Speed Secrets......

[Red]

Hey all...

Well, I've been bitten by the pylon bug agian. I guess I was never really cured, but this Warbird series is a blast! My new plane bit it today in a spectacular midair , but I'm not too bummed. I already have another one on the way! (Thanks Steve)

I thought if there was enough interest, I would do a series of posts about tuning you and your airplane for racing. All of the things I do, with the exception of a couple of areas, directly apply to even the most basic racer and will help you and the plane go faster.

I love to race, it is my favorite aspect of R/C. I have participated in or flown just about every class of racing there is including Giant Scale, Gliders, F5B, F5D etc... I have picked up many tricks along the way as well as developed quite a few of my own. So if you'll put up with my ramblings, I'd be glad to share as much as I know. I'll just touch on a few things in this first post and see how things go.

First I cannot stress enough how important it is to have a properly set up aircraft. You cannot take a sport aircraft to the field on race day and expect to do well with it. They don't fly the same at all. I would suggest if you want to sport fly your racer, make a separate program for racing and sport flying.

Second, is small things add up. Some of the things I'm going to suggest may seem trivial and not worth the effort. I couldn't disagree more. Think about it this way, if all the stuff you do nets you a 2-3% increase in speed, that may seem like a small amount and not worth the effort. Over the course of 15 laps however that 3% adds up to 30-45% total or about half of a lap! That's a bunch and can certainly make the difference between first and last. The devil is in the details and here is where it counts most.

If anyone saw my plane fly before its demise, you could see that attention to detail made that thing boogie!

Here are some of the things I would like to touch on:

Aircraft theroy
Aircraft & radio setup
Pilot (you!) setup
Flight testing & trimming
Tricks: Prop, batteries, aero mods etc....

Please ask questions and provide feedback.

Aircraft theroy
When I look for a plane to race, if it's not already chosen for me like the T-28's or Q-500, I take many things into consideration. First an airplane has to be stable above all else to be fast. It doesn't matter if you have the fastest plane on the course, if it is all over the place its worthless. You have to be able to fly a clean course to go fast.

I like planes with a long tail moment. They groove better, turn better and will allow for a faster more agressive setup in the end. (more about that later) I also like a high aspect ratio wing, they turn harder without scrubbing speed as much. If you think about it, a turn is just a loop on its side. So a plane that will loop tightly by making lift effeciently, will consequently turn harder. A clean profile is also nice. This usually means no radials for me, but I made an exception in my latest aircraft because the other than that it fit my needs perfectly. Light weight to keep the wing loading reasonable is also a major factor Now you can begin to see why I liked my Ki-84 so much, and why I ordered another.


Aircraft & Radio setup
A racer has to fly smooth to be fast. First, make sure your CG is correct. (we'll talk more about CG during flight testing) Not only does it make your plane flyable but an aft CG is faster. Why? Well if you understand that the stabilizer's job is to provide down force to counter act the natural pitching moment of the wing, it stands to reason that with a more aft CG, the less down force the tail will have to provide. With a nose heavy airplane, you have to carry up elevator trim to compensate. Not only does this make the aircraft slower because of the added trim drag, but it also makes it harder to fly because the trim will change with speed. This assumes the aircraft has the proper incedence set. Most of the time you will want it at 0-0. If this is out, it is a bit harder to change, but it can be done. You want your racer to fly with zero elevator trim. If the incedence & CG are correct, you can also play with down thrust to make it not climb at speed.

Next up is control throws. Dual rates and expo should be you be your best friends. If they aren't, get aquainted! You want your racer nice and soft on the controls. You should only have enough throw to make a turn with about 3/4 deflection. The extra 1/4 left over is only for emergency. Some guys think they want alot of control to get out of trouble and end up over controlling. You won't get into trouble in the first place if you have soft controls! I run about 30-40% expo and only use high rate for landing. I set my plane up to dive slightly at full throttle with a very slow roll rate. The reason is if you pull to hard on a turn and start heading for the middle of the course, all you have to do is while you are still knife edge is to let off the elevator and the plane will naturally push back out onto the course. The other thing is I get rid of nearly all of the down elevator throw. This goes hand in hand with over controlling. I have seen more people smack their planes by getting on the down elevator too hard and getting out of shape. You only want enough up elevator to round the corner smoothly without scrubbing speed. Any more than that is a complete waste and is asking for trouble.

