[Bob Axsom]

I did a max fuel burn rate test on my O-360-A1A for the AirVenture Cup Race today. It is a 407 nautical mile race and I have 36 gallons usable with the tip tanks removed for racing. The only thing of interest here is at 2720 RPM, 3,000 - 3,500 MSL, MAP 26.5 - 27 (respectively), OAT 29C, leaned for 1300F hottest EGT, the test fuel burn rate was 14.5385 gal/hr.

From years of flying this and several other stock O-360-A__ Lycomings the 75% burn Rate is a solid 10 gal/hr.

If you are curious the, the ground speed required to make the 407 nautical mile trip with a half hour VFR reserve AT FULL POWER is 206 kts. With no reserve the speed required is a little over 164 kts.

Bob Axsom

 

[Bob Axsom]

An altitude selection table

I generated a spread sheet with a pesimistic climb rate of 500 fpm for climbs to 2,000 ft (700 agl) to 15,000 ft in 1,000 ft increments. It is always a little tough in a remote location to decide what altitude to fly at. The table has columns for race completion times (remaining distance divided by level flight speed, times 60, and the climb minutes). I can't put the spread sheet in here but I'll put in a couple of columns. The usage requires getting the best wind data and deciding on the expected cruise speed at each altitude and comparing the appropriate cells from the to find the shortest predicted race time an executing the plan. Oh yeah, and when everything does not match predictions, improvise (read you lose - I am familiar).

Bob Axsom

Race time with remainer at 210 kts
117.0571429
118.0857143
119.1428571
120.2
121.2285714
122.2857143
123.3428571
124.3714286
125.4285714
126.5714286
127.5142857
128.5714286
129.6285714
130.6571429

Race time with remainer at 220 kts
111.8272727
112.9
114
115.1
116.1727273
117.2727273
118.3727273
119.4454545
120.5454545
121.7272727
122.7181818
123.8181818
124.9181818
125.9909091

 

[hevansrv7a]

I don't get it

If you are curious the, the ground speed required to make the 407 nautical mile trip with a half hour VFR reserve AT FULL POWER is 206 kts. With no reserve the speed required is a little over 164 kts.

Bob Axsom

Bob, from a fan..

If you have reserve on a given distance then your burn mpg was better than with no reserve (fewer gallons burned for equal miles) and that usually means lower speeds. So 164 kts should give you better mpg than 206 kts. Didn't you indicate that 180 kts was about as fast as you could go? What am I misunderstanding?

 

[Bob Axsom]

The problem next week is picking the right altitude

181.2 kts is my current max speed but I'm hoping for another fraction of a knot by using Gaffer's Tape to reduce the hinge access holes in the tail. The real parameter of interest in this problem is overall ground speed from takeoff to crossing the finish line. I want to go max power and rpm all the way which I have satisfied myself I can do but I do have to really stay on top of the situation to make sure I do not run out of fuel (I have a procedure with a safe abort plan in worst case).

In past AirVenture Cup Races I have done best by staying as low as legally possible because the winds are lowest there and the Dayton west bound are usually headwinds. But this year we have an east bound race and everyone is thinking tail winds. I have been looking at the wind charts and they are showing head winds at the surface and tailwinds on the order of 45 kts at 15,000 ft. With my clipped wingspan I have two concerns one is the climb rate will drop off faster than a long wing as I go higher and there will be extra induced drag to maintain level flight at higher altitudes. I am going with the short wing so I will just live with the extra induced drag (I have done no tests on this). As far as the climb rate is concerned I took a conservative 500 fpm at 100 kts to estimate the climb cost or race performance (no wind effect considered here just a conservative SWAG).

Climb Climb 100Kt Remain.
K ft min. dist dist.
2 2 3.3 402.7
3 4 6.7 399.3
4 6 10 396
5 8 13.3 392.7
6 10 16.7 389.3
7 12 20 386
8 14 23.3 382.7
9 16 26.7 379.3
10 18 30 376
11 20 33 373
12 22 36.7 369.3
13 24 40 366
14 26 43.3 362.7
15 28 46.7 359.3

Now for the distance remaining the wind is considered with a nominal 180 kt cruise (another ballpark number for prerace decision making). At 2K ft I get there quick and get into cruise mode fast but I have a long way to go (402.7 nm) into what now would appear to be a 10 kt headwind. So my predicted speed for the remainder of the course is 170 kts ground speed. If I check my table for 2K ft and 170 kts GS it shows 144.129 minutes. At 15K ft it takes 28 minutes to reach cruise altitude but the remaining distance is 359.3 nm and if the predicted wind is a direct tailwind of 40 kts then I can go to the 15K ft row and 220 kt GS column and read the predicted race time of 125.991 min. and see that under those conditions 15K ft is a better race altitude than 2k ft.

Race times (minutes) with 2k ft - 15k ft alt with cruise GS at 170 kts
144.1294118
144.9294118
145.7647059
146.6
147.4
148.2352941
149.0705882
149.8705882
150.7058824
151.6470588
152.3411765
153.1764706
154.0117647
154.8117647


Race time with cruise GS at 220 kts
111.8272727
112.9
114
115.1
116.1727273
117.2727273
118.3727273
119.4454545
120.5454545
121.7272727
122.7181818
123.8181818
124.9181818
125.9909091

This is a first cut tool that does not address the obvious speed gain descending from 15 K ft, etc. but it is a start.

Bob Axsom

 

[Christopher Murphy]

nice data

Bob,
Not being as tech savy or an engineer, I have used a similar approach for the last 6 races. I did alot of testing when I had my Sensenich FP prop restricted to 2600 rpm. I had good data for that prop.

I have a different prop (Catto) now and have not done much testing with it except to run it in the races. I have noticed that my metal prop was better at higher altitudes than the lower pitch catto is. My unknown quantity is the point at which I will be better off altitude wise considering the performance loss of the prop and engine horsepower.

My hope it that the winds at 9-11,000 feet will make the above a mute point.

by the way,

my performance matrix is created the morning of the race with an E-6B

RACE 34

 

[Bob Axsom]

Basically that is my approach as well

Chris, with this straight line one leg race it is fairly easy to run all the practical options out ahead of time in this matrix and react to the winds of the day. But basically it is the same process. Hope all goes well the weather looks good for the flight into Mitchell on Friday. I will work some more on the preparation but except for taping the holes in the tail I'm ready to go. See you there next weekend and good luck in the race. 9 to 11 thousand looks pretty good based on what I'm seeing lately.

Bob Axsom

 

[Howie]

Bob,
I've followed your modifications and admire your methodical approach to making your plane faster. After reading this thread I had a bunch of questions, which you need not answer, but here they are.

How long does it take for you to shed airspeed gained in a descent? Does the speed gained make up for the climb to altitude? If you did go to 15,000 for a large tailwind, how long can you stay there for the large shove from behind before starting your descent?

What about turbulence? If the wind changes direction that much, at what altutudes will you be shoved around without any gains? Would that (turbulence) preclude operation ,at the high speed you would normally expect on the descent if in still air?

Questions and choices... anyway, thank you for the early morning brain twisters and good luck in the race.
Respectfully,
Howie

 

[Bob Axsom]

Bob,
How long does it take for you to shed airspeed gained in a descent? Does the speed gained make up for the climb to altitude? If you did go to 15,000 for a large tailwind, how long can you stay there for the large shove from behind before starting your descent?

What about turbulence? If the wind changes direction that much, at what altutudes will you be shoved around without any gains? Would that (turbulence) preclude operation ,at the high speed you would normally expect on the descent if in still air?

Howie

Actually the descent effects need to be added to the table.

It seems like the plane slows to cruise speed very quickly after leveling out with no sustained effect from the previous high speed.

My gut feel is you can never get back all of the time lost in the climb by descending.

I have a portable oxygen system if needed so I can stay at altitude as long as necessary myself. The thing I have to consider is the best rate of decent to get the most out of the potential energy acquired in the climb. I'm thinking something like 100 fpm down to the 200 ft AGL finishline requirement. I need to work out the best rate of descent yet but some testing years ago indicated a slow rate of descent is best.

Turbulence has seldom been a problem for me personally but if it becomes a serious concern I will have to slow down below the yellow arc and search for smoother conditions at a different altitude. It certainly can be a problem.

Bob Axsom

 

[Christopher Murphy]

questions

Howie,
The points you raise in your questions are the points that make cross country racing interesting and not just another trip from point a to point b.

Closed course racing involves variables with the airplane and its compontents. ie. One prop vs. another on any given day or how the engine is tuned. On the cross country race a large part of it is getting the most out of what you have. Using data like Bob's and knowing where your plane performs the best. Flying a tight course and executing your plan can make your performance better than planes that are all out faster.

RACE 34

 

[Howie]

Bob and Chris,
Thanks for the replies. Your choices are what the race hinges upon, the different choices make the race.
Can you do the runup,shutdown, get towed to the runway, fuel up then take off? Bob, I won't ask for your secrets, but the tip fuel would make me feel better. What about fuel from undergound tanks vs. the truck (temp. of fuel)?
The large tailwind, depending on the shear layer is tempting but, I think 9500 would be my limit and except any tailwind I could get. That long shallow descent to the finish seems like the hinge pin of the race to me.
How about a traffic avoidance device and watch for the aircraft in front and try to dope out what he/they are getting/doing. If I got passed, I'd know the error of my thinking/choices.
I'd also try to get a pirep from somebody aproaching Dodge for actual winds aloft vs. forcast. Is this legal?
I can see the attraction of this type of event, the hindsight/disection will be lots of fun. Another item, what about taping the canopy flange seam after you get in?

