Mig29 V4M3e3 Hi alpha testing... :)

Hi everyone -

Had two of my favorite Russian park jets out at the field again today blasting through several more batteries.

Fortunately, it was almost dead calm, so I managed to get some sustained high alpha testing with my Mig29 V4M3e3... :)  Unfortunately, epic fail on my part as I didn't expect it to be calm so I forgot my camera at home to video any of the flying... :/ :(

As Stephan mentioned in his post with his excellent flying video, http://migsrus.blogspot.ca/2014/08/flight-highlights-of-mig-35-namc.html it is a bit more of a challenge turning right with this plane in high alpha than it is left.  This is true for most planes in my experience, but this one seems to be a bit more of a challenge... :/

So, I did a little research to understand why this is common to all planes in an effort to figure out how we might go about alleviating some of the challenge of turning right in high alpha with this plane.  Before I go any further, I must caveat by saying that I am certainly not an aeronautical engineer, so how I am interpreting things primarily just comes from my own logic, from considerable park jet flying experience and closely observing how my airplanes respond and behave in given situations.

I found an excellent website that explains a few contributing factors to why most planes are a bit more challenging to turn right in high alpha quite simply, or at least simply enough for me to understand it... :) http://wiki.flightgear.org/Understanding_Propeller_Torque_and_P-Factor  Although these theories are primarily for front mounted or tractor prop type planes, much of it still applies I think to our mid mounted "prop in slot" park jets and in fact might be exaggerated a bit due to the stronger prop wash in close proximity to the empennage (tail assembly of rudders and elevons/elevators).

From what I have read from the link above, there are probably three main things going on that cause making it more difficult to turn right in high alpha with our park jets.

• Prop wash - "A propeller pushes air not just horizontally to the back, but more in a twisting helix around the fuselage (clockwise as seen from the cockpit). As the air whirls around the fuselage it pushes against the left side of the vertical tail (assuming it is located above the propeller's axis), causing the plane to yaw to the left. The prop wash effect is at its greatest when the airflow is flowing more around the fuselage than along it, i.e., at high power and low airspeed, which is the situation when starting the takeoff run."  This is quoted directly from the link above.  Although in our situation we don't have a fuselage in the way of the prop wash, our prop wash when it hits the rudders is still fairly strong as it has not had much distance in which to dissipate.  Also, perhaps high alpha exaggerates this behavior since it is a low airspeed situation, not necessarily high power, but certainly low airspeed;
• Propeller torque effect - "Torque effect is the influence of engine torque on aircraft movement and control. It is generally exhibited as a left turning tendency in piston single engine propeller driven aircraft.  According to Newton's law, "for every action there is an equal and opposite reaction," such that the propeller, if turning clockwise (when viewed from the cockpit), imparts a tendency for the aircraft to rotate counterclockwise. Since most single engine aircraft have propellers rotating clockwise, they rotate to the left, pushing the left wing down.  Typically, the pilot is expected to counter this force through the control inputs. To counter the aircraft roll left, the pilot applies right aileron.  It is important to understand that torque is a movement about the roll axis. Aileron controls roll. Prop torque is not countered by moving the rudder or by setting rudder trim. It is countered by moving or trimming the aileron.  This correction induces adverse yaw, which is corrected by moving or trimming the rudder (right rudder)."  Although when we turn left in high alpha, we don't normally want the plane to roll, having this added benefit does seem to make it easier to turn left and probably contributes to the challenge at times of turning right.; and
• P Factor - 

"P-factor is the term for asymmetric propeller loading, that causes the airplane to yaw to the left when at high angles of attack.

• Assuming a clockwise rotating propeller it is caused by the descending right side of the propeller (as seen from the rear) having a higher angle of attack relative to the oncoming air, and thus generating a higher air flow and thrust than the ascending blade on the left side, which at the other hand will generate less airflow and thrust. This will move the propeller's aerodynamic centre to the right of the planes centreline, thus inducing an increasing yaw moment to the left with increasing angle of attack or increasing power. With increasing airspeed and decreasing angle of attack less right rudder will be required to maintain coordinated flight.
  This occurs only when the propeller is not meeting the oncoming airflow head-on, for example when an aircraft is moving down the runway at a nose-high attitude (in essence at high angle of attack), as is the case with tail-draggers. Aircraft with tricycle landing gear maintain a level attitude on the takeoff roll run, so there is little P-factor during takeoff roll until lift off.
  When having a negative angle of attack the yaw moment will instead be to the right and and left rudder will be required to maintain coordinated flight. However negative angles of attack is rarely encountered in normal flight. In all cases, though, the effect is weaker than prop wash."  Again, this information is considering a normal tractor type airplane, but I had never considered that the prop itself might not be generating even thrust with both blades until I read this part of the article.

