I was going to post this in the delta/speed thread but figured I'd better start my own since this is veering off topic...

I know F3D (FAI) racers benefit from retracting the gear to prevent the "toes out in the wind" syndrome. However, in the events in which I'm involved (Q500 and Q40), the rules specify that the designs must have fixed gear.

My question is, what is better... To have the gear tucked in tight so that it hangs in the prop wash or should the designer try to place the undercarriage in the "clean" air away from any prop wash? I've heard opinions both ways and would like your opinions.

Thanks very much!

You liked that "toes" bit didja?

There was an F3d model a few years back that used closely spaced tandem wheels in a belly fairing. The design was actually published in RCM. It was the world champ that year.

Unfortunetley I believe that Q500 and maybe Q40 have landing gear rules that specify non tandem wheels and spacing that precludes "cheating" like that.

What can you do within these rules? Probably not much other than preventing gaps that turbulate the air. The conventional gear places the wheel close to the final vertical part of the aluminium gear. This will create a small gap that will generate some turbulence in the grand scheme of things. All I can suggest is keep the wheel VERY close to this last little vertical bit and file that part so it's flat on the wheel side and airfoil shaped on the inner face with a countersunk bolt for the stub axle. Arrange some setup that then allows the axle to be firm but keeps the wheel within a paper thickness of the vertical part and has a blank outer axle cover. For the rest of the gear just file and polish it to a nice airfoil. Lots of work so don't land hard on it.....

You also need a minimum cross section. If it allows in the rules make the fuselage a bit pod bellied so the landing legs are shorter. And use the minimum sized wheels they allow and make them out of aluminium with a small O ring tire...... Starting to sound like Formula 1 isn't it?

I don't know how much of this you can get away with as I understand that the rules are tightening up to make the racing less gimmicky and more sport minded. But all in all the little things help. Using internal surface actuator linkages for example.

With fixed gear away from the prop wash you have drag equivalent to the freestream velocity. BMatthews' comments of how to reduce the drag are well thought out.

The drag of each component of the airplane doesn't care how fast the complete airplane is going, just what the local component velocity of the airflow is.
If you put the gear in the prop wash you are essentially wasting power. The high velocity of the propwash against the gear converts into drag that is higher than the out of propwash drag.

It would seem then beneficial to have a longer gear that is streamlined as he described. The drag of the extra gear length is small compared to the close in drag effect.

That comment Ben, makes me think seriously about the benefits of pusher power.

What do you think?

Thanks for the responses, guys!

Bruce,
Yeah, I admit it. I'm a sucker for a catchy phrase. All of the "tricks" you mention are things that I'm familiar with and have seen "at the races" so to speak. I haven't seen a lot of aluminum wheels lately; most guy use what we call "pizza cutters". These wheels are the min diameter and are made of the same material as bowling balls. When viewed from the edge, they are diamond shaped -- .25" thick at the hub tapering down to about .0625" or less at the outer radius. And the hub countersinks into the wheel! Pretty nice really.

You're also correct about filing an airfoil on the gear. Man, that IS a lot of work! That is the part I dread most about putting together a new racing plane. I am usually content to just round the edges everywhere I can on the gear; getting the aft edge down to a sharp TE for a proper airfoil just takes too much work! Now we have CF gear available that is already shaped. I keep meaning to get a set or two to try but never seem to get around to it.

Ben,
That's a good explanation that I haven't heard before. Thanks! What do you think about wheel pants? I think that when the rules allow, using pants is to the racer's advantage even when you pay the small weight penalty. Of course, since Q40 is supposed to be a scale event, you can't put pants on a plane when the originals didn't have them. Any of the P51s--Foxy Lady, Miss Ashley, Red Barron or planes like the Napier Heston or Caudron or Firecracker for example.

Any thoughts about how to optimize the design of wheel pants?

Some of the other "little" things we do that add up... Yes, all linkages are concealed. "Skin" hinging techniques where there is no visible hinge line on the top surface of a control surface. Sealing the underside of the hinge line on the bottom of the surface. Fudging the wing tip shape to try to minimize the vortex. Area rule fuses etc.

