Feel the "Need for Speed"?!

Hi everyone -

Yes, I know, another corny cliche from Top Gun, but hey, we probably all feel the "need for speed" once in awhile if we are building park "jets"... :)  Before I go any further, I wanted to recognize Frederic (fmt2bx from the RC Powers forum) who contacted me via PM and got me motivated to write this post. Thanks Frederic  :)

So there are lots of ways to get a plane to go fast.  I know of folks who have built hybrid or concept planes that have been up in the area of 120+ mph by playing with different size props, etc.  Or other folks who have taken planes and put huge motors, props, ESCs and batteries which required significant modifications to the plane to achieve some crazy speed.

However, I think Stephan and I tend to be more "old school" where we like to take a stock plane and tweak and modify it to get it to go faster with essentially a stock setup.  I think that is kinda how NASCAR got started, and I would say they are pretty popular and successful... :)

I should also caveat that speeds that I mention in this post are collected using an app on my phone called HK Datalogger (used to be called RC Speedo available for Android and probably iPhone as well https://play.google.com/store/apps/details?id=com.neon.rcspeedo&hl=en )  I am sure there are inaccuracies in how this program collects and calculates the speeds, but it is a bench mark and fun to see.  There are other methods, I know folks at my field have telemetry suites in their planes and transmitters that will give speed based on GPS, but for our use, that adds more weight to the plane which if you want more speed is not a good thing (more on that later).

So much of what I am going to discuss is based on countless hours of thought, research, building and tweaking backed up by hundreds of flights at the field which is the ultimate testing ground.  Neither Stephan nor I have wind tunnels to show scientific proof of our findings, but we fly and test our planes repeatedly before we decide that modifications and changes contribute to greater speed and/or new behaviors in our planes.  We are always completely honest in our assessments, even when things don't work out as planned, I think it is the only way to keep the integrity of what we are doing.

I should also say that while this post might be "exhausting" :/ to read, it is not "exhaustive" in it's information about how to make a plane go fast, these are just lessons learned from mine and Stephan's personal experiences.  The added bonus is that many of the philosophies and techniques that we have tested make your plane fly smoother and more efficiently through the air which are good things whether you want a "rocket ship" plane or not... :)

Anyway, enough of all the caveats and disclaimers, on to the fun stuff! :)

I am pasting in this link and this video that I made recently as you can find many of the links to different components I talk about in the following paragraphs.  Also, in the video, I did a very detailed section on my thoughts and experiences with a couple different pretty fast power setups.  The power system discussion in the video starts at 1:15:29 if you want to fast forward to that.  http://migsrus.blogspot.ca/2014/10/my-park-jet-workshop-video.html

Plane selection

Selecting a plane if you are seeking some real speed is very important.  Any plane can be made to go fast, but certain planes just take speed to a whole other level if you want that "show off", "impress your friends" kinda speed.  Stephan and I have had the most success with the RC Powers Mig29 V3 and V4 when seeking out some serious speed, even in their stock (built exactly to plan) configuration.  I won't go too heavily into modifications in this post, but will touch on them briefly in the build portion of this discussion.  So for sake of simplicity, I will discuss mostly stock builds but with a few important tweaks that lend themselves to better speed and overall performance.

If the Mig is not necessarily to your liking, here are some key things to look for in a plane to go really fast;  drag, weight, strength and stability.

Drag

Is the model you are going to build naturally sleek?  Models like the F18 and F35 tend to have a lot of drag due to either wing shape in the F18 or just bulky, draggy profile like the F35.  However, planes like the Mig29, Su35 and F15 tend to be sleeker and with their swept back wings more conducive to higher speeds.

Weight

This needs to be looked at from two viewpoints.  Is the plane complex and heavy in it's construction?  If so, you are right away fighting one of speed's worst enemies, too much weight.

Can the plane handle the weight and wing loading inherent in big, heavy power setups?  The F18 for example again has a long straight wing and is very sensitive to weight, wing loading and g forces.  I found this out the hard way with my first F18 V3 where too much weight and too much power caused me to snap my wing and that is with 4mm carbon rod for reinforcement.  Within reason, the Mig29 and F15 can handle much higher wing loading without too much problem.

Strength

This goes hand in hand with weight, certain airframes and certain wing shapes are stronger or can be reinforced better than others, so it is something to consider.

