Rotary Performance in Sharpie Race

[Tcrook]

I've got to get out of the machine shop more often, I had'nt heard of this race. I've been having withdrawal pains ever since the SUN 100 was killed off by the lawyers.

Got lot's of questions about the race but I'll log onto the race project site for answers.

I was hooked after my first race in 2003 (1st place in category 8, 160 HP RV). Race data is a great place to get documented performance data. Been trying to compile some comparative numbers between alternative engines and Lycomings for a long time.

This might be a good point to ask other RVers for a specific number. Do you know your fuel flow at top speed near SL? Pilots always gasp when I tell them that I burned 17 GPH during the last SUN 100 but I maintain that a Lycoming powered RV going the same speed (217.57 mph average ) under those conditions would burn about the same. Anyone willing to give a number?

Tracy Crook
Mazda Rotary powered RV-4

[rv6ejguy]

Welcome Tracy!

It would be great to have you post some real world numbers on your conversion. Many here are interested in alternatives.

[flyeyes]

Originally Posted by Tcrook

This might be a good point to ask other RVers for a specific number. Do you know your fuel flow at top speed near SL? Pilots always gasp when I tell them that I burned 17 GPH during the last SUN 100 but I maintain that a Lycoming powered RV going the same speed (217.57 mph average ) under those conditions would burn about the same. Anyone willing to give a number?

Tracy Crook
Mazda Rotary powered RV-4

Hi Tracy.

I know it's comparing a bigger orange to a smaller apple, but our Cirrus with an IO-550 (310 HP, balanced injectors, very accurate fuel flow) goes through about 28 gph with everything forward at sea level. This is about 2-300 degrees ROP at 2700 RPM. Economy cruise (67%, 50 LOP) is about 14 gph.

My RV-8 indicates about 20 gph (200 hp IO-360) under the same conditions, but I'm not as confident in the accuracy of the fuel flow.

Either way I think your fuel flow numbers are entirely reasonable for rich, high-power settings. What I'm really interested in is fuel specifics for economy cruise ;-)

[1:1 Scale]

Stumbled upon this, thought it might be good for a chuckle or two

[Geico266]

Guys, that was a really cool race & economy run. Very well done. Maybe we can repeat this at LOE?

[zkvii]

Originally Posted by Tcrook

This might be a good point to ask other RVers for a specific number. Do you know your fuel flow at top speed near SL? Pilots always gasp when I tell them that I burned 17 GPH during the last SUN 100 but I maintain that a Lycoming powered RV going the same speed (217.57 mph average ) under those conditions would burn about the same. Anyone willing to give a number?

Hi Tracey,

Well I wasn't at the race, didn't see the TV show, and don't live near a holiday inn - but..... TMX-IOF360 (parrallel valve) WOT 28.7", 2600 at around 1K DA - about 60 lph, which I make 15.8 US gal per hour on takeoff.

Carl

[Rodoc]

I know this is an old thread, but I just stumbled across it and though I might add my input about the race and shed some light on what was going on with some of the aircraft.

First, great job and thanks to Sharpie for putting on a great event. Many thanks to Condor for being the Referee/time keeper. Unfortunately, the results are no longer posted on the web so bear with me if there are minor gap in my memory of the outcome.

This was not your random collection of RV's. Not a single plane had smaller than the 180 hp -360 engine and most were of the 200hp. variety or larger. My rotary engine was the only fixed pitch prop... get the picture?

The race had 2 main winners, the fastest Mo-Fo and the most fuel frugal. Obviously it would be really hard to be competitive in both categories so Sharpie tried to make a formula that would reward speed and economy. To some extent his formula worked, but still if you were significantly slower you had an advantage in this category too. In the end there were 2 groups - those competing for fastest mo-fo and those going for the economy win.

Sharpie's formula does do a good job of helping to look at overall aircraft efficiency among aircraft of slightly different engine sizes/types. I agree that this race was all about LOW DRAG.

So this is how it shook out... No one knew ahead of time who was going for speed and who was going for efficiency. Turns out that of the 9 aircraft who entered the race, only 2 were going purely for the efficiency win. They were obviously much more fuel efficient and much slower. That left 7 aircraft going for the fastest mo-fo including me - yet, no one wanted to look like a fuel burning pig either. Everyone in that group flew at WOT the whole time, but many at reduced RPM and/or LOP to keep fuel consumption under control.

