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Phil & Margie Hodge
New member
Username: N410mp
Post Number: 20
Registered: 01-2000
| | Posted on Monday, June 14, 2004 - 08:47 pm: | |
A less scholarly answer to the original question:
If the weather is favorable I usually fly my CT IO550 MT 4 blade at around 9000', 23 squared, 150 KTAS, 14 gph. She'll go a lot faster, but at 2300 rpm she's efficient, smooth and quiet. |
Reinhard Metz
New member
Username: Reinhard_metz
Post Number: 41
Registered: 03-2003
| | Posted on Tuesday, June 15, 2004 - 10:20 am: | |
Sorry folks, I just can't resist, after reading this discussion. The other day I ran across an interesting web page defining the "seven levels of photographers":
http://www.kenrockwell.com/tech/7.htm#measurebator
Check it out - I suspect one could do a direct translation to pilots/kitbuilders - The category of "measurebator" surely exists in our realm as well! |
michal
Unregistered guest
| | Posted on Tuesday, June 15, 2004 - 11:30 am: | |
Thanks for everyone's input.
I figure an aircraft that has a ceiling of 20,000 is probably going to fly fastest and most efficiently at some higher teens. Just a common sense.
On the subject of whether the 'fastest' altitude is also the best economy altitude I suspect again they are fairly close. I happen to have Piper Merdian's POH and for this aircraft the fastest altitude is about FL250 but the best economy is around FL300. But esentialy there are only a few knots of difference between its peak speed at FL250 and at higher altitudes. I know Merdian is turbine but I bet the same thing applies to other airplanes - namely their best economy altitude is not far from their altitude for best speed. |
Jim Butler
Unregistered guest
| | Posted on Tuesday, June 15, 2004 - 12:06 pm: | |
When east of the rockies and making a cross country trip of at least an hour, I like to cruise between 8000 to 10,000 feet with an IO-540, naturally aspirated, 300 hp engine. At 9000 feet, I usually am getting 21.5 inches of manifold pressure, 2400 rpm and if you add the headwind component to the gps groundspeed, you almost always come up with 186 knots. Larry Olson had told me the best altitude for this airplane (IO-540 equipped) would be from 8000 to 10,000 feet. I have never done any actual testing to determine what is best. Like Wayne, I didn't build it to go slow, and try to get there as fast as I reasonably can. Being able to pick the best altitude for wind seems to make the biggest difference. You probably could make a graph of the loss in horsepower against the increase in efficiency due to altitude and determine where you want to fly for either speed or economy, based on actual test runs, but I have never taken the time to do it. |
Paul Fagerstrom
New member
Username: Paul
Post Number: 7
Registered: 12-1999
| | Posted on Tuesday, June 15, 2004 - 12:15 pm: | |
Michal,
Ok, here goes my 2 cents.
The CAFÉ assoc. has documented that the CT with a 210 hp engine can cruise at 170 KTAS that was 75% power I believe. When I flew 511 I could get 190 KCAS at density altitude of 5,000 which was full throttle and max rpm (2700 or 2750 I forget now) but the book had it at 75 % power. But that seemed to be a high rpm to cruise at and after talking to many people I still am not sure if it is any worse wear on the engine than when cruising at a lower RPM but 75% power, perhaps we will have a discussion on that.
For your next question of best specific range (nm/per lb of fuel), I can't answer that exactly but I will bet the answer will involve flying at a very slow airspeed that is just impractical. Jim Warner flew from Ca to OSH non-stop in his CT I believe he has 210 hp but may be only 200. The big variables are the engine option, weight of the plane and how aerodynamically the plane is finished. I typically flew at 9 to 10,000 ft and got an airspeed of about 180 KCAS burning 15-17 gph depending on how the engine temps looked I did not like to get them too hot and errored on the side of caution. I chose that altitude because it was very easy to get to and still gave me good speed, Larry usually flew at 12,000 on long trips and I believe about 175 KCAS. By the way I am calling KCAS what I read on the airspeed indicator, after I compensated for altitude & temp.
Reinhard, I think your right.
Michal, now that I typed the above I just read your latest post. I do not think the 2 are close in normally aspirated engines otherwise range max range in miles would be at fastest cruise, unless I am missing something then I am sure someone will let me know.
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michal
Unregistered guest
| | Posted on Tuesday, June 15, 2004 - 12:22 pm: | |
Jim,
I know, wind is a big part of the equation.
Actually Richard Collins in the latest issue of FLYING offers some wisdom on the subject of high flying:
Most normally aspirated airplanes find their best crusing speed at the highest density altitude where the engine will still generate the percentage of power used for cruise.
