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Air Racing

paul330

Well Known Member
For the last 2 years I have taken part in Race for Rhinos in Botswana. Great fun but I want to move myself up the results table this year. It?s not Reno - it?s a handicapped navigation exercise over about 320nm on each of 2 days. Major mods to the aircraft are not permitted so I am trying to come up with a couple of ?tweaks? that might give me a knot or 2 - bearing in mind that an extra knot over the 2 days equates to about 1.5 minutes......

- ballast in the baggage hold. Reduces trim drag by shifting CG aft. However, ups the gross weight.

- rig ailerons (say) 1 degree up. Less lift produced in the outboard wing section and more inboard so reduction in tip vortex drag.

- I have Slick mags with a fixed 25 deg advance. Retime to (say) 27. Possible downside - increased CHT and possible starting problems.

Comments/analysis from the team welcome ......
 
Remove any external Nav or second com antennas that are not critical to the flight, this should be good for at least a knot.
Take a good look at your gear farings and wheel pants. We know that you get 12 to 15 knots by using the farings so it only makes sense that a bit of tape might also pay with some speed
And most importantly, get a good weather briefing regarding upper winds. Practice some climbs and find out how much of a tail wind is worth climbing for. The reverse also applies to headwinds. Keep in mind that moderate turbulence can cost 2 to 3 knots.
 
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Removal of equipment and taping not allowed under the rules.

Yes, of course, winds are taken into account. Worked that out as a break even of about 5kts per 1000'. Some seriously low flying last year on the into-wind legs - this is Africa after all ....;) On the other hand, you have to take into account ease of navigation. This is map, compass and stopwatch with GPS disabled so low flying to gain a couple of knots can be counterproductive if you wander off track.
 
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On the other hand, if one were to remove the extra aerials, amend the equipment list, collar CBs and label the COMM "ILS/VOR Inop" before submitting the entry...... Hmmmmm......

Not entirely in the spirit of the rules but :D

However, analysis of my other potential tweaks still welcome....
 
Push the throttle and prop controls all the way forward! This might seem obvious, but based on Airventure Cup results over the years I suspect there were a few pilots who were not really racing. Along the same lines, lean for best power (and be prepared for some really impressive fuel flow numbers!).

Ideally you should test the effects of any changes, but it's extremely difficult to accurately and reproducibly measure changes of 1-2 knots. It might be worthwhile to do some climb tests however, and to test true airspeed at different altitudes (see the archives for how to do this). Keep in mind that the best race altitude depends both on the wind, and on how long you'll be there.
 
Thanks guys. Without wishing to sound ungrateful for the comments, I?ve learnt all this stuff over the last couple of years. I?m looking for some sort of guidance as to whether my less obvious ideas above are likely to be practical or will actually achieve anything.
 
Not sure what altitude you are going to fly but I can tell you that the timing bump you have proposed will probably not have a measurable effect on speed. I did a timing sweep at best power mixture and 8,500 feet and found the power essentially flat. My 540 D4A5 did peak in speed at 27 degrees, but that was only worth one knot over the standard data plate setting of 25 degrees. If you fly lower, this slight "peak" will diminish even further. Of course, the inverse is true as well. If you do bump the timing hoping for a slight edge up high, you will be too advanced while climbing to that altitude so the net result might be zero gain.
 
Thanks Michael.

Actually, the base is at 3000' and we don't really climb much. Last year, I didn't get above about 1500' agl catching tailwinds on the down-wind legs. (you can probably add about 1000' for density altitude).

One knot is good! Any issues with starting with the retard breaker firing 2deg earlier?
 
I'll revise my statement. At your altitude, you will not see any gain in speed due to an ignition advance. I have done full throttle, sea level testing and I've swept from 20 to 30 degrees and found ZERO speed difference. Said another way, with the rich mixture and fat air you will see, even a 5 degree retard from data plate values does not hurt speed.
 
Thanks Michael.

That's just the sort of info I need. I find that surprising but scratch one idea. Saves me all the time and effort taking the cowls off, making the adjustments, testing it .......

How about the other 2........?