Stability

Is the plane going to just bury the nose and go straight ahead when you hammer the throttle, or twitch and wobble all over the sky?  Any instability or wasted movement in a plane like excess torque roll, zooming or diving at high speed or tendency for the tail to wag or oscillate will all rob you of speed. Planes with vertical stabilizers that are perpendicular to the wing plate like the Mig29, Su35 and F15 are far more stable at speed and in the wind than airplanes with angled vertical stabilizers like the F18, F22 and F35.

Again, because of the Mig's strong performance in all these categories coupled with the fact that it is also the most stable and versatile over a wide speed regime, it is probably the best choice for a fast, stock plane in our humble opinion.

Power system selection

I will break down each part of the power system (prop, motor, ESC and battery) briefly here, there is far more detail in the video including how I like to balance my props and setting throttle points for your ESC, so I encourage you to watch that.  The four components of the power system need to be matched properly to ensure you are getting maximum effectiveness out of the system without pushing any one component of the system too hard.  Other than the prop, the prop doesn't really care... :)

In this section I will discuss the FP (Focal Price)/DX (Deal Extreme) 2700 and NTM 2700 motors as discussed in the video as they are our two favorite power systems and components are readily available around the world.  I also wrote this article awhile back discussing thrust to weight ratio and motor/prop selection for some more light reading... ;) http://migsrus.blogspot.ca/2014/07/some-thoughts-about-thrust-to-weight.html

Prop

Any power system starts with the prop.  It determines how much air gets moved, and how hard your motor and the rest of the power system has to work to spin it.  Ensuring your prop is in good shape, balanced, no nicks or damaged is very important to squeezing the most effectiveness out of the prop and power system.  Additionally, an unbalanced or damaged prop can cause vibration which can lead to structural damage of your plane.  At about 1:21:00 in the video above, I relate a story of mistakenly putting an unbalanced prop in my plane and the resulting damage... :/

What I would say is that taking the time to balance your props pays big dividends in how smoothly and efficiently they run which in my experience relates to more speed.  Starting at about 1:21:40 in the video above I demonstrate the method I like to use to balance my props.

I essentially only run the 6x3 EMP or 6x4 APC props on the two motors that I use, each motor and prop combo having their own function.  What I would say if you want to experiment with a prop/motor combo that is not common is to first see what the motor manufacturer says and/or test your power setup with a watt meter before taking it flying.  I have a discussion on watt meters starting at 1:55:00 in the video.  A watt meter is like a dynamometer of sorts for your power system in that it gives you good feedback on amps being drawn, watts being produced, etc to know whether the components of your power system are matched well.

Motor

I already mentioned the FP/DX 2700 and NTM 2700 motors as being our two favorite motors.  I have the FP/DX motor in most of my planes as it gives pretty amazing top speed (over 80 mph in my Mig29s and Su35) and it is fairly light weight (about 3 oz lighter than the NTM 2700 setup).  Lots of motor and prop combos are available that can possibly produce the same thrust and speed as these two, but for the cost and ease of setup, it is tough to beat these two motors in my experience and opinion.

One key number I want to emphasize and that is the 2700.  The FP/DX and NTM motors are 2700 Kv, meaning with a three cell battery, 2700 x 11.1 = 29970 RPM.  Most of the other popular park jet motors (the D2826/6 from HK for example) are 2200 Kv motors, so they will theoretically produce much less RPM 2200 x 11.1 = 24420 RPM.  It is tough to beat that much extra RPM matched with the right prop if you are really seeking out some serious speed.

It is important to know the limits of the motor either from what you know from the manufacturer or your own or other people's testing before pushing it to or beyond it's limitations on a regular basis with either too much prop or too many volts (4 cell or higher battery) or you will quickly burn out your motor.

For true blinding speed, it is tough to beat the NTM 2700 motor on 4S once in awhile.  It is plenty fast on 3S, regularly pushing planes up and over 90 mph, but 4S is like having afterburners for occasional dragster type speed.  Both Stephan and I got our modified Mig29 V3s to 109 mph... :)  There is nothing quite like watching some foam and glue you have put together yourself go by you that fast...truly exhilarating... :)

ESC

Probably the most important thing about matching your ESC to your power system is that it can safely and effectively provide the amps that your motor/prop combo needs.  For example, if your motor pulls 36A at full throttle like the FP/DX motor does, you don't want to use a 30A ESC.  It will provide the power for a short period of time, but eventually your ESC will burn out.  Better to have some overhead like a 40A ESC so that everything runs efficiently and safely.  Now, "overhead" does not mean "overkill".  No need to go overboard and use a 60A ESC, yes it will work fine, but you are adding excess weight and taking up more room on your plane for no reason.