Well...thats all for now. Lemme know if this is helpful and I'll keep going.

Cheers
Red

[patrick_ohalloran]

Thanks for posting. I know I've got more than one setup issue, so I'm listening. Hope the other racers might join in too.

Patrick

[Red]

Aileron differential is also something you may want to consider. We want a bit of diff to pull the nose to the left as we enter a turn scince we don't have a rudder.

Thats about it for aircraft setup. In general you want a smooth easy to fly plane, so you can concentrate on making good turns and clean passing.

Pilot

This is probably the place to make the biggest improvement in your lap times. You need to have a well setup plane, but then you've also got to be able to get into the groove. There are many tricks to flying a tight clean lap. These are things I don't normally share, so pay attention!

I have a minimalist attitude when it comes to racing. I fly with the least amount of control input possible, meaning I try not to move the stick if I don't have to. If the plane is setup correctly it will go where you point it. This gives you time to set up for the turn, watch for traffic or make a pass. Done correctly, it will feel like a long time between the turns.

I break each lap into segments and concentrate on doing the best I can on each part. The straight away sounds like the easy part, but in reality it is the heart of the lap because it sets you up for the turn. You blow the entry to the turn, its tough to get back on track and you end up zig zagging all over the place trying to get it back under control. In too tight and you have to pull hard to make it and scrub speed. To loose and you have to fly wider which is slower.

That brings us to the turn. I break the turn into three parts. The entry. The turn, & the exit. These are separate distinct control inputs. You enter, roll up to knife edge, and release. You pull through the turn and release, then you roll out on your heading to take you to the next turn. DO NOT under any circumstances try to make a turn in one fluid motion! I see many people making wild swooping turn and invariably this is the method they are using. I cannot stress enough about how smooth flying is the key to a fast lap.

Gotta run...but the next post will reveal the tricks to ripping off fast consistant laps.

Red

[Red]

How do the fast guys make turn after turn that looks like its going through the same chunk of sky? Magic? Super human vision? Nah...nothing that magical. Nothing beats practise, but there are a couple of things that will work like a crutch.

Practise the start a bunch from the same height. Know that height and start from the same position every time. A count in your head will work fine for practise. The starts are a great place to get in the lead quickly. A good start will go along way to winning a race.

First, if it is sunny out, watch the shadow. Most of us have a hard time judging depth to the first pylon. When you pass the end of the runway, roll up and pull. If that is getting you cuts, pull at the first grape stake after the end...whatever works for you. The point is to give yourself a visual clue as to when to turn. Pretty soon you will be able to anticipate the pull and be ready for it. Just don't push it. Thats what happened to me and I cut out.

If its overcast, another trick is to count. Position yourself on the flightline in the same spot for each heat. As the plane passes you, count 1,2,3..pull. The count will vary with your particular plane, but once you figure it out, it will be very consistant...use it!

Pylon 2 seems to be a bit easier, but there are still a fair number of cuts down there. Most of the time people over rotate turn 1 and end up heading to the inside of the course. If they don't correct, they go long enough but come inside turn 2 and cut. To most people it looks like they are flying parrallel to the couse, but it is deceiving. The correct line will appear to take you outside the course, but in reality it will set you up nicely for the turn. This is where a slight diving trim to your plane will come in handy. After turn 1, you let it drift out and set up for 2.

Dinner time....more later.

[Jon_Stychno]

    Thanks for sharing some of what you have learned over the years! I appreciate the fact you are willing to give up "speed secrets" which will make the races more exciting and enjoyable for all participants, without fear of giving up your "competetive edge"!

[adam_clement]

 Red, thanks for taking an interest in the forum and helping the other racers(and me) fly better. The forum really has not gone to well but this is just what we needed.  . Since I have joined the club there have been a few different racing classes but these are by far the two funnest classes I think we've had. . I do have to say though, now that Red is racing in warbird it has gone to a whole new level !!!!

Adam

[Red]

    Thanks for sharing some of what you have learned over the years! I appreciate the fact you are willing to give up "speed secrets" which will make the races more exciting and enjoyable for all participants, without fear of giving up your "competetive edge"!

...

[Red]

Red, thanks for taking an interest in the forum and helping the other racers(and me) fly better. The forum really has not gone to well but this is just what we needed.  . Since I have joined the club there have been a few different racing classes but these are by far the two funnest classes I think we've had. . I do have to say though, now that Red is racing in warbird it has gone to a whole new level !!!!

Adam

......

[Red]

Only a few minutes before work today, I'll try to get more up soon.

OK...so now you have your plane built and ready to go. Double check the CG per the manufacturers instructions, and accurately mark the location of your battery. Keep in mind that with these small wings that the CG range is very short and moving it 1/8" can have a large effect.

Fly around and trim it for level flight (or slightly decending) at full throttle. Roll it inverted and let go of the stick. It should continue straight for a bit before diving. If it heads for the dirt right away you are nose heavy. After landing the up elevator trim will confirm this.

Now on most models, with he Alpha ones in particular, you will actually be carrying a bit of down elevator trim. This is because of a positive incedence (they do this to make it easy to fly). The best thing to do is to check during building and make adjustments then. If your kit requires you to glue on the tail, this is the best place to make the change. If the tail is already glued on like the Alpha's, you have to shim the wing. Double sided tape with only one side affixed to the fuse works well for this.

Some planes will change trim with airspeed. Even if you have the incedence correct they will climb while at full throttle. The cure here is down thrust. It will only take 2-3 degrees.

So now your plane is trimmed, CG'd and the incedence is correct all good to go right? Nope. Now fly it level at full throttle and pull a full up elevator loop. Remember a pylon turn is just a loop on it's side? If you are like most, it will loop way too tight scrubbing lots of speed and possibly even snapping out. Reduce throw until you can make a loop of about 50-60' radius at full deflection without scrubbing or snapping. Remove most of the down elevator throw, add 30-40% Expo and this is you low rate racing setting. You can increase it a bit on high for landing.

Next do a series of rolls. You should look for the rate to be about 180-200 deg/second. Once you get that, add the expo and make that your low racing rate. Next, watch for heading changes during the roll. We want it to roll perfectly axially. If you look closely, most wardbirds will want a bit of rudder in the direction of the roll. Scince we don't have a rudder, you need to add differential. You want the upside aileron travel to go about twice as far as the downside.

Gotta run, but I will explain how the differential works and a few ways to get it next time.

Ciao
Red

[Red]

Ok....all about differential. A wing makes lift by changing the pressure of the local airflow. When air passes over an airfoil shape, the top curved section causes the airflow speed up relative to the airflow under the bottom of the wing. This sped up airflow over the top has less net pressure and lift is the result of this pressure difference. Contrary to popular belief, the air does not have to "match up" at the trailing edge...but that, along with fully symmentrical airfoils and how they make lift is a whole nother thread.

Back to differential. Lets assume that your aircraft has the same amount of up and down aileron travel. Imagine in the differential pressure situation I described above, when you apply aileron for a left roll, the left aileron goes up into the lower pressure and the right goes down into the higher pressure. The right aileron down into the higher pressure causes more drag than the left and causes the nose to swing in the opposite direction. To cure the opposite nose swing, you reduce the amount of down aileron travel vs. the up. Usually you need to reduce it by about 30%, but I have seen it be as much as 70% on a plane with a high aspect ratio wing.

There are many ways to achieive aileron differential. If you have a two servo wing, the simplest way is to just adjust your end points. If your plane has standard torque rods with the servo on top, you can bend the torque rods toward the trailing edge, or use a wheel or X shaped servo horn and attach the pushrods behind of the arm screw hole. If your plane has the servo underneath, do the oposite of the above. The torque rods go forward and the attach points go aft of the arm screw hole. If your plane has one servo with a flexible rod, it is a bit more difficult but can be done. You need to use a V shaped horn and hook the right aileron to the left side of the horn and vice versa. I hope that makes sense. If not drop me a line and I will explaine it better. You can also reglue the horn on the aileron so the attach point is ahead of the hinge line. What it amounts to, is do whatever is necesary to make your plane roll cleanly.