Last year there were 5 RV-10s. We were all pretty much equal in speed - the 2-bladers might have had 1/2kt on me. Except one who was about 4 knots faster and came in 3rd out of 115. Supposedly stock build with standard engine and mags - nobody could work out why he was so quick......
 
ailerons and flaps

...

- rig ailerons (say) 1 degree up. Less lift produced in the outboard wing section and more inboard so reduction in tip vortex drag.
...
I've heard that this can help, might want to consider the same for flaps, if it's possible on the -10.

Can you restrict intake or exit air? Build a cowl flap? Smaller tires and smaller wheel pants?
 
Ok, not allowed to tape gaps. Anything in the rules about sealing them from the inside? Sealant, foam tape, flap seal?

Some gap and fairing joggle drag is external, but some of it is excrescence drag, jets of air escaping from inside the airframe. The jets are similar to hard parts sticking out into the airstream.

Same problem; different aspect: airflow through the interior of the airframe is exactly like cooling drag, i.e. drag = mass x loss of velocity. Reducing the mass flow or the velocity loss is a drag reduction.

Cooling drag itself can be big factor. Given a typical GA cooling system, any air which enters the front and exits the rear suffers a loss of velocity. Mass required for cooling is, well, required. However, any which bypasses a trip between hot fins is pure drag; it did nothing useful. So examine your engine compartment sealing very carefully, in particular if you have flap seals. Most leak badly. Reduce leaks to reduce mass flow.

Is your propshaft sealed?

Now address loss of velocity. Fair any structure in the cooling outflow, like nose gear support tubes. Remove or move garbage hanging in that flow.

Do you have any cooling headroom? If CHTs are low, reduce the cowling exit area.

Want to get serious? There is very likely some energy loss at the underside of the cylinders and heads, where the two cooling streams (imagine one from each side of a cylinder) meet at the exit of the baffle wraps. Decreasing that loss would result in higher exit velocity. It's one of the reasons the water cooled guys can get less cooling drag.
 
Sealed everything I can in the cowl. CHTs run at just around 400F during racing so I don't want to reduce cooling any more. This is meant to be fun and I don't want to compromise the life of the engine. I experimented last year with putting in a bigger oil cooler and reducing the flow from the #6 baffle. Oil temp is fine and I reduced #6 (highest) temp by about 15F - so I can run a bit leaner ie closer to best power. Nothing else to be done there ...... I'll put a boroscope in the cowl and make sure all the seals are good - pretty sure they are but worth a look.

I agree that aft ballast should help but can't quantify it.

I will try rigging the ailerons up a bit - that's easy to do and to undo. Flaps can't be changed.

Thanks for the inputs!
 
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It's a matter of adding up little improvements here and there. Racing has always been like that. You can find more.

Back in the 80's I had a friend with an identical production racebike. All season long, I could draft past him with about a half a mile per hour to spare, which had him muttering and throwing things in the pits and threatening to file a protest, as surely I was cheating. At the end of the season I showed him a photo another friend had taken of us on the main straight at Roebling Road. We looked like twins, both tucked tight...except I was sliding my feet back so just my toes were on the pegs, then pulling my heels in tight. Tony's toes were in the breeze, like little speed brakes ;)
 
I've heard that this can help, might want to consider the same for flaps, if it's possible on the -10.

The RV-10 airfoil is designed in such a way that in high speed flight angle of attack the ailerons and flaps are already in a reflexed position.
At high speed they are very slightly loaded on the upper surface already. Reflexing them any more would probably be adding drag, not reducing it.
 
The RV-10 airfoil is designed in such a way that in high speed flight angle of attack the ailerons and flaps are already in a reflexed position.
At high speed they are very slightly loaded on the upper surface already. Reflexing them any more would probably be adding drag, not reducing it.

Thanks Scott

The idea was to re-rig the ailerons only up by 1 degree. The thinking was that this would effectively slightly reduce the AoA and lift at the outer wing thus reducing the tip losses. Of course, it would be necessary to increase the inboard lift slightly to compensate but, again, the thinking was that the inboard section of wing is more efficient.

But you are the expert and if you think it would be counter-productive......
 
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