99.9% of motors run best on low timing setting on the ESC which is luckily what 99.9% of ESCs come set at when they leave the factory.  So I would just leave the timing and other settings as is.  I discuss more of this in the video starting at about 1:41:30.

One thing you will always want to do is ensure that you set your "throttle points" to ensure your ESC realizes where 0 and 100% throttle is.  Sometimes an ESC may think that even though your throttle stick is at 100%, it will only give you 80% for example unless you tell it where 0 and 100% is.  I do a quick demo on how to do this starting at 1:44:10 of the video.

Battery

It is very important to ensure that your battery can provide the amps that your motor will need to work at best power and efficiency.  I discuss this in more detail in the video starting at 1:48:15.

I would also use a battery that is in pretty good condition that already has a few cycles on it to ensure it is performing at peak performance.  Brand new batteries or batteries that are getting old and a bit puffy may not provide long, strong bursts of power that you will want for some good speed runs, so use those for some slower cruising.  Again, this is where having "overhead" on the amps without "overkill" is important.  No need to have a 60C discharge battery when 30C will do, the higher discharge rating again normally means a bigger, heavier battery and more weight which you don't want or need.

Build process

How you approach, conduct and finish your build is probably the most important area where you can set yourself up to have the fastest plane possible if you are seeking good speed.  It is also the simplest and cheapest way to upgrade your plane without changing power systems, etc, it simply requires some thought, some of your time and patience, but the dividends are well worth it from my experience.

If you are going to build a serious speed plane using a motor like the NTM 2700, then a bit of a commitment to that is required right from the outset.  The FP/DX 2700 motor combo does not require quite the commitment of planning as you can pretty much build the plane per stock directions and you will be OK.

Here again is where you have to factor in drag, weight, strength and stability and what sort of compromises you might need to make.  Adding strength often adds weight and sometimes adding stability might cause both weight and drag increase, so it is a compromise to get best performance.

If you are going to use the NTM Prop Drive motor on 3S or potentially 4S where speeds can be approaching or exceeding 100 mph, it is important to ensure the plane is reinforced or you will literally tear it apart.  Oscillations caused by weak areas of the plane will also detract from your speed.  In the following pictures, you can see the carbon reinforcements I am using in a modified Mig 29 V4 I am currently building where eventually I will be using the NTM 2700 motor.  It is "necessary weight" to make sure the plane performs at it's peak.

With a lighter setup like the FP/DX motor, you don't need the reinforcement in the back plate or the vertical stabilizers, but experience has shown I would still put the reinforcement along the leading edge of the wing and elevon to handle the g forces of extra speed.

KF airfoils

When Stephan and I first joined forces in the NAMC, Stephan was doing some very exhaustive testing on different thicknesses and configurations of KF airfoils on his KNex (Knife Edge experimental) Mig.  He also did considerable research trying to find data on what the ideal percentage of chord should be used on swept wing planes like the Mig.  Unfortunately, conclusive data was not available for KF airfoils and swept wings, but through "seat of the pants", consistent testing, we found that about 40% of wing chord is best for speed and stability when using KFs on the Mig29 (for simplicity, this means adding about 1/4" onto the trailing edge of the KF templates included in the plans).  Also, extending the top KF forward along the LERX (leading edge root extension) added to the smoothness and stability of the plane.

We both like to use paperless dollar store foam for our KFs.  We find this gives us the right thickness (approx 4.5 mm) for the airfoil.  We also both prefer KF4 for best speed and stability, so we end up with 4.5 mm thick KFs top and bottom.  I know others have experimented with thinner on the bottom and thicker on the top, but from our experience this can cause imbalance in lift and we are looking for rock solid stability, especially as speed rises.

One of the most crucial things to ensure when using KF4 airfoils is that the trailing edges match exactly.  If one is shorter or longer than the other, it can cause significant imbalance in lift and cause the plane to zoom (climb on it's own) or dive as speed increases.

Although the angle of the camera may make these KFs look like they are not symmetrical in where the trailing edge is, they are.