Given those factors it is easy to see how airframe/prop efficiency played the major roll in both speed and the fuel economy points contest. In fact, with some exceptions, those who were fastest also burned the least fuel. Given that, it is only fair to consider the tandem aircraft in slightly different classes (even the -4 and -8 are different enough from each other to warrant separate classes). Wingnut's 180hp. light slick RV-4 with a C/S hartzell prop ran away it, followed not too far behind by "Special" Eddie and his very nice "Eddie Special" 200hp RV-8. Not to take anything away from their excellent aircraft and flying skills, but comparisons are much harder because they were each in his own class.

So to compare more apples to apples, that leaves only 5 rv-6/7's (S-B-S) who were competing for fastest Mo-Fo. The run-away leader was clearly Sharpie with a solid margin victory in BOTH speed and efficiency. Personally, I think he filled his wings with pixy dust :-)

The rest of us all have excuses.. My rotary was middle of the pack at 3rd, but only lagged Groucho for 2nd by a fraction of a MPH(...eh.. if I had been faster, I would have beaten him.. :-). As I recall, Groucho was also second in efficiency (again, of the 5 S-B-S going for fastest MF).

Somewhat further back were Tooth and Bad Rivet. Tooth (200+ h.p. RV-7) accidentally kept his carb heat on throughout the race (doh!) and Rivet was running rich of peak because his engine was still relatively fresh and he was uncomfortable with LOP ops. That is why Rivet burned way more gas than anyone else. If it weren?t for him I would have been last place of the 5 in economy - having been just edged out by Tooth. Tooth was also hindered by a 3rd blade on his prop and Rivet was hindered slightly by a training wheel in front.

So what about props? It is very well established that the 3-bladed props are less efficient. For me it is worth it because of the smoothness. I think it is pretty well established that the Hartzell/Sensench props are slightly more efficient than most composite props. - Enough said.

I disagree with your position on C/S props. With respect to overall time here in a race that includes starting from a standstill and climb to altitude, a C/S prop would make a ton of difference. No one in this race stayed below 6500 feet (it was just too hot and bumpy down low and the outbound leg had better tailwinds up high). Supposing a climb rate difference of 500fpm over that 3000' of climbing is a whole minute of difference right there not counting acceleration. A C/S prop will for sure change outcome in such a race. For example, if I had a C/S prop I would have easily made up the few seconds on Groucho and moved in second, and I would have been giving Sharpie a run for his money.

But while a minute or two makes all the difference in a close race like this, it matters not in the overall scheme of enjoyment of your RV and making it to grandma?s house (or grandkids house) in a timely fashion. But that is a different debate...

So what about performance of the rotary engine? It is difficult to say exactly because of the confusing factors of prop, turbocharger, extra cooling drag and flight profile.

There is little doubt that the surface/volume ratio of the rotary's combustion chamber makes the rotary less fuel efficient than a piston engine. I have heard (uneducated?) estimates that it is as much as 30% worse. Not sure where that came from except maybe skewed car data. The calculated difference in surface/volume ratio is less than 10% (depending on which piston engine you are talking about). Given my handicap of a 3rd prop blade and larger cooling drag, I think I have shown pretty definitively that the difference is not particularly significant - and nowhere near 30%. Compare my efficiency to the only other 3-bladed prop in the race (Tooth). Within a couple tenths of a gal. we burned the same amount of fuel despite the fact that he flew slower.

What about cooling drag? It is true that most water cooled RV's are going to have increased cooling drag over the air cooled engines. But that has more to do with our choices than the nature of water cooling. As others have pointed out, proper ducting for water cooling requires long diffusers to slow and re-accelerate the air. When done right (like in the P51), cooling drag for water cooling will be significantly less than for air cooling. Alas, the RV is not particularly amenable to long ducts and most of us flying water cooled RV's are going to pay some amount of cooling drag penalty. I think that this race shows that it is not excessive. And don't forget that of the 5 S-B-S FMF racers, my gas COST the least because the rotary loves MOGAS. (I regularly use it while the others cannot or choose not to).

To address some of your other questions about the race:

Extra distance around the pylons: You have a point could have made a small difference. I am sure everyone used his GPS which made it easy to be sure to stay outside the pylon without going too far. A difference of a few seconds at most. Also, we departed the airport in a direction opposite in heading to the first pylon. Some may have made that x-wind turn a little earlier than others and made some time.

Altitude flown: Played some factor. There was a moderate quartering tailwind that day that got a little stronger and more of a true tailwind with altitude. Because of that I climbed to 9000 for the outbound leg and descended on the way back. That scheme no doubt hurt my overall time but helped with efficiency.

An alternate engine conversion is definitely not for everyone. But the rotary is a feasible, lower-cost alternative that has its strengths and weaknesses without sacrificing any of the major factors important to an aircraft installation (power, weight, efficiency, reliability).

Hopefully we will do the race again someday.. Maybe a flying start to help neutralize out the f.p. prop factor.