He also writes:
.. A better tailwind or smooth air is the only reason to vault up into the flight levels. Even then, if the airplane is ot pressurized, it is hardly worth the trouble and expense of breathing supplemental oxygen
But it is still good to know what your airplane would like best. |
Charles M. Robinson
New member
Username: F15epilot
Post Number: 19
Registered: 02-2004
| | Posted on Tuesday, June 15, 2004 - 01:04 pm: | |
Michael,
Your assumptions are a bit broad and might not be as accurate as you assume. There are complete aircraft design courses that address the topic of designing an air vehicle and we don't have time/space on the forum to cover that. Consider an automobile 4-cyl engine that can fit in both a sleek, sports car and into a boxy pickup. As RPM increases against a fixed load resistance, the the fuel flow goes up. Increase the load , and you have to increase fuel flow to compensate and likely increase RPM. Putting the motor in the sleek sports car, means you'll go a lot faster than the boxy pickup, but you eventually hit a horsepower limit for both that will pretty much limit your top speed. Using a dynamometer, you can plot the increase in fuel flow vs. the increase in delivered horsepower, work out the associated RPMs and will probably find that your maximum range speed in the sleek car will be a lot faster than the maximum range speed in the boxier truck (why some cars get far better fuel economy at 80 mph than others--my Toyota Supra gets max range at about 78 mph; my Avalanche shows a marked decrease in mpg at speeds above 70). That's because drag increases roughly by the square of the velocity. Hence, while a little bit more throttle (fuel flow) gets you more RPM to the wheels, as you get faster, that 'little bit more' starts becoming a lot more until the extra throttle doesn't affect your speed. Go take a look at the number of cars that use the same engine and you'll see each has a different economy because each has different gear ratios, aerodynamics, tire sizes, etc.
For planes, it's more complicated (and for experimentals where there are more variables in shapes, even more so). Small changes in shape can make big impacts on aerodynamic forces (the F-15 has a limit of imperfections within the first 9 inches of the nose since at higher AOAs, those imperfections can generate yaw forces of 3 tons or more...yes, I said tons). Weight on the car only is a factor when you're stopping, starting, or going up and down hills--but as lift has to be equal to weight in the plane, continuously generating that lift results in continuous induced drag. Wing size and shape are chosen for where and how the plane will operate (aerodynamics to generate lift) but also to hold fuel, gear, and lights. The prop you choose isn't like a tire on the road in translating RPM into forward propulsion--it has the same physics as a wing to generate lift, but a few others from the fact it's rotating and translating at the same time. Hence, there are a lot more dynamics between increasing the throttle and seeing the A/S indicator increase. Adding variable pitch (constant speed props) complicates that even more; ditto for adding a turbocharger or comparing a turbine to a piston. The power curves are very different. The 'turbo' engines can operate higher than normally aspirated piston engines, and consequently their power curves extend a lot further than a NA motor. That allows them to get up into the thinner air where drag is reduced.
Referencing a Beech Siera POH, the 75/65/55% max cruise speed altitudes are 7500/9000/12,000 feet at 163/153/140 mph, burning 12.1/9.1/7.9 gph, respectively. Checking the range charts will show you that your max range in the Siera comes by flying anywhere between 6 and 9000 feet at 55% power, and drops off if you fly higher (no wind). But as others mentioned, you do need to consider wind. Into a 100 mph head wind at 9000 feet your ground speed would be only 40 mph making the miles/gal of fuel a lot less. If that was a 100mph tailwind, then you'd have 240 mph ground and going higher would increase the mpg.
Part of the fun of the experimental (to me, at least), is I get to develop that POH and determine those numbers for my plane. And while other planes of similar designs will be 'similar' they will not be the same. I'm looking forward to determining those numbers and finding that 'magic' combination of manifold pressure, RPM and altitude for each weight/configuration that gives me the best performance. As some guys used to say on another aircraft forum, "a knot increase here, a knot there, and pretty soon you're going supersonic."
Good luck with your plane decision...I believe you'll find the Express at the top of a very short list when you're done.
Chuck
BTW...the Siera service ceiling is 14,350'...and the best thing that happened to me in buying my first plane was only getting the POH (a gift) and not getting the plane
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Jim Butler
Unregistered guest
| | Posted on Tuesday, June 15, 2004 - 02:30 pm: | |
To throw in a bit of completely useless information, the cars at Indy this year were generating somewhere around 850 horsepower, but "only" had straight away speeds of around 230-240, with lap speeds of around 220, plus or minus. The Express, with only 300 hp, has straight away speeds of around 213, but doesn't have to create enough downforce to be able to turn left at the corner. Big difference in drag.
By the way, when I am in cruise flight, I lean the engine until the hottest cylinder is at about 380-390 degrees. At 9000 feet, this usually is around 16 gallons per hour. I have GAMI injectors, but when I go lean of peak, I lose so much horsepower and airspeed that it isn't worth the fuel savings to me. Like Wayne said, we didn't build these airplanes to go slow. |
Douglas Hoff
New member
Username: Dhoff
Post Number: 15
Registered: 05-2003
| | Posted on Tuesday, June 15, 2004 - 04:00 pm: | |
I see that Phil & Margie's plane is equipped like mine (IO 500 with MT 4 balde prop). Normal cross country speed for me (GPS) is about 150-165K, depending on winds. Is this about the same for you Phil? I usually find that I can burn about 11.9 to 12.2 gph by slowing to 2050 squared, and lose very little speed according to the GPS, so if I'm not in a hurry this is what I do. Is it any easier or harder on the engine to go at lower RPMs? Is there an optimum range for engine life? |
michal
Unregistered guest
| | Posted on Tuesday, June 15, 2004 - 04:52 pm: | |
Your assumptions are a bit broad and might not be as accurate as you assume.