So, if you are still awake after reading this rather technical data, here are a few things I thought about after reading this that I have observed at the field over time when practicing high alpha with my RCP F18 V3 with which I have logged many high alpha flights

In an effort to keep this plane light, I did not reinforce the vertical stabs, so I often get a lot of "vertical stab flutter" on my right vertical stab where the left one stays static.  This is in direct relation to the prop wash point above I think, showing that the right stab gets more prop wash and since the prop wash is going that way, when trying to turn right, perhaps there is less force on the left rudder surface. 

Also, I notice that when in sustained high alpha that the prop sound is different, the power setting stays fairly constant, but the prop does sound different, perhaps this is caused by the P factor and the imbalance in thrust between the two blades?  With this F18 V3, I normally had to keep a tiny bit of right rudder input in at all times when doing high alpha or the plane would naturally want to drift left to the point where I could do almost a complete wide 360 turn if I left the rudder alone and just kept the plane stable in the pitch and roll axes in high alpha.

So since these factors above are always going to be present in any park jet, I started to wonder if there was something in how I redesigned the vertical stabs and rudders on the Mig29 V4M3e3 that may be causing it to be a bit more of a challenge than other planes to turn right in high alpha.  

So while at the field yesterday, I started to think about several things and decided to see what was happening.  I don't want anyone to get the idea that this plane is impossible to turn right in high alpha, but it definitely requires more lead time, more steady rudder input to turn right than to turn left.  Is this manageable?  Of course it is.  And considering that the way the rudders and vert stabs work so amazingly well in other parts of the flight envelope is it really a matter of great concern?  Well, no probably not, but one of our cornerstones at NAMC is to constantly pursue that "perfect park jet", especially where our favorite park jet the Mig29 is concerned.  While we understand this might not ever be achievable, when there are other options to explore, why not explore them?

So I wanted to see if the plane drifted left like my RCP F18 V3, so I set it up to fly high alpha and then just took my thumb off the rudder.  It in fact stayed very straight and tracked true, only wobbling if a bit of breeze hit it.  So, no bad tendency to drift left on it's own, but still a bit more challenging to get it to turn right.  Now, comparing the F18 V3 to the Mig29 V4M3e3 is a bit of "apples and oranges" as the Mig29 vert stabs are perpendicular to the wing plate, while the F18's are angled outwards, perhaps this causes more left drift on the F18, but the F18 is also a bit easier to turn right in high alpha.

With my plane in particular with the CofG about 3/4" ahead of stock, I can almost pull the nose up to about 70 degrees for short periods of time (say maybe about 5 seconds or so).  I am not trying to hover it by any means, I think a "hovering parkjet" is a bit of a stretch of scale performance (sorry RC Powers... :/), but that is just my humble opinion.  What I did notice though was as I got the nose up that high, my ground speed came down to almost zero, putting into basically a "power stall".  This in fact then did really funky things with the rudder control... :/  Not only was turning right then almost impossible, in fact I got reverse yaw several times where putting the rudder hard over to the right actually caused the nose to go left...yikes!  Fortunately I was high enough to recover or that could have been costly... :/

So I started to wonder, did something I did in the design of the rudders/vert stabs help exaggerate the problems with prop wash, prop torque and P factor?  As Stephan had mentioned in one of his build/walkaround videos, I had moved the leading edge of the vertical stab forward when redesigning the vert stabs/rudders.  Unfortunately, this was not based on any kind of "aerodynamic epiphany", rather pure aesthetics so that the top of the vertical stab didn't look too narrow or "pointy"... :/

So I got out some of the most valuable tools of an RC park jet test pilot's arsenal, a copy of the plans, a pencil, eraser, ruler and protractor and got to work looking at what was going on and drawing up possible alternatives. 

I drew up this diagram of the stock vert stab/rudders with the Mig 29 V4M3e3 vert stab/rudders drawn over them highlighted in yellow as well as maybe some other options highlighted in orange and blue.  I tried to highlight the original leading edge of the stock vert stab in green, but it doesn't show up too well... :(  However, hopefully you can see that there is quite a bit more surface area forward of stock with the adjustment in the angle of the leading edge..  So I started to wonder...does this extra surface now block some of the prop wash to the right hand rudder when trying to turn right?  If the left hand rudder is already less effective because of the prop wash properties, maybe I have made matters worse by physically reducing the amount of air that flows to the right rudder when trying to turn right in high alpha... :/

So I started thinking about another plane I have been flying a lot lately that has very effective rudders, the FRC Su35 MK2.  The rudders were one area that I did not modify when I built this plane.  And since I already had a copy of the plans on my work table as it is a plane I am building right now, I did have a bit of an "aerodynamic epiphany"... :)  Although I have said that the rudder hinge and trailing edge of the rudder on the Su35 are vertical, you can see in the picture below they are not purely vertical, they in fact tilt back about 3 degrees which is much less than the stock Mig29 which tilt back about 13 degrees.  Now, before I go too far down this road, I must admit that I have not been able to fully assess the Su35 rudder design in high alpha as for me, the Su35 MK2 is not a good high alpha plane.  I find that as soon as I get the nose up to about 25-30 degrees, wing rock comes on quickly and violently, that has been my experience with all three Su35s I have built (two stock and one modified), but again, just my experience.

So in the plan diagram (I just pasted it in below again to make it easier to follow), I drew a line that angles back 3 degrees from our V4M3e3 rudder, shown in orange.  Then keeping the same dimensions at the top of the vert stab, this allows the leading edge to be angled back considerably, almost to stock (again this is shown in orange).  Although it is angled slightly, it still gives an almost vertical appearance on the back edge, allowing me to keep the Mig29M/Mig35 profile that I really like and the still relative clean back end that reduces drag and turbulence over the much more angled vert stabs/rudders of the stock Mig29 V4.

I also drew in the 4" by 2" rudder that we have been using on the Mig29 V4M3e3, again shown in orange.  Then for another option, I drew in the rudder styled exactly after the Su35 style rudder, the hinge line shown in blue.  Although this option makes for a narrower rudder in relation to the V4M3e3 rudder, it ends up being approximately 8.5 sq inches, where the V4M3e3 rudder is 8 sq inches.

So here are perhaps a few options I am considering for my next Mig29 V4M3e3/Mig35 NAMC.  I don't plan on cutting up my current plane as I think it is due for a motor upgrade to the NTM Prop Drive 2700 for some kick ass speed where high alpha performance is not as important as just "haulin' the mail" and doing everything else at greater speed and higher wing loading... :)  Besides, if I modify the rudders and vert stabs on my next Mig, I want this to compare it to.

• Option 1 - leave everything as is.  This probably won't happen as I won't be able to test and evaluate a different setup if I did... :/ ;
• Option 2 - angle the V4M3e3/Mig35 back 3 degrees and keep the 4" by 2" inboard rudder.  This still gives pretty much vertical rudder hinge and trailing edge, but allows the leading edge to be pulled back closer to stock and hopefully prevent as much prop wash being blocked from the right rudder as I'm trying to turn right.  This rudder design gives great crisp, clean rudder input without the adverse behavior of roll like the angled stock V4 rudder;
• Option 3 - again angle back 3 degrees, but use the style of rudder that the Su35 uses, taking the hinge all the way to the top and not having an inboard rudder.  This reduces the chord or depth of the rudder surface from the V4M3e3, but increases total rudder surface by 0.5 sq inches and perhaps extends the effectiveness of the rudder in the prop wash as it expands after leaving the prop.  This design works very well on the Su35 as it provides very crisp, clean rudder input without adverse roll behavior.  As mentioned before though, I have yet to be able to assess this rudder design in high alpha with my Su35 MK2 in any configuration.

So, in the spirit of testing and evaluation, it is going to be either option 2 or 3.  As with any change or modification, there is always a risk of adversely affecting other performance qualities of the plane, but if we don't try, we won't know... :)

The Mig29 V4m3e3/Mig35 NAMC is still the best park jet I have ever flown, and even if neither Option 2 or 3 changes it's right turn performance in high alpha without adversely affecting it's other performance qualities, I will be very happy with it.  As I said before, this is a small part of the whole flight envelope and it can be dealt with using the right anticipation and finesse in piloting.  But then again, one never knows what might happen with that next little tweak...I think the risk is worth taking... :)

Cheers,

Scott