wrong, but the wing strike(s) I recall during landing occurred after touchdown, and were gust related, not from a slip during the flare.

My limited experience (compared to Jim) in cross wind landings is I run out of rudder to keep the fuselage aligned with the runway before the side slip puts the wing low enough to be near the ground. When I do run out of rudder, I have to use some crab to stay on centered on the runway as I descend on final; typically, the wind diminishes enough on short final that I don't have to crab during the flare. The down wing never seems close to the ground. The only time a wing got close to the ground was taxing fast on the runway after landing, when a gust lifted the upwind wing. I now taxi more slowly in wind, spoilers out, and keep the stick into the wind. all things I'm sure Jim mentioned during training

 

[edwalker]

I think that aviators may have created a false dichotomy about crabs and slips in landing. In fact, I believe most experience pilots seem to learn a combination of the two and use an intermediate mix of both on landing, constantly adjusting between the two modes as they tune the landing flare and touchdown. There is a risk to extreme adherence to ether method. The slip camp is at risk for banging a wingtip, while the crab group risks side loads to the gear if the kick isn't just right (and it rarely is perfect) as the float down starts to drift. I've had a number of owners remark about laterally asymmetric main tire wear on their planes, and I suspect that this is one of the contributors.

Human learning is about what works. You start with a model and then variations on the theme tend to modify understanding and practice. Motor memory is even more tricky because sometimes you're not even aware that you're learning. Here's where your SV data logger can help. I practice cross wind landing a lot, and I find that what I'm actually doing is wing-level crab on final but it transitions to a slightly upwind-wing low slip during the flare that softens the crab angle allowing for a much more gentle kickout. The basic idea is enough slip/crab combo to stay aligned with the centerline without a too-low wing or a lateral drift. I didn't realize I was unconsciously doing this until I started looking at my SV black box data where you can actually see the roll and heading data. I have to admit that the combo provides a much more gentle and controlled landing than either method alone.

No matter what, Jim is right that you have to get all three wheels solidly on the ground and going in a straight line (preferably the runway centerline). I would also add that gusting conditions and orographic turbulence near the runway make it even more difficult, so having more degrees of freedom by continually tuning the balance of the two styles give more options.

The final thing to remember is that the Phoenix has an amazing thrust to weight ratio, so when it's all going to hell the throttle will bail you out for a go-around.

[Eric Greenwell]

Has anyone measured the bank angle of side slip steep enough to require full rudder? A glance at the attitude indicator while in the slip should give the degrees you are maintaining.

Measuring from a head-on photo of my Phoenix, it would have to tip 10 degrees to the side before the wing tip would touch (by comparison, my ASH 26 E wing tip would hit the ground with only 8 degrees of bank).  I don't know what 10 degrees would look like while sitting in the cockpit on the ground, but I think it would seem like "a lot". Someone needs to do some flying and measuring, so we have an idea of how close the wing tip could be to the ground in a side slip to landing.

Jim's comments about landing in gusty crosswinds with 1/4 spoiler got me to thinking: 1/4 spoiler lets you touch down at a lower air speed than with 1/2 or 3/4 spoiler (as I tend to use), then opening the spoilers fully reduces the lift more than it would going from, say, 3/4 spoiler to full spoiler. So, the glider will be less sensitive to gusts.

[Dave]

Thanks for the input Jim.

[mikeschumann]

I, unfortunately, was the pilot associated with one of the wing strikes.  It was two weeks after I took delivery of my Phoenix, when I was flying it from FL to NC.  I was landing and had a moderate cross wind. As I was touching down my power flight training kicked in and I instinctively slipped towards the crosswind.  BAD move.  I didn't realize that I had a wing strike until the next day, so I barely touched.  Unfortunately it was a paved runway, so it scraped up the gelcoat quite a bit.  This would have been a non-event on a grass field.

I downloaded all the user logs from the Dynon, so I was able to analyze in detail what happened.  It turns out that as I touched down I had a 10 degree roll to the left.  It turns out, the way the dihedral is set up on the Phoenix wing, when you have a wing strike, the main wing hits the runway exactly at the point where the wing tip extension connects.  The extension is angled so that it is parallel to the ground when the strike occurs, so you end up scraping the entire width of the flaperon on the extension.  There is a very minor scrape on the end of the main wing section.

One thing that would be nice would be some "training wheels" that could attach to the tie-down attachment points on the wing extensions that would help protect you from a wing strike on solid pavement.  This could be as simple as a block of wood, just thick enough to prevent you from scraping the bottom of the wing.

Unfortunately for me, I wasn't paying enough attention during my training to Jim's repeated warnings about keeping the wings level on landing.  Sometimes, you just have to learn the hard way.

[Dave]

1. Good to hear experience of other Phoenix flyers. On subject of "training wheels"...lots of older pure gliders have wing-tip skids or tiny composite wheels since their natural posture on the ground is with a wing down.  Perhaps an old pair of in-line skates are waiting in my basement for a new role in life?....Well, maybe not.  Enjoy the flying season.  My wife and I are planning an outing from PA to Bar Harbor in July. Too bad we can't fit the bikes in the plane...I see rentals in my future.  Any interest in a September/October Northeastern or Mid-Atlantic fly in?  Was lately in Edenton, NC....great airport and extremely charming little town.  Saratoga Springs, NY is great too.  Ideas?

Dave Glosser

[Martin Stepanek]

Hello dear friends!

"Training wheels"? Interesting request....

Of course I prefer to keep wings levelled enough to precede "touch" with RWY.

Maybe we could somehow use the nut for tie-down to attach some small wheel...

I just don´t want to make it "fixed" on the plane - I am sure you will not need it after some experience :-)

Best regards!