There is another thing that I'm not sure about... The RX on/off switch. What would be the best way to reduce drag for this necessary item? There really are just two popular ways that guys are handling this. First is to have a small piece of music wire that protrudes through the side of the fuse that you can pull for "on" and push for "off". The other thing I've seen is to drill a small hole through the switch knob, thread a fine piece of fishing line through the hole and knot it, then have the line exit both sides of the fuse. Then you pull on one side for on, and pull the other side for off. This requires about 3" of line exit each side so you have something to grab. So you have about six inches total of this fine fishing line flapping in the breeze. This compared to about a half inch of music wire for the other method.

Which of these methods would you say is more advantageous?

Thanks again guys!

When the basic form of the model is set by the rules all you have left is "tricks" to optimize the details.

I have never looked at the Q40 rules so I don't know what is allowed but if it's a scale type event and you're allowed retract gear then the racing type P51's would be better than something that uses fixed gear even with the wheel pants.

Wheel pants that use good airfoil outer shapes are probably a good second choice. And finally those "pizza cutter" wheels you mention sound like the modern equivalent to the aluminium racing wheels of days gone by.

For the switch how about a wire that sticks out the side of the model with an L shape. Push it in to turn on and catch the leg of the L to pull for off. When ON the leg for the L would be very close to flush against the side of the model with just enough give to hook it with your fingernail. Similarly using such a wire and hiding it in the wake of some other item that you're stuck with like the muffler or a wing bolt head or near the landing gear or something else will reduce overall drag.

If you're stuck with a canopy or similar design elements then a hinged snap down or velcro down hatch or door could be used for the battery charging connector as well as the switch. It all depends on the model design.

But don't neglect the basics and then try to compensate with details. Use a proven low drag "brand name" airfoil selected for the proper Reynolds number and lift coefficient so the minimum drag bucket point occurs at your operating speed and load (turn load) range.

How fast are you going, anyway? Area ruling is only helpful for aircraft in the highly compressible regime, i.e. transonic. The whole idea of area-ruling is to reduce the transonic drag rise, and delay the drag-divergence mach number. By their very nature, propeller aircraft can't usually get up to this level.

The biggest, best thing you can do is clean up and reducethe area that's in the propwash. "Scrubbing" drag is a tremendous factor in aircraft speed, as the small, high-rev racing propellers are moving the propwash at very high speeds. Get the area of the fuse down by decreasing the cross section, move the tail back farther and make is smaller, all of the things you see on the Formula 1 full-scalers. If you can come up with a way to cool it, properly cownling the engine would be tremendous boost.

.02 deposited.

Thanks Bruce,

I forgot to mention that in Q40, retracts are prohibited. Most Q40 racers feel the P51 design is the fastest there is right now. So, all those Ashleys out there would be faster if they could only do something with the gear.

The L shaped wire is a good idea. I've done it myself but have always had it set up as "pull for on". Having it sit flush to the fuse side when on is something I'll try.

Thanks!

Yeah, it's contrary to the "proper" and safer way of using the pull for on option. You'd have to be careful to not bump it on. A snapover bit of tubing or plastic with a 6 foot long "Remove Before Flight" ribbon would ensure that there were no accidents....

Exposed gear can be fast if you're allowed to fair or shape the legs but a cleaner design with wheel pants may be a better option if the rest of the model can be made to match the shape of the Ashleys and such.

But this is half the fun, right?

Starfire,

I fell into a trap and you caught me. Most guys call the popular fuse shape "Area Rule". What I should have said was "laminar" where the portion of the fuse aft of the wing's TE is pinched slightly (concave).

To answer your question, the current record for Q40 is 0:59: and change for a 2.5 mile course. If you do the math, this works out to over 150 mph. But you must remember that because the course is ten laps around three pylons, the actual distance traveled will be considerably longer than that because of the radius of each turn around a pylon.

Generally speaking, Quickie 500 racers are around 160 mph, Q40s are around 180 mph and F3D (FAI) racers can top 200 mph.