Evaluating a plane's performance - the Basics

Hi everyone -

In the pursuit of building and flying the "perfect park jet", Stephan and I are constantly assessing and evaluating the performance of our planes to see if there are certain characteristics that still need improvement or perhaps they are just behaviors that will always be there and we just need to adjust our flying accordingly.

As you can imagine, it takes some time to fully assess how an airplane is going to react in certain maneuvers and it takes time and repeated execution of that maneuver to honestly assess what a plane is doing.  Some of these things are evaluated hand in hand with the process of "dialing in" a plane that I described in a previous post http://migsrus.blogspot.ca/2014/07/some-thoughts-about-dialing-in-plane.html.  But then once the plane is "dialed in", more intermediate and advanced maneuvers can be practiced and assessed already knowing that for normal flying around the plane has been trimmed and balanced to the best of my ability.

Additional factors are also the limits of a person's flying skills, there are certain moves I am still working on, but currently don't have the skills to execute properly on a regular basis to be able to assess the plane's performance.  Additionally, I don't always repeat the same loop or same roll each and every time, pilot error, or slight differences in stick movement can make a big difference, so it takes some focus and discipline to try and repeat the same maneuver over and over the same way.  Weather and wind conditions are another big variable over which we have no control and have to factor in whether or not it is the plane that has bad behavior or if it is the wind or maybe a combination of both.  Often we find bad behaviors like the negative impact of Elevon Polar Pitch Effect (EPPE) caused by wind which has led to Stephan and I working hard to reduce elevon and tail plate size on the V4M3e3 to reduce this behavior.

Stephan and I both love flying foam park jets, there are few types of RC planes that can be such fun to build with inexpensive materials and components that give back such exciting performance.  We love the speed, agility and how we can push our planes and our skills for such little investment of time and money when compared to other types of planes in the RC realm.  I like to call it a very high "fun to dollar" ratio.

To put my flying style into perspective and I think Stephan is in the same ball park, let me try and describe how I like to fly as this has a large impact on the framework within I assess my planes.  For the past year I have really been focusing on trying to fly in a more scale fashion.  I have watched countless videos on Migs and other Jet fighters flying at airshows and have been trying my best to repeat turn rates, climb rates, loops, rolls, etc.  I find it a very challenging and satisfying way to fly as repeating a move that a real plane has executed is a lot of fun.  I'm not a fan of crazy rifle bullet rolls or flip loops, I did do this at one time, but found there was very little challenge to doing these as they just require slamming the sticks around without requiring much finesse on my part.

When I look at a plane, I am looking at it as a whole of it's parts, not just control surfaces, but weight, wing loading, thrust to weight ratio, acceleration, top speed, etc.  For the most part since I build with only two different power setups, the Focal Price 2700 and the NTM Prop Drive 2700, if the weight, thrust to weight ratio, acceleration and top speed are off, then either there is a problem with one or more components of the power train, or I have simply built too heavy.

So for this first installment, I would just like to discuss how I evaluate a few basic handling characteristics of a plane, ones which will be repeated over and over during the life of the plane, but are probably some of the most crucial.  Those are launch/takeoff, climbing, descending, turns, landing.

Launch/takeoff

Hand launching a plane is still one of the most unpredictable things for me about flying park jets.  It is that few seconds in every flight where I don't feel like I have any control of the plane right after it leaves my hand and starts to fly away.  What I am looking for here are any bad behaviors, too much torque roll (perhaps I am launching with too much throttle), sudden nose dive or nose pitch up, I find most of my planes will pitch down slightly, but that is normal until I get get a bit more speed and the plane stabilizes.  I normally launch all my planes with about 45-50% throttle, normally on the higher end if it is a bit windy.  Most planes that are fairly stable at slow speed like the Mig29 are not too much of a problem.  The RCP T50 V1 which does not like to fly at slow speed can be a real crap shoot on take off, so I really have to be careful with it.  I am looking primarily for the plane to at least stay airborne for a second or two after it leaves my hand until I can get both hands back on the transmitter and start flying it away.

Climb out

When a plane is climbing out after launch or at anytime during the flight, I am looking for it to respond smartly with no pitch up or rolling tendencies.  As I discussed in a previous thread, I find that 1.7:1 thrust to weight ratio is ideal for how I like to fly and expect my planes to perform http://migsrus.blogspot.ca/2014/07/some-thoughts-about-thrust-to-weight.html  I am also looking to make sure it tracks in direct response to the power and control inputs I give it and not drift one way or the other (unless it is due to wind).  If it does, either it needs some trim or something is wrong with how I built it.