Again,best wishes.
Howie

 

[tin man]

Actually the descent effects need to be added to the table.

It seems like the plane slows to cruise speed very quickly after leveling out with no sustained effect from the previous high speed.

My gut feel is you can never get back all of the time lost in the climb by descending.

I have a portable oxygen system if needed so I can stay at altitude as long as necessary myself. The thing I have to consider is the best rate of decent to get the most out of the potential energy acquired in the climb. I'm thinking something like 100 fpm down to the 200 ft AGL finishline requirement. I need to work out the best rate of descent yet but some testing years ago indicated a slow rate of descent is best.

Turbulence has seldom been a problem for me personally but if it becomes a serious concern I will have to slow down below the yellow arc and search for smoother conditions at a different altitude. It certainly can be a problem.

Bob Axsom

A very wise and accomplished test pilot and world record holder told me that your best descent rate will be a constant rate that puts you at VNE at the finish line. It'll take a lot of trial and error but should be worth the effort. Of course you know all about different climb profiles right?
tm

 

[Bob Axsom]

Answers - as much as I can

Bob and Chris,
Thanks for the replies. Your choices are what the race hinges upon, the different choices make the race.
Can you do the runup,shutdown, get towed to the runway, fuel up then take off? Bob, I won't ask for your secrets, but the tip fuel would make me feel better. What about fuel from undergound tanks vs. the truck (temp. of fuel)?
The large tailwind, depending on the shear layer is tempting but, I think 9500 would be my limit and except any tailwind I could get. That long shallow descent to the finish seems like the hinge pin of the race to me.
How about a traffic avoidance device and watch for the aircraft in front and try to dope out what he/they are getting/doing. If I got passed, I'd know the error of my thinking/choices.
I'd also try to get a pirep from somebody aproaching Dodge for actual winds aloft vs. forcast. Is this legal?
I can see the attraction of this type of event, the hindsight/disection will be lots of fun. Another item, what about taping the canopy flange seam after you get in?

Again,best wishes.
Howie

The start is controlled by the race organization under the leadership of Eric Whyte. There is a morning briefing and the pilots go to their planes and wait for the engine start direction from officials. The planes are directed to take off in fastest to slowest class order to minimize passing (and very little passing actually occurs). The taxi into position and hold and release for takeoff are given by personnel adjacent to the runway hold position. The planes generally start the takeoff roll when the plan ahead rotates up off of the runway.

Fueling is to be done before the morning briefing so you are ready to go as soon as the briefing is over. I would like to have the tip tanks on as well but the plane is a full 3 kts slower with them on. It will require much morecareful fuel management to complete the race without them.

Altitude selection and point of descent and rate are critical. In the 2005 AirVenture Cup Race. I interpreted some information from FSS when I called from the motel before the race and decided to climb to 8,500 ft for the race. as I was climbing I could sense this was a very bad idea and it was confirmed by a significant head wind after leveling off. I descended of course but the damage was done. Since then I have tended to fly at minimum altitude.

Some may have outside data but in our case it is just me and my wife and the Bluebird - I listen to ATIS and 122.0 at altitude but if yo are 500 AGL there's not a lot of helpful info. This race will be different.

See you in Dayton next year?

Bob Axsom

 

[Bob Axsom]

A very wise and accomplished test pilot and world record holder told me that your best descent rate will be a constant rate that puts you at VNE at the finish line. It'll take a lot of trial and error but should be worth the effort. Of course you know all about different climb profiles right?
tm

That does indeed sound like wise advise and I am listening. I do not know about different climb profiles. Many years ago when I worked at McDonnell I read in the company paper about the climb profile in the Streak Eagle operation when the F-15 set several Time-to-Climb records but I am not knowledgable of ones that would be a benefit to me. I'm listening though.

Bob Axsom

 

[MCA]

Bob,

These data are useless unless you can take them out to a few more decimal places!

Have a great race. Wish I could be there this year.

 

[hevansrv7a]

This confuses me

A very wise and accomplished test pilot and world record holder told me that your best descent rate will be a constant rate that puts you at VNE at the finish line. It'll take a lot of trial and error but should be worth the effort. Of course you know all about different climb profiles right?
tm

Maybe this works, but I don't see how..

In my experience and, I think, in my theory, a constant descent rate would produce a constant indicated airspeed. Faster airspeed requires more horsepower although in severely diminishing proportion - the cube root and all that. I have no opinion on the best way to fly a race, just trying to understand this thread. So it seems to me that while faster is better, it takes exponentially more energy. The total energy is your THP plus your additional power from gravity. OK, so as you descend you have more power available and your indicated airspeed gets closer to your true airspeed. Won't that mean that for a given indicated airspeed you will be going slower as you get lower?

So, bottom line, will an airplane at constant (full) throttle go faster as it descends, given a constant rate of descent? Or will you have to keep pushing the nose down if you want to accelerate?

Safety Note: Remember that Vne is valid for TAS, not IAS. See flutter article by Van in the RVator a while back.

 

[Bob Axsom]

I wish you were too Marc

Bob,

These data are useless unless you can take them out to a few more decimal places!

Have a great race. Wish I could be there this year.

It should be a very good race. I looked through my 2006 AirVenture Cup Race Program and checked out your airplane. Putting that IO-390 in it jumps you into a very hard class but what the heck that gives you a higher target to aim at and it is nice to have all that power for the rest of the year. I'm not sure where you live but check out the www.sportairrace.org web site for other cross country air races this year. Off the top of my head there is one in Wichita on August 22 & 23; one in Sherman, TX on October 4; one in Memphis on October 18; and one in Taylor, TX sometime in November.

Bob Axsom

 

[MCA]

We've been exhibiting at OSH in 07 & 08 so that precludes doing the race on Sunday, unfortunately. But it was a blast.

Frankly, Bob, I'd like to petition someone to get the -390 engines considered in the same class as the -360s because there were several -360s in the AVC that had more HP than a stock -390, and were able to race in a competitive class with other RVs rather than against Glasairs, etc. No time nowadays but perhaps in the future.

 

[tin man]

Maybe this works, but I don't see how..

In my experience and, I think, in my theory, a constant descent rate would produce a constant indicated airspeed. Faster airspeed requires more horsepower although in severely diminishing proportion - the cube root and all that. I have no opinion on the best way to fly a race, just trying to understand this thread. So it seems to me that while faster is better, it takes exponentially more energy. The total energy is your THP plus your additional power from gravity. OK, so as you descend you have more power available and your indicated airspeed gets closer to your true airspeed. Won't that mean that for a given indicated airspeed you will be going slower as you get lower?

So, bottom line, will an airplane at constant (full) throttle go faster as it descends, given a constant rate of descent? Or will you have to keep pushing the nose down if you want to accelerate?

Safety Note: Remember that Vne is valid for TAS, not IAS. See flutter article by Van in the RVator a while back.

In my experience your indicated will not stay the same. You can try it yourself. Start at 8k or so, do a gentle push over, maintain ROD, and watch your airspeed climb.
YMMV
tm

 

[tin man]

That does indeed sound like wise advise and I am listening. I do not know about different climb profiles. Many years ago when I worked at McDonnell I read in the company paper about the climb profile in the Streak Eagle operation when the F-15 set several Time-to-Climb records but I am not knowledgable of ones that would be a benefit to me. I'm listening though.

Bob Axsom

This is from Klaus Zavier who is no stranger to cross country races. There are two objectives in racing, getting to altitude and getting downrage. Even Catto propellers don't have constant thrust curves so one might find that with thier propellor/engine/airframe combination that climbing at vy + 10 or 20or more will get you downrange and at altitude quicker than a straight vy climb. This will be different for each airplane. In my testing with my propeller/engine/airframe combination that number turned out to be pretty close to the carson speed number. Other RVs were not. YMMV
tm

 

[hevansrv7a]

OK, I'll try it, but..

In my experience your indicated will not stay the same. You can try it yourself. Start at 8k or so, do a gentle push over, maintain ROD, and watch your airspeed climb.
YMMV
tm

If your ROD is constant and your TAS is increasing (hypothetical) then geometry suggests your angle is getting flatter. But that's the TAS.

If as you descend your IAS increases then it could mean that your TAS is staying the same. As I said in mine, they will converge as you get closer to standard conditions. That still won't get you to Vne, will it?

If the TAS and ROD stay the same then the horsepower added by recovering the energy stored in the climb will be constant, too. The available engine thrust will increase as the air density increases, but so will the parasite drag. Our airplanes go fastest on 75% at the point where the air is thinnest but still able to support that power. Therefore at least some of the additional thrust from being lower/denser will be consumed in drag. Maybe the TAS will increase but I'm still wondering.

On the other hand, the max speed is usually measured at sea level so there is some support for the idea that you go faster lower down. But that doesn't resolve the ROD issue.

Since I have a GRT, I can watch them both. Perhaps I'll remember to do it. And if so, I'll report back.

 

[tin man]

Well I was lucky to get thru geometry in high school and lots of numbers seem to confuse me , I'm pretty sure that anytime you lower the nose you're gonna pick up speed.
tm