Great, diff and rates are taken care of, it is time to do some laps and see how it behaves. It should fly level, roll cleanly and pull through the turns hard. One thing that shows up once in a while is some planes have a tendancy to dive into a corner. You roll up 90 deg and pull only to find it heading for the grapes. That can be a bit unnerving! Pretty easy fix here. The problem is almost always the plane being heavy on one wing. Tape a dime to the outboard wing and try it again. If it helps but doesn't cure it, increase the weight (nickle, quarter etc.) till it grooves. You can slot the tip and glue it in to make it look real nice.

So all of the above should get you a clean easy to fly well behaved plane. You will be amazed at home much different your plane will fly, and how much easier it is to fly. Nothing beats practise, so get out there and turn some laps until you are comfortable. You will get faster, I garantee it!

Speed Mods

You'll notice that in all the above chatter, I haven't mentioned anything about mods to make your plane faster. Well...I've saved the best for last. I must stress one more time however that your plane HAS be set up properly to make the most from these mods.

Airframe

Lighter is better. Period. Your plane will accelerate faster, corner harder and have a higher top speed all else being equal. Use the lightest equipment you can that will still get the job done. Small servos, small receiver and epecially the batteries (lots more about this and motor/prop combo later).

Drag is the enemy. Anything, no matter how small produces drag. Coil up your antenna and get nearly all of it inside the plane. This is a huge and often overlooked drag producer. Use a big spinner, it will streamline the fontal area and clean up some of the dirty air around the hub of the prop. Consider internal linkage if you don't already have them. Clip the wings to the 31" limit. These plane are light enough that you won't be giving up anything by clipping. Cut the canopy down, sand the wing, fuse and tail all smooth etc. Skin friction is another problem area. A smooth plane will be faster! Make the edges of the tail, and the trailing edge of the wing as sharp as possible. You can inlay 1/64th plywood to make a hard edge to sand down to and retain the structual integrity.

More to come where I will focus on motor/battery/props. There is much to be said and learned here, so it will be a post all its own.

Red

[Jon_Stychno]

 :)Keep it coming Red!

[Red]

Sorry for such a long time between posts. I was working some crazy hours, but I'm home now 

Ok where were we? Oh yeah....the power section. As much as having your plane set up correctly, having the correct balance between motor/prop & battery is just as important. Since the motor is selected for us, battery and prop selection become more critical.

In most race setups, you want a small diameter prop for max rpm, and hefty pitch for speed. We give up some low speed acceleration for top end velocity. After all, it's racing right?

Lets talk a bit about the motor before we go too much further. The Park 450 motor has a Kv rating of 890. (rpm's per volt) Simple math will tell us that 11.1v X 890 =  9879 rpms. Thats pretty low for a racing application. This motor is about 75% efficient giving us a no load rpm of about 7400. It is not going to turn much faster than this no matter what. You cannot exceed the Kv rating. This means the normal small diameter, big pitch prop is not the way to go.

Now those of you that have tached your motor with say a 9x9 APC prop on it will note that it will turn about 71-7200 rpm. This means is that we are not working the motor very hard at all, as it it very close to the no load rpm even with a good size prop on it. Current in flight is in the 10-12 amp range. The motor won't turn faster than the Kv, so we need the most effecient prop to make use of the limited rpms we have available. At this rpm, the motor is very torquey, and will spin a pretty hefty prop. A 9" is about the right diameter, and 9" pitch gets the job done. But what else? Pitch is what gives us the speed we want, and since it won't turn any more rpm's, more pitch is the only answer. I have retwisted 9x9 to get another 1" pitch, picked up some speed and didn't drop any rpm. That means I'm headed in the right direction, and more pitch is in the future. There is more testing to be done here.

This is where motor timing may help. I hear a lot of people bragging that they are running "high" timing thinking that it will give them more rpm. Wrong! We are already up against the Kv limit of the motor, so high timing does nothing but create heat and pull more current. I go the other way, med or low timing and load the motor up with more pitch to get the current up. This gives me more torque to turn more pitch.

As far as batteries go, I want the least capacity (for the lightest weight) that will get the job done. This is where "C" rating comes into play. A battery with a higher C rating will give higher voltage for a longer period of time. Here is a simple example to illustrate the point:

A typical 2100 Thunder Power can have a 20C rating, meaning it can deliver 42 amps continous.
A more modern 1600 pack has a 35C rating, meaning it can deliver 56 amps! We are not pulling anywhere close to that many amps, so this smaller, lighter pack is more than adequate and may actually deliver better punch for the duration of the race. It's a win win all the way around.

