G8-2-130r

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xxbgxx
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G8-2-130r

Post by xxbgxx » Sat Aug 06, 2005 10:58 am

hi!
can anyone one tell me how much hourse power the G8-2-130r and the G8-2-80 makes
and how i can turn the pis to hp

Al Belli
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re: G8-2-130r

Post by Al Belli » Sat Aug 06, 2005 3:09 pm

Hi,

The engine will produce no horsepower, when it is at rest, since horsepower is defined as moving a mass through a distance in a specific amount of time.

The horsepower equivalent for a jet engine at rest can be derived by using the choked nozzle velocity of the exhaust at the local speed of sound, and the pounds of static thrust.

Equivalent HP ( static ) = thrust + ( 0/375 x thrust )
Then we have: 1Lb. = 1 Hp. static equivalent

For dynamic thrust the speed of the vehicle is used with the static thrust.

HP ( dynamic ) = thrust + ( speed in MPH/375 x thrust )

Example:

A Dynajet with 4.5 pounds of static thrust ( 4.5 Hp. equivalent )
is powering a plane at 200 MPH.

Dynamic Hp. = 4.5 + ( 200/375 x 4.5 )

= 4.5 + ( 0.5333 x 4.5 )

= 4.5 + ( 2.3999 )

= 6.9 Hp. ( dynamic equivalent. )

The formula was derived using information from Irwin Treager's book.

See the attachment.

Al Belli
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racketmotorman
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re: G8-2-130r

Post by racketmotorman » Sat Aug 06, 2005 10:43 pm

Hi Al
The only problem I have with your answer is , what is the actual thrust of the engine at the speed you are using , is it the same as static , or has ram air had an effect , and the forward speed of the aircraft changed the net :gross thrust equation .
4.5 lbs at 200 mph will only be the equivilant of your 2.4 hp of work done , can't quite get my head around the 6.9 hp you mention .
Cheers
John

Al Belli
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re: G8-2-130r

Post by Al Belli » Sun Aug 07, 2005 12:04 am

Hi Racket.....,

I am assuming 1 Hp./ Lb. of thrust. The increase in apparent Hp. is added to the static ( assumed ) Hp. to get the figure I stated.

From airplane ( speed ) racing experience, the 6.9 Hp. figure is much closer to the value expected from a 2- stroke racing engine at 170 to 200 MPH.

The whole exercise is based on a " touchy - feely " desire to equate apples to oranges.

If we consider the heat released by the operation of the pulsejet, we can equate it to horsepower.
746 Watts ( Joules ) per second = 1 Hp./ Sec. = 0.7062 Btu. / Sec.
Gasoline = 20,750 BTU. / Lb.
A dynaJet consumes 0.03 Gal. / Min. or about 0.2 lb. / Min.
This is about 4,150 BTU. / Min.
424 BTU. / Min. = 1 Hp.
4,150 / 424 = 9.79 Hp. at 100 % energy conversion.
How much heat, released by a pulsejet, is used to propel the combustion products through the system, and out the tailpipe ?
Assuming 50 %, the apparent Hp. is about 4.9 Hp.

This might be refined by searching the literature available to determine the estimated energy efficiency of a pulsejet.

Al Belli

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re: G8-2-130r

Post by racketmotorman » Sun Aug 07, 2005 3:36 am

Hi Al
Please correct me if I'm wrong :-)
but a prop producing thrust in model scalings will only be converting horsepower to thrust at about 75% efficiency max , whereas a lb of thrust is still a pound of thrust no matter what ,and doesn't have the same "conversion" problems.
When using your BTU consumption to convert into horsepower you need to factor in the fact that most of your fuel burn will be simply used to heat the exhaust gases , very little will actually be turned into useful "work" because of the inherent low compression ratio and poor thermodynamics , your 50% would be way off , an auto engine is lucky to achieve 20-25% conversion , the very best large capacity power generation plants can only approach 50% thermal efficiency .
Assuming your 0.2 lbs/min fuel burn ,then you'd need 0.2X15 =3lbs/min of air , assuming peak combustion temps of 3,000 deg F , Sph of air 0.24
Therefore 3 lbs/m X 3000 x 0.24 =2164 BTUs /min to simple heat the air required for combustion .
Assuming mass flow is 3.2 lbs/min or 0.053lbs/sec and thrust 4.5lbs then you'd need an efflux velocity of around 2700 ft/sec, which would be impossible, therefore the mass flow needs to increase which means more air needs to be heated , but as the fuel flow is fixed the extra air simply brings down the peak combustion temperature to a point where the mass flow times the efflux velocity give your thrust level .
The increased mass flow will mean more of your btu's will go out the exhaust pipe as simply hot air without doing any work .
I hope this makes sense :-))
Your thoughts ..
Cheers
John

