Some thoughts about "thrust to weight" ratio and motor/prop choice

Hi everyone -

Awhile back I wrote a post about my favorite all round weight for the V3 and now V4 size planes as well as the FRC Su35/37.  You can read more in that link here http://migsrus.blogspot.ca/2014/02/rc-powers-v3-size-planes-and-weight.html

Essentially what I found for my style and preference of flying was that between 20-21.5 oz in AUW (including battery, in this case a 2200 3S) was where I liked to be for these planes when I use the Focal Price 2700 motor.  As I mention in the article, this gives me a thrust to weight ratio of about 1.7-1.8 to 1 which still gives me good speed, acceleration and punch out power for best overall performance in matching my flying style and skill level.

So as I was flying my Mig29 V3M3e3 the other day comparing it to my Mig29 V4M3e3, I was trying to think of something they might have in common and it turns out to be the thrust to weight ratio.  The NTM Prop Drive 2700 with a 6x4 APC prop on 3S produces about 42 oz of thrust.  With a 2200 battery, my V3M3e3 weighs 25 oz, so a thrust to weight ratio of 1.7:1.  One thing I have learned over almost two years and 40+ park jets is that thrust to weight ratio is an important component of how I build and fly my planes.  Fortunately for me after testing multiple motor and after literally thousands of flights over 2 yrs, my two favorite park jet motors are the Focal Price 2700 for lighter planes and the NTM Prop Drive 2700 for heavier and very fast planes.

So here are some interesting things I started analyzing.

1. While thrust to weight ratio has a lot to do with top speed, the prop used has a big impact on initial acceleration and top speed characteristics.
2. When I speed tested my Mig29 V4M3e3 the other day with the FP 2700 motor and 6x3 EMP prop, my weight was about 20.5 oz, so a thrust to weight of about 1.75:1, top speed of 88 mph.
3. When I speed tested my Mig29 V3M3e3 with the NTM Prop Drive and 6x4 APC prop, my thrust to weight ratio was 1.7:1, top speed of 94 mph.
4. Both setups gave me the overall speed and acceleration (give or take depending on the motor/prop setup) that makes for exciting flying and the scale performance and aerobatics that I enjoy, so the common thrust to weight ratio of about 1.7:1 is an important number to me for how I build to enjoy the performance I want when I fly.

I had found last summer/fall when I tested 8 different motors on the bench and at the field in my RCP F18 V3 along with different props 6x3/6x4 EMP and 6x4 APC that the 6x3 EMP prop was excellent for initial acceleration and punch out power as it spooled up to max RPM quicker, but for true top end speed, the 6x4 APC didn't spool up as fast, but delivered best top end speed.

Why I bring this up is that for our next NAMC modification of the RCP Mig29 V4, Stephan and I are going to try and build a hybrid plane that has enough strength to handle the punishment and weight of the NTM 2700 power train without making it too heavy so that we can still effectively use the FP 2700 as well.  Basically one air frame with two different power train options.  It will be an interesting challenge, but one that I'm sure we are up to... :)

Essentially once the air frame is completed, the difference in power trains will be as follows based on weights I took last year when I did all the motor testing.  Your weights may vary depending on what ESC and/or battery you use.

Focal Price motor setup

• FP 2700 motor with 6x3 EMP prop - 65 gr/2.3 oz + Turnigy Plush 40A ESC - 41 gr/1.5 oz + Turnigy 1600 Mah 3S 30-40C discharge battery - 147 gr/5.2 oz - total weight of 253 gr/9oz.  If we use the 2200 Mah battery which I like to use depending on airframe weight, it brings the weight up to 303 gr/10.7 oz

NTM 2700 motor setup

• NTM 2700 motor with 6x4 APC prop - 99 gr/3.5 oz + Turnigy Plush 60A ESC - 70gr/2.5 oz + Zippy Flightmax 2200 Mah 3S 40C discharge battery - 197gr/6.9 oz - total weight of 366 gr/12.9 oz.

So here is where things get kind of interesting...

With the 1600 Mah battery option for the Focal Price motor, the power train alone produces 4 oz of thrust for every ounce that the power train weighs.

The NTM 2700 setup produces 3.3 oz of thrust for every ounce that the power train weighs.

More than likely, oz for oz, the NTM setup will produce a greater top end speed because of the prop, but the Focal Price setup will have better initial pop and acceleration for some quick agile moves and aerobatics because of it's prop.  Both these setups I find give me about 5 minutes of flight endurance.  Of course, using 4S with the NTM 2700 for some blinding speed is a lot of fun too for those of you with a bit more experience, this gave me a thrust to weight ratio of 2.2:1 and a top speed of 109 mph... :)

Unfortunately, the Focal Price 2700 runs way too hot when trying to use a 6x4 APC prop and trying to use a 6x3 EMP prop on the NTM Prop Drive wastes horsepower, so it is not really an option to swap out props with these two motors as a way to have different thrust to weight ratios.

So you might say, well that is all good for you as you have a couple years experience and like to fly a little faster (probably 90% of my flying is at 50% throttle or higher) and more scale than some folks do, what about someone just starting off or someone who doesn't like to fly as fast or expect the performance that you do?

Interesting question and while I am well on my way to another good "ramble fest", why not look at some different numbers dependent on what you might want your airplane to do or what it is capable of doing dependent on your build and flying experience.  As beginners, we probably all build a little heavier than we should, but that is fine, all part of the experience.  I have learned that after building 40+ park jets, that light but strong is the best setup.  It takes some time and experience to know where to spend the air frame weight to gain the strength and where to save it which is why scratch building is so much fun and rewarding, each new build is a learning experience and a chance to experiment.