Props are sort of a black art, and the subject is really too deep for me to go into really great detail. Prop design has always facinated me and is the simplest way to up the performance of any airplane. You would be surprised how a tweaked prop of the exact same size with make a huge difference. If you want to learn more specific information, drop me a line, ask a question here or give me a call and I will be glad to help. It takes a good deal of patience to work a prop, but the rewards are worth it.

Some basic prop theory might be in order. A prop is just a rotating wing. It has all the same advantages and suffers the same setbacks as any other wing, but it flies through the air much faster, so small improvements pay off big here. Airfoil shape, airfloil cleanliness, planform and tip design all come into play.

First above all else your prop must be balenced. An unbalenced prop can cost 2-300 hundred rpm, which is close to 20% of your thrust! Trust me here, it is huge. If you don't have a balencer, beg, borrow or steal one and learn how to use it! I won't go into much detail here, all the balencers come with good instructions.

You can not bolt on a prop out of the bag and expect to go fast. I spend on average at least an hour for each race prop I work. Here are the steps in order I use to get a prop ready to go. If this is your first time, start checking balence after step two to make sure it's not way out. Most APC props have the center hole drilled off center, so make sure you fit and balence them with the included bushings.

1. Take an Exacto knife and scrape the flashing off all around the perimeter of the prop.
2. If you are going to clip the prop, measure each blade carfully and cut with a razor saw or sand to length.
3. If you are going to change the planform, use heavy grit (80) sandpaper and a sanding block to make the new        shape, including the tip.
4. Use 150 grit and a block to sand the LE, TE and tip till they are nice and thin. The same principal applies here as a wing airfoil. You want a sharp trailing edge, a smooth but not blunt leading edge and a thin tip.
5. Go over the areas you just sanded with 220 grit further smoothing the entire blade. At this point I'll usually throw it on the balencer to check progress.
6. Final sand it with 400 wet. Nake sure all edges are super smooth. It should feel like satin all over. Dry it off and check the balence.
7. Mark the heavy blade with a sharpie and sand the back side at the tip till it balences.
8. Wax it. Yes you heard me, wax it. Several coats, buffing in between.

I have some ideas that I'd like to try with props. If any of them work out, I'll be sure to share them here. One I have in mind is pretty radical, so if you see me flying some wierd looking prop....look out!


Well, thats about it. I'm sure there are some details along the way that I have missed. Please feel free to share some of your progress here, ask some questions or make a comment.


Go fast & turn left

[Red]

All I hear are crickets in here....that must mean you all are tweaking your planes right? 

It occurred to me that I could spend some more time talking about props. Afterall, it is probably the single biggest speed improvement you can bolt on. Well, that is sort of a misconception, you can't really just bolt on a prop and go. They require quite a bit of work, both to actually prep and to learn how. I can't tell you haw much $$ I've ruined trying to figure out how to do it on my own. I would have killed to have a forum like this growing up where I could see what others had done before me. It was all trial and error for me! But, it was a great learning experience and I still read anything I can get my hands on regarding prop design.

Here are a couple of good prop resources:

http://www.mh-aerotools.de/airfoils/
http://www.supercoolprops.com/

Put your thick glasses on, they are an egg-head read!


When working a prop I pay attention mostly to LE shape (nice and thin), TE (sharp) and tip. I usually sweep the tip more than they are stock and sand them quite thin. Stock prop airfoils are pretty blunt and not the most aerodynamic. Reshaping makes a big difference here. If I can't get close to the right size I want, I pick first the right pitch and then cut down to the right diameter. For instance, the best results so far have come from an APC 10 X 10 cut down to 8.75 x 10. Like I said above, there is more work to be done here, but I feel there is a little speed left in the right prop combo.

The bottom line is, just like with any form of racing, they guys who put in the extra effort are the ones going the fastest. I'm not saying you have to do all the things I said above, but don't be upset when those who did pass you! 

[patrick_ohalloran]

For those of you following you can hit the "notify" button and you'll get an email on a new post, hit it again to turn off notifies on this topic.

[Red]

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