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herbgh(at)nctc.com
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 Posted: Sun Feb 21, 2010 11:04 am Post subject: Valley Engineering Big-Twin - Any one flying with one? |
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12 hp per gallon per hour.... pretty good liars formula...Pretty
accurate estimation...Herb
At 11:00 AM 2/21/2010, you wrote:
| Quote: |
2 gph x 6 lbs/gal / .48 bsfc = 25 hp
2 gph x 6 lbs/gal / .42 bsfc = 28.57 hp
--------
Thom Riddle
>>>>>>>>>>>>>>
So using these numbers the rotax 912ul burning 4 gph is only producing at
best case 57.14 hp and worst case 50 hp. Lets say 53.57 for the average.
Add 7000 ft of altitude with a loss of 4.25 % per thousand. For a loss of
29.75 %. leaving at altitude 37.63 hp at cruse. Don't sound right. Or
is the 4.25 % per thousand wrong? I have had my mkIII912 to 13,000 and
using the same numbers the loss is 55.25% leaving only 29.6 hp are these
numbers essentially correct or am I missing something?
Boyd Young MkIII
Checked by AVG - www.avg.com
02/21/10 07:34:00
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Dana
Joined: 13 Dec 2007
Posts: 1047
Location: Connecticut, USA
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| Posted: Sun Feb 21, 2010 5:11 pm Post subject: Valley Engineering Big-Twin - Any one flying with one? |
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At 07:59 AM 2/21/2010, Thom Riddle wrote:
| Quote: | --> Kolb-List message posted by: "Thom Riddle" <riddletr(at)gmail.com>
The early FS w/ 377 I once owned, had plenty of climb performance.
One way to guestimate the actual horsepower being delivered from a 4-stroke engine is by knowing the actual fuel consumption rate of the engine under power. Most 4-stroke engines burn in the range of .42 - .48 pounds of gasoline per hour... |
Another way to calculate the actual thrust horsepower (including any propeller efficiency loss, which is significant) is to measure the power off rate of sink, and the full power rate of climb, t the same airspeed. Add these two rates together, divide by 60 (to convert feet/minute into feet/second), multiply by the aircraft weight, and divide by 550. The result is the actual THP your engine/prop combination is producing. The actual engine shaft horsepower is probably around 50% more than that, assuming about 66% prop efficiency.
-Dana
--
Always keep clothes and weapons where you can find them in the dark. [quote][b]
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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Mon Feb 22, 2010 6:52 am Post subject: Re: Valley Engineering Big-Twin - Any one flying with one? |
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...The result is the actual THP your engine/prop combination is producing. The actual engine shaft horsepower is probably around 50% more than that, assuming about 66% prop efficiency.
-Dana ...
Two questions, Dana:
1) By "power off" do you mean idle power or engine stopped?
2) Do you mean the result is the thrust horsepower at that airspeed at full throttle climb? or some other specific regime?
My Slingshot at 65 mph climb and idle power glide are 900 fpm during normal summer density altitudes. My typical flying weight is 825 lb.
900+900=1800
1800/60 = 30
30x825/550= 45 THP
45 THP x 2 = 90 BHP
With engine off and prop stopped the descent rate goes up to 1200 fpm.
1200+900=2100
2100/60=35
35x825/550=52.5 THP
52.5 THP x 2 = 105 BHP
My 80 hp at 3,300 rpm Jabiru engine produces about 75 hp at WOT climb at 3,000 rpm at 65 mph at sea level, a bit less during summer.
Maybe my prop is a lot more efficient than your assumption. Your thoughts?
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Thom Riddle
Buffalo, NY (9G0)
Don't worry about old age... it doesn't last very long.
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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Mon Feb 22, 2010 6:55 am Post subject: Re: Valley Engineering Big-Twin - Any one flying with one? |
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OOOOPS!
I just looked at the last line of each of my examples and realized I multiplied by 2 instead of 1.5 My bad.
45 thp x 1.5 = 67.5 bhp
52.5 thp x 1.5 = 78.75 bhp
Sorry for not proof reading better. Makes more sense now.
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Thom Riddle
Buffalo, NY (9G0)
Don't worry about old age... it doesn't last very long.
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Dana
Joined: 13 Dec 2007
Posts: 1047
Location: Connecticut, USA
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| Posted: Mon Feb 22, 2010 10:15 am Post subject: Valley Engineering Big-Twin - Any one flying with one? |
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Thom, power off would be prop stopped... there is still a significant amount of thrust from an idling engine.
The thrust horsepower (I see you corrected the X2 to x1.5 in the later message) calculated is the THP you're producing in the full power climb. Actually there are some minor variations, since the stopped drag has some small but significant amount of drag that isn't a factor when under power, so to be strictly correct, you'd have to have a fully feathered prop (or removed, but then I guess you'd have to do deadstick glides of a mountaintop!).
The 1.5 factor is from a VERY off the cuff guesstimate of 66% prop efficiency. I have no idea what it really is... but if you calculate as I described and know the exact BHP of your engine, you can calculate the exact prop efficiency (or combined prop/redrive if the engine specs give BHP before the redrive). Sounds like I was pretty close, though.
The power off rate of sink at a given speed times the sink rate is the "power deficiency"; this is the power required to maintain level flight at that speed. Any excess power over that required for level flight, divided by the weight, equals the climb rate, again at that same speed. To be more technically correct, it's at that speed _and_ angle of attack... with a high thrustline there will be trim issues with power that cause changes in drag, for additional inaccuracy... it's an approximation, but as you can see, a pretty good one. That 78.75 is close enough to 75 to be well within the limits of error, even better if you figure the drag of the stopped prop... 3.75HP is 15# drag at 65 mph, which is quite believable.
These both come from the same simple relationship: force X velocity / 550 = HP. It work for weight as the force and vertical speed, or thrust (= drag) and airspeed (speed converted in both cases into ft/s and force in pounds, of course).
-Dana
At 09:52 AM 2/22/2010, Thom Riddle wrote:
[quote]Two questions, Dana:
1) By "power off" do you mean idle power or engine stopped?
2) Do you mean the result is the thrust horsepower at that airspeed at full throttle climb? or some other specific regime?
My Slingshot at 65 mph climb and idle power glide are 900 fpm during normal summer density altitudes. My typical flying weight is 825 lb.
900+900=1800
1800/60 = 30
30x825/550= 45 THP
45 THP x 2 = 90 BHP
With engine off and prop stopped the descent rate goes up to 1200 fpm.
1200+900=2100
2100/60=35
35x825/550=52.5 THP
52.5 THP x 2 = 105 BHP
My 80 hp at 3,300 rpm Jabiru engine produces about 75 hp at WOT climb at 3,000 rpm at 65 mph at sea level, a bit less during summer.
Maybe my prop is a lot more efficient than your assumption. Your thoughts?
--------
Thom Riddle
Buffalo, NY
Kolb Slingshot SS-021
Jabiru 2200A #1574
Tennessee Prop 64x32
I prefer the company of peasants because they have not been educated sufficiently to reason incorrectly.
- Michel de Montaigne
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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Mon Feb 22, 2010 11:06 am Post subject: Re: Valley Engineering Big-Twin - Any one flying with one? |
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Thanks for the clarification, Dana.
I recognized where your were coming from once I saw the 550 figure, a number burned into my gray matter over 4 decades ago. Many ways to skin this cat, and all work pretty well with good numbers.
do not archive.
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Thom Riddle
Buffalo, NY (9G0)
Don't worry about old age... it doesn't last very long.
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