So, if I look back to all the testing I did with my RCP F18 V3, I ranged in weight from 17.3 oz to 21.5 oz depending on the motor/ESC/battery combo I tested.  I ranged from 1.2:1 to 1:8:1 in thrust to weight ratio.  With the F18 top end speed is not a real factor since at a certain point, drag becomes too much, but having that "hole shot" power is helpful for aerobatics and powering out of high alpha.

So at 1.2:1, the plane flew OK, would move along well, but had very limited vertical capability, maybe about 50 ft straight up in the vertical before it just ran out of "oomph".  Also, punch out power was fairly limited, so I didn't have that emergency acceleration I might need to get out of trouble.  I was light at 17.3 oz with the Grayson Hobby MJV3, but very limited power range and got bounced around quite a bit in the wind.

Since I have already discussed the higher end thrust to weight ratio and what power setups I use to achieve this, let's look at the mid range where a lot of folks might be if using the recommended power setups from RC Powers.

Hobby King Turnigy D2826/6 motor setup

• D2826/6 motor with 6x4 APC prop - 67 gr/2.35 oz + Turnigy Plush 30A ESC - 34 gr/1.2 oz + 1600 Mah 3S battery - 147 gr/5.2 oz - total weight - 248 gr/8.75 oz.  The thrust per ounce of power train with this setup is 3 oz of thrust per oz of power train.

In bench testing, I found that with a 6x4 APC prop, the best setup was with medium timing on my ESC, the thrust produced is 750 gr/26.5 oz.  On my F18 V3, this setup took me to 17.8 oz, so a thrust to weight ratio of 1.5:1.  For most folks starting off, this is a pretty light weight and if you are using hot glue, you might only get to this weight by using two servos and elevons only and keeping the paint application to a minimum (magic marker works great for a quick paint scheme and adds zero weight).  The plane flew very well with this motor, much better punch out, overall performance and speed than the MJV3 and would be an excellent overall starter package for a beginner or folks not really needing or wanting to fly too fast.  In effect, your plane could weigh up to 22 oz with this motor setup, which would give you a thrust to weight of 1.2:1 and about the same performance I describe getting with the MJV3 with my F18 V3.  Much heavier than that in all honesty and you are going to have to fly around at a pretty high throttle setting and won't have much power in reserve to get yourself out of trouble which is always a good thing, especially when starting off.

As a side note, when bench testing the D2826/6 motor and the 6x3 EMP prop, I got about 30 oz of thrust and much better pop and acceleration at the field when flying.  Top end speed difference was not noticeable, but again I was flying a "draggy" plane in the F18 V3.  Additionally, the motor ran much cooler and more efficiently with a 6x3 EMP prop.

So, to try and make sense of this all, "thrust to weight" ratio should be considered an important part of how a plane is going to perform and a factor that should be considered when building a plane.  It is always a trade off, normally the lighter the motor, the less power it produces, the heavier the motor the more power it produces.  Where the balancing act really becomes interesting is that with a heavier motor, normally a heavier ESC and battery are required and more reinforcement is required to handle the wing loading and punishment the big powerful motor will produce.  As mentioned earlier, greatest success and flexibility is achieved by keeping the build as light as possible (using light, strong glue like Foam Tac) while keeping it strong.  I would rather have a light plane that I can have the flexibility to add weight by putting in a heavier battery than have a heavy plane that will only fly and perform well with a light battery.  With bigger motors like the NTM, heavy batteries are pretty much a must as you need the capacity and C rating to feed that beast of a motor.

So you might say, well if I am going to build heavy and want extra power in reserve, why not just start off with the NTM Prop Drive?!  Well, I can tell you from experience this is really not a good choice for beginners as the extra torque, weight and power can get you into more trouble than it will get you out of.  The Turnigy D2826/6 package recommended by RCP is probably one of the best all round choices to start off with.  If you want to give yourself flexibility for starting off, get a 40A ESC for your D2826/6 and then for just about a $12 investment, you can upgrade to the Focal Price 2700, gain about 10 oz of thrust without having to change anything else...like flying a whole new plane with just an easy motor change and the increased speed and acceleration is big time fun... :)

If you are looking for some more help in choosing a motor/prop combo or want to have a better idea of what your current setup is giving you (as long as it is on my spreadsheet... :)), post #127 of this thread on the RC Powers site has the latest version of the spreadsheet I produced when I did all the bench testing of popular park jet motors last year http://www.rcpowers.com/community/threads/parkjet-motor-bench-testing.15685/page-7  I would encourage you to go have a look at this and download it, it can be very helpful in determining motor and prop choice dependent on what your airplane might weigh or what you think it might weigh at completion.

So hopefully this rambling has been helpful in demonstrating how thrust to weight ratio might be important in how you like to build and fly.  While many of the numbers I quoted and calculated are fact, how they performed at the field is primarily from my opinion and assessment of the plane/power setup and it's performance in the framework of how I like to fly.

Please don't hesitate to post questions or comments on this or any of our posts if you are looking for help with a build or just want to discuss something.  We don't have all the answers, but might be able to shed some light on another way of looking at things if you are looking to try a new power setup or are looking for some different performance characteristics from your plane(s).  And besides, we both just love planes are are more than happy to talk about them... :)

Cheers,

Scott