Build in general

So once you have committed to the build, regardless of what motor you decide to use, it is always best to build as light as possible without sacrificing strength.  I know this sounds easy and I can't give you a lot of guidance, but using lighter glues like Foam Tac can save you a lot of weight over the build of a plane, probably 1-1.5 oz in my experience.  It takes a couple builds to figure out what is enough and what is too much, that is about the best advice I can give you :/

As a former card carrying member of  "overbuilder's anonymous", I can tell you it is tough to avoid the temptation sometimes of "well, a little extra dab of glue won't hurt", but please do!  All those extra dabs add up to grams, then oz and before you know it, your plane is close to 900 grams and flies like a brick!  Well, maybe that is extreme, but it can happen... :/

It is also important to use the servos and pushrods that do the job without adding extra weight.  Another couple of articles of light reading follow

Servos - http://migsrus.blogspot.ca/2014/07/metal-gear-servos-on-park-jets.html

Pushrods - http://migsrus.blogspot.ca/2014/07/thoughts-on-scratch-built-pushrods.html

Also more on these subjects in the video starting at 49:31.

Finish and Paint

I did a section on this in the video starting at 1:58:20.  I like to consider a good sanding job on my plane as "free speed".  Since Stephan convinced me to use symmetrically shaped leading and trailing edges on my wings, horizontal stabilizers/elevons and vertical stabilizer/rudders, I have gained on average about 8 mph just from this simple change.  If you look at the picture of the green and brown Mig above again, you can hopefully see the symmetrical shape to the leading edge of the wing.

All it takes is some some time, patience and a sheet or two of sandpaper and without any other changes, the speed and efficiency of your plane is dramatically improved.  Also as I mention in the video, a good overall sanding job not only makes your plane look better, but saves on average about a half oz of weight.  And even if you don't want your plane to go faster, having it go through the air more efficiently and more slippery just makes sense regardless of your plane's speed.

Paint is something again that you need to be careful with as a heavy paint job can very quickly add a lot of unwanted weight.  I was getting a bit "slap happy" with my paint recently and found paint jobs were adding almost 40 grams... :/  So I experimented with thinning the paint as I described in the video and managed to reduce this down by 10 grams.  If you want a "paint scheme" that adds pretty much zero weight for a really fast plane, magic marker works really well also.

Modifications

As mentioned, I won't go too deeply into the modifications that Stephan and I have tested.  Primarily they are concerned with smaller, more efficient control surfaces, smaller elevons and reduced back plates, all which help reduce drag.  As Dave Powers mentioned in his promo for the Mig29 V3 "drag can't be caused by things that aren't there", so reducing the span of our elevons and the size of the ailerons and elevons has helped our planes be faster and more efficient and precise in their handling which is important at higher speeds.  Also, by having smaller surfaces, they cause less turbulence and less drag when they are deflected than large surfaces.  Anyway, you can read through our build logs and post flight reports for more information on these many modifications we have experimented with.

Dialing in for speed

So now your plane is complete and time to dial it in.  Just like a race car, the final tuning still has to be done.  A little more light reading on some of my thoughts and processes on dialing in a plane http://migsrus.blogspot.ca/2014/07/some-thoughts-about-dialing-in-plane.html

To get maximum speed out of a plane, it is important to take the time to balance and trim it as close to zero as possible to prevent drag and/or adverse behavior as your plane gets moving really fast.  Also when pushing your plane to really top speeds like close to 100 mph, it is critical to ensure hinges and linkages (especially the elevons) are solid with as little slop as possible.  I have had one plane with elevon flutter where the hinge started to come loose, fortunately I chopped the throttle and landed in time, but any longer and the elevon would have ripped itself off.

It is also important to check your speed plane over on a more regular basis for cracks in the airframe, motor mount, loose hinges, linkages, etc as the increased stress can be hard on the plane and at top speed all these little weaknesses can be highly amplified.

So if you are still with me, I hope this has been helpful if you are looking to build a really fast plane from scratch or are just looking for a way to make your current plane a bit faster and more efficient without much outlay of money.  Again, there are many other philosophies and setups to make your plane go exceptionally fast, but Stephan and I agree the setups we use are pretty simple and easy to get working well without a huge outlay of money and crazy prop/motor/ESC and battery combos.

The FP/DX setup is fun and exciting and if you don't have a large area to fly in, a good choice for inexpensive speed, pop, acceleration and fun.

The NTM setup especially on 4S needs a bit more room and probably a bit of flying experience before taking on that much speed and power, but if you have the room and the courage, the speed is pretty breath taking... :).

Hopefully we have given you some things to think about, it just takes a little planning, proper plane and power system selection, focus on detail when building and finishing and you can very easily turn your park jet into a real speedster... :)

If you have questions or comments, please leave them below or if you don't have a Google account, please feel free to e-mail us at our new e-mail address scott@migsrus.com.

Have fun and good luck satisfying your "need for speed"... :)

Cheers,

Scott