Descent

When a plane is descending, especially with a bit of speed on, I want to ensure it responds well to the elevator input so that I can pull out in time, I'm also watching again that it doesn't drift or want to roll as it gathers speed in descent and that I am not seeing any wing flex as I pull out of the bottom of a descent or loop to ensure that my build has been strong enough to compensate for the wing loading and g forces I plan to put on my plane.

Turns

As I have mentioned in a previous post, when you think about the makeup of your average flight, the one maneuver you will do the most is turn.  If a plane misbehaves or is a handful in turns, it is more than likely going to be a handful in most other maneuvers as well.  Normally I will set my throttle at about 50% and fly several figure 8 patterns in front of me, alternating direction so that I am not always turning left to the left of me and right to the right of me (almost sounds like the song "Stuck in the middle with you"... :)).  By flying at about 50% throttle and inputting about 50% stick deflection in the turns, I am simulating how most of my turning at my flying field will happen as I re-position for my next aerobatic maneuver/high speed pass, etc.

In the turns I am looking for how the tail is handling, is it bobbling around, or following the rest of the plane like it is supposed to?  How does my plane hold it's energy?  Does the tail want to drop in the turn requiring rudder input?  Does the nose drop indicating that the plane is losing energy in the turn and needs more power to sustain an even turn?  How does it track?  Does it maintain even, uniform turn rate, or does it dig in it's heels and "bat turn", or wash out like a drift car?  Some of this may be due to the fact I don't have my throws and expo dialed in just right yet, so it is a good indicator that more adjustment is needed, or it could be something I need to adjust in how I fly the plane or adjust for the next time I build it.  If I am finding negative effects from too much surface area in the back, or Elevon Polar Pitch Effect, this can be compensated with a gentle acceleration through the turn, or as mentioned, adjusted through modification in the next build.

I am also looking to ensure it is not too sensitive in the roll part of a turn or the aileron input.  Since I fly my planes with aileron and elevon working together, they can get a bit more touchy in the roll, so I want to make sure I adjust this properly so I don't over bank when trying to do a simple turn.

Landing

Landing like take off can be an exciting phase of flight as the plane slows down and again comes in close proximity to terra firma... :/

Before I talk about what I am looking for on approach and landing, I should discuss the landing technique that I have developed (I won't say perfected, as I don't know if I will ever be there... :/) over thousands of park jet flights.  Most of my planes are in the 21 oz/595 gr region.  For my heavier 25 oz/710 gr planes with the NTM Prop Drive, everything stays the same, I just give myself more room to land as the heavier planes tend to glide further.  This is certainly not the only way to fly and approach and land safely, but one that works for me... :)

I like to do my base leg turn at about 50% throttle, that way as I am turning final, I have some decent speed and momentum.  When the plane is about eye level (or what I perceive to be about 6 ft off the ground), I chop the throttle to zero.  I always fly with the ESC brake "off", so I normally get a bit of free spooling which I find helps keep my plane more stable as it slows down.  I allow the plane to descend on it's own using rudder and or roll inputs to keep tracking as straight as possible down the runway.  I like to allow it to keep up some speed as it goes forward to help penetrate ground effect, that spot about 1-1.5 wingspan lengths from the ground, so in the case of these planes, about 2-3 ft off the ground.  Once I am about 1.5 - 2 ft off the ground, I will feed in very gentle elevator input to flare the plane and use some aerodynamic braking for a nice smooth landing.  Of course all bets are off if it is windy, this may require the power to be held on slightly to help with forward momentum or maneuvering, all a matter of practice and knowing how the plane will react.

 On approach to landing, I am looking at how the plane slows down, does it just stop dead when I chop the throttle or maintain decent momentum?  I am looking for it to maintain some momentum or I will have to keep power on through most of the landing phase to keep from "landing too high" i.e. landing two feet off the ground and then just thumping down... :(  Tip stalls can occur quite easily on approach to landing if a plane is susceptible to this problem.  In other words one wingtip will stop flying and drop (often quite quickly) and often the plane then ends up cartwheeling... :(  I'm also looking to see how a plane handles ground effect, sometimes if a plane is a bit "floaty", it will almost bounce off the ground effect layer meaning perhaps I need to keep a bit of power on to help it penetrate the ground effect.  Many of my worst landings have been caused by landing on top of the ground effect layer where I am basically at zero speed still a foot or two in the air and then the plane just drops in whatever fashion it wants to... :(.

So hopefully this explains the basic guidelines I look for when evaluating a plane and some of the basic maneuvers I use to be able to repeat the same maneuver numerous times to truly assess what my plane is doing.

Next installment of this series, I will look at some more intermediate maneuvers, rolls, stalls, loops, stall turns, slow and high speed flight to further evaluate a plane's performance.

Cheers,

Scott