Martin

[mikeschumann]

5 hours ago, Martin Stepanek said:

Hello dear friends!

"Training wheels"? Interesting request....

Of course I prefer to keep wings levelled enough to precede "touch" with RWY.

Maybe we could somehow use the nut for tie-down to attach some small wheel...

I just don´t want to make it "fixed" on the plane - I am sure you will not need it after some experience :-)

Best regards!

Martin

A removable wood skid that bolts onto the tie-down nut would be a nice option for new pilots to have until they get used to keeping the wings level on landing.

[Gfeldman]

Provided that a wheel as far inboard as the tiedown eyelet hole would protect the rest of the wing outboard from that point this seems like a good idea.

George Feldman

[Guest airusa]

I will share an experience and a technique that will surprise all of you. And you will probably not believe it. I do have witnesses and these are the facts. 

I flew into Angle's Fire, NM to visit a friend that wanted to see a Lambada.  I was on my way to Indiana and decide to stop in.  The wind was reporting 18Kts G29kts West to East 90 degree to the runway.  The x-wind max component on the Lambada is 9 knots. I have about 10,000 hours experience in tail wheel airplanes, and I had a very good flight instructor way back when I received my certificates.  He explained and demonstrated how to make a tail wheel airplane, and the Lambada is similar, stick to the runway like a suction cup.  I arrived at Angle's Fire after a long day, and needed to put it there for the night.  My friend Johnny Smith and his family were waiting for me.  The wind sock was stiff, and standing straight out and was 90 degrees to runway on the most part. AWOS was reporting 18kts G29kts like is does most of the time there.  

I decided on landing and did a normal traffic pattern considering the extreme gusty conditions.  The theory of landing with a tail wheel type aircraft and keeping it on the ground and under control has been planted in my head a long time ago.  Over the approximate 10,000 hours of tail wheel hours and landing in various wind conditions, the technique always worked without well.

The technique is simple.  Land with all three wheels touching at the same time. Regardless of how much the aircraft is being bounced around with turbulent gust changing the pitch angle, roll of the aircraft in and out of proper bank, crabbing, rolling always quickly and abruptly return the aircraft to the three point position, and use as much force on the controls as necessary to do so.  At that landing speed you are not stressing the aircraft with abrupt movements of the controls. With the elevator control input stick held hard back against the stop at touchdown so that the elevator is applying maximum full down weight onto the tail wheel, at the same time the aileron control has to be at max into the wind. held hard against the travel stop.   How does it stick like a suction cup? Well if you think about it very closely, and analyze what are all of the forces applied to the aircraft you will see that the following occurs. 

With the tail wheel hard against the asphalt with all its weight, the tail wheel will resist sideward movement and prevent weather veining of the fuselage.  If the pressure is let off of the stick and the elevator is allowed to move downward reducing the load on the tail wheel and relieving weight loading so there is no side friction, then the aircraft becomes a weather cock like on a barn roof and is easily turned into the wind. Thus skidding sideways down and across the runway.  That is not the suction cup part though.

The suction cup effect is because the Center of Gravity of the tail wheel aircraft is behind the main landing gear legs and wheels.  I am talking about the lateral axis of the aircraft from which pitching of the aircraft rotates around. When the aircraft has a change in pitch it rotates around the lateral axis.  With the main gear firmly against the pavement and the tail wheel is firmly against the pavement, the aircraft can not rotate around the lateral axis. It cannot rotate therefore the suction cup effect. If the airplane were to rotate around the lateral axis the rotation point at the center of gravity would have to rise upward while it is rotating. With the tail wheel firmly against the asphalt with all of its weight pushing down, it cannot rotate to change pitch of the aircraft.

As a dealer for Lambadas and SunDancers, I now have seen lots of mishaps while landing these aircraft. The root cause is the pilot touches down on the runway, and while at or near flying speed stops flying the aircraft. By that I mean, the pilot relaxes his hard back pressure on the stick that forces the tail wheel to the asphalt during this critical stage of landing, the horizontal tail produces enough left to relieve all of the weight off the tail wheel any amount of gust or cross winds turns the aircraft into a weather cock, and off ya go.  I am sure there is still a maximum cross wind component. Katrina that went through New Orleans may prove me wrong.

Joe Kulbeth CFIG  Owner of SunDancers.  I will see you all at Minden.  My last word on this, you want need training wheels if you listen to me.

[Eric Greenwell]

On 7/22/2016 at 9:18 PM, airusa said:

Land with all three wheels touching at the same time.

My tail wheel time is 350 hours - far less than yours - but I've come to the same conclusion. Even if I flare somewhat high, the plane won't bounce significantly as long it is in a three point attitude when it hits the runway (the tail wheel touching first seems to be about as good as a three pointer). Landing in negative flap also seems to help - my guess is the wing has lower lift potential at it's "on the ground" attitude than if the flap is in zero, and is therefore less affected by wind and gusts.

[Guest airusa]

I will add to the long instructional thing I wrote, and that is, when the Lambada, SunDancer has reached a three point touchdown and the aircraft is still at or near stall speed, the efficient wings of the glider are not completely stalled. When back stick pressure is slightly released it reduces the angle of attack of the nearly stalled wings and merely flies up and away. Most people say it is bouncing, but in my opinion it is flying again.  And when a slightest gust of wind is present the up wind wing will rise.  Stick held hard back against the stop, and stick pointed into the wind, it will stick like glue.

Joe Kulbeth

[Tim Blofeld]

Hi Martin,

Love the Phoenix and love the idea of electric propulsion. How practical do you think it might be to retrofit a Rotax powered Phoenix with electric propulsion sometime in the future (like TBO)?

Thanks, and all the best with new developments

Tim

P.S. Apologies for hijacking an existing topic!