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re: G8-2-130r

Post by xxbgxx » Sun Aug 07, 2005 7:06 am

hi al and racket
hmmmm i got lost lol
so can you tell me how much the G8-2-130r and the G8-2-80 hp it makes
thnx alot

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re: G8-2-130r

Post by Al Belli » Sun Aug 07, 2005 12:34 pm

Hi,

Thrust is the important quantity for propulsion.
Horsepower is best applied to rotating systems.
I think that trying to equate the two for pulsejets is going to be very frustrating, and basically will require an extensive study by a thermodynamicist. Way too much calculus for Me !!!!

Al Belli

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re: G8-2-130r

Post by Dave » Sun Aug 07, 2005 4:06 pm

Guys
This is how Gene described the conversion in his "Technical Manual" on his G8-2 engines.

"Horsepower versus Jet Thrust
Most people are used to evaluating power on the basis of an engine's horsepower rating. However, an engine that produces pure thrust (rather than torque at a rotating output shaft) is rated in pounds of thrust. Jet thrust cannot be stated in horsepower, unless a specific speed is given. By calculation, the horsepower rating of any jet engine operating in a static condition is zero, regardless of the amount of thrust. It's possible, however, to make comparisons on the basis of tests performed on conventional power plants in similar applications. This would be referred to as "Equivalent Horsepower" (HP1).

For example, if tests on a conventional reciprocating engine showed that a 10 hp engine with a fixed-pitch propeller produced a static thrust of 50 pounds at full throttle, then a value of 5 pounds of thrust per horsepower might be used. Since the G8-2-40 engine develops 40 pounds of static thrust at full power, HP1 would be 8 hp. The shortfall of this approach, however, is that the thrust of a conventional engine/propeller system declines with increased speed, whereas the thrust of the G8-2 jet increases with speed (using an air scoop). So the two power systems are not directly comparable (click to see graph). In addition, one would have to be confident that the comparison horsepower value was actually representative of a typical ICE/propeller power system...

...If a G8-2-40 jet were moving at 400 mph (586 ft/sec), the foregoing equation would result in 42.7 hp (HP = 40 x 586/550 = 42.7). The same engine moving at 600 mph (954 ft/sec) would result in 69.5 hp (HP = 40 x 954/550 = 69.5). In order to achieve meaningful results, however, the value for velocity must be appropriate for the particular application. In other words, the velocity must be based on the result of the particular thrust acting to overcome the resistance of a particular body. The speed at which the two factors become equal (thrust and resistance) will be the maximum speed attainable in the particular application. This would be the appropriate velocity to use in the foregoing equation."

Sorry but the formulas, etc would not paste properly, but hopefully this help.

I would also suggest that you run this past Luc and Viv to get their take on it as they are working full time with related concepts.

Dave

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re: G8-2-130r

Post by racketmotorman » Sun Aug 07, 2005 10:10 pm

Hi Dave and Al and xxbgxx
It is a complex subject with no simple answers I agree , in full sized aircraft any residual jet thrust on a turboprop has thrust in pounds divided by 2.5 to get an estimated shaft horsepower which is then added to the actual shaft horsepower , this 2.5 :1 factor is used even though the normal cruise speed of a turboprop is getting close to the point where one pound of thrust should be worth one horsepower, but other factors are coming into play by then such as the reduced difference between jet efflux velocity and airspeed into the engine, and altitude .
The problems with only measuring static thrust with a fixed pitch prