For those that may remember I wrote about issues with my Sig Shoestring last year on take off rolling in at me a bit.  I learned to deal with it, but it was never really quite right.  This year I tried out a Sig Buster and it handles the take off much better.  At some point I will look at them together and figure out what the difference is.

But this is not what I wanted to know about this time around.  This year we were allowed to use LA 15s on the airplanes so that is what I put on the Buster.  The engine run seems pretty strong and pulled the airplane through the air without issue.  Initially I thought it was faster than the Fox 15 equipped Shoestring, but the watch says that they are about the same.  I looked over some of the scores for the Brodak Clown race (limited to the LA 15) and found that the fastest guy for long race was turning about 3.7 seconds a lap with all of the pits included.  Now this is pretty close to what I did with the Buster during a recent race.  So I was thinking, what are the limiting factors on airplane speed during a race?

I know that there is horsepower pulling everything along and then airplane drag and line drag slowing it down.  How do things like prop diameter and pitch affect our speed?  In my case, I was getting about a 3.2 second lap with a 7x5 APC on the LA/Buster.  I switched to a 7x6 APC and got about the same.  I had anticipated a marked improvement in speed, but it stayed about the same.  Are the lines or the airplane the limiting factor?

For what it is worth, for a mile run using the Shoestring/Fox 15 I managed about 56 seconds.  For the Buster/LA 15 I managed about 58 seconds.  From this I have developed a better respect for the Fox 15.  We are using 0.015" x 52' for our runs.  We also did a race on the same set of lines with pretty decent results.  Fairly fast, but slow enough that most people can keep up with the airplane.

Any input on what "wall" I am running up against on improving speed would be appreciated.
John

Hi John

"So I was thinking, what are the limiting factors on airplane speed during a race?"

Usually the pilot and the pitman:))

Seriously though, it could be one of many different variables. It all depends on how many of them are affecting the airspeed. The main culprits are, and this is in no particular order, Engine, leadout position, Prop size, fuel tank (positioning and venting), and to a certain amount, balance point. I left out the lines since everyone has to race on the same size lines anyway.

Since you already have a ball park figure for airspeed, optimizing your airplane/engine combination for maximum speed can be done with an afternoons flying, providing all the previously mentioned variables are close to optimum or can be changed easily.

First up is the balance point. For racing, around 12-15% is ideal, it gives enough response without being twitchy, and it helps keep the model a bit more stable in flight and during landings.

Second; In order for the model to track the circle properly for maximum speed, the leadouts must be positioned correctly. I have use of a computer program that can determine if they are. I would need to know: Speed of the airplane in MPH. Line length, and diameter. Half of the wings span. Weight of the model. If you email me, I can send you the program (it's PC based) along with some basic instructions and you can get the location spot on.

Third; are you getting the most from your engine? If your local rules state that the engine be stock, then there's really not much that can be done other than fine tuning. If your rules allow, remove the head shim. Since it is a sport engine, the LA 15 engine has woefully low compression in stock form. A boost in venturi size would also be beneficial if the rules allow.

Fourth, and one of the direct questions you asked about...Props. The 7x5 APC is pretty good, but probably not the best fit for the low compression and small venturi of the LA 15 engine when used for racing. Try and get some APC 6.5x5, or 6.5x5.5 props. This series has worked very well for us on .15's for sport Goodyear and Clown racing. I would start with the 6.5x5. This will allow the engine to wind up more, and should bring you a speed increase, as well as increased acceleration off the ground.

You may not have experienced any speed increase with the LA 15 over your Fox 15 due to both of them making similar power at the rpms you were running with the 7x5-7x6 props. The LA 15 certainly has more potential than the Fox, and I'm certain would favour the increased rpm's of the smaller diameter props more than the Fox.

I know this is more info than you asked for, but it is all related when it comes to trying to increase an airplanes speed.

I hope you find this helpful

best regards, Les Akre

If you want to go fast the first thing you need to do is drop the one lap times and start talking like a speed guy, in half mile times.

On 52.5' lines a half mile is eight laps.
On 60'  lines it's seven laps.

There's an error of about 0.2 seconds starting and stopping a handheld watch and it averages out better on a longer distance.  Besides, you're just flying the whole tank, so you might as well get an accurate time.

25.7 = 70 MPH, what you need for 15 Profile Carrier or Cartier Combat.

24 = Jack Bauer, CTU, also 75 MPH, a decent time for Clown Race, LA25, Foxberg, and AMA Speed Limit Combat.

22.5 = 80 MPH, MAAC Speed Limit Combat.

21 = 85.7 MPH.  If you do this with an LA15 or LA25, they will look inside your engine, and if they don't find anything, change the rules next year. Ref: Brodak Clown and Toronto 1400 Lap Race.  Marvin Denny's claimed best effort with a G21 Big Iron.  A winning hand in blackjack.  The price of gold as arbitrarily dictated by FDR (from a base of $15) to justify printing more currency.  A bad habit.

20.0 = 90 MPH, formerly, the minimum competitive speed in Profile Carrier.

19.0 = 95 MPH, at various times, a good rat Racer, Fast Combat, and Slow Combat.  Currently, an F2D that needs engine work.

18.0 = 100 MPH.  The Holy Grail, at least until you get your first taste.