Charles,
Not sure what assumptions. Actually I made no assumptions except the one when I stated that most likely there won't be big difference between the best altitude for fast cruise and the best for range (though power settings clearly will be different in both cases). So far I see nothing that would prove me wrong. I am actually susprised that with so many Express owners no one yet tabulated results and effectively put together their own POH with perfromance charts. But I understand that supplemental oxygen makes the whole enterprise difficult. And since this is not a certified production aircraft there is no factory-provided POH. I understand all that.
Best to all,
Michael J. |
Charles M. Robinson
New member
Username: F15epilot
Post Number: 20
Registered: 02-2004
| | Posted on Tuesday, June 15, 2004 - 08:59 pm: | |
Michal,
Wasn't trying to disagree with you or say you were wrong in any way. In fact, I fully agree that it would be nice to see some examples of POHs for our Express aircraft to include performance charts. I believe the POH project has already been addressed in the archive and there is one avaialbe for the CT version sans performance charts. Perhaps we could archive performance data for both a CT and an FT/Auriga/Series 90 with a 210hp (IO-360), 250hp (IO-540), 300hp (IO-540-K) and 310hp (IO-550). Normalizing the performance charts to a baseline would be the most difficult task to account for different aerodynamics (finish, fairings, weight) and physical factors, especially the prop. Perhaps a simple data base to start where owners could post their performance numbers based on the above categories with a few sub-categories for 2-, 3-, and 4-blade prop. We already have Jerry's CAFE data for one configuration as a start.
I'll be happy to send you a copy of my flight test info for my Auriga with a Hartzell Q-tip 3-blade and IO-540-C4B5.
As for the assumptions I mentioned, I wasn't trying to say any were wrong, just potentially inaccurate. For example:
1) ... a ceiling of 20,000 is probably going to fly fastest and most efficiently at some higher teens. Not necessarily. Depends on the determinant for the ceiling. For example, without pressurization, flying above 25,000 feet requires pressure breathing, not just supplemental O-2. For kits and most GA planes, the manufacturer probably won't test the plane to an aerodynamic limit that high (if the plane can get there) since there's little likelihood it will be flown there. Hence, the 'ceiling' of the plane published by the manufacturer will probably be lower than the aero limit. For that matter, I've talked to many AA-1 owners who've STC'ed their planes with larger motors and tail wheels. Those planes will easily climb (reported from actual flight) above 15000. Yet their max range cruise altitudes are in the 7 to 9K range.
2) ...the 'fastest' altitude is also the best economy altitude. From the Sierra example I gave, it depends on your definition of close and which way the wind is blowing. The maximum True airspeed cruise altitude is 7500', while the maximum range altitude is at 55% power between 6000 and 9000: a range of +/- 20%. Now if you want fastest Indicated A/S, you'll need to do that on a cold day, at sea level in calm weather and on the deck.
3) .. turbine but I bet the same thing applies to other airplanes. You really can't compare a turbine to a piston engine. They are two separate animals, even if you're comparing them on the same airframe. The turbine engine's power curve is sloped differently and peaks at altitudes well above a NA piston engine.
Again, I will be glad to share my data with you and anyone else interested. The gentleman who is purchasing my Auriga's sister plane (the previous owner built a CT and an FT before starting the project I'm finishing) and I plan to do a similar data exchange to give each of us an idea of how our planes are performing.
Regards,
Chuck |
wayne
Unregistered guest
| | Posted on Wednesday, June 16, 2004 - 01:12 pm: | |
WOW, to many #s and variables, we all have a different set up,so untill everyone that is FLYING an Express would just post a TRUE airspeed figure at max weight at 5000' density altitude and 75% power we could talk all day and still not be able to say who's set up is the best. and that still won't mean anything.
BTW, when I flew cancelled checks, speed was everything, we ran the snot out of the planes and engines. they like it fast,wide open, 26/26 we didn't crack jugs and they would make TBO. The guys that try to baby their engines usually end up doing more harm than good.
Get busy building, you could of had the stab closed by now. |
Reinhard Metz
New member
Username: Reinhard_metz
Post Number: 42
Registered: 03-2003
| | Posted on Thursday, June 17, 2004 - 02:26 pm: | |
OK, stop measurebating, and start flying/building! or, as the kid in the first episode of "Six Feet Under" said, "OK, zip down your flys, get out the ruler, and let's get this over with!" |
Michael Jastrzebski
New member
Username: Michal
Post Number: 13
Registered: 03-2004
| | Posted on Tuesday, June 29, 2004 - 12:39 am: | |
Ok, I think I finally got my original question answered. I simply managed to get a hold of a few pages from Mooney Ovation's POH. Ovation's engine is very similar since it is normally aspirated 280 HP (derated from 300HP I think).
Based on the charts - for best true air speed sea level is best but you would be burning fuel like crazy.
For best range - the higher the better. At 20,000 Ovation can fly almost 28% farther then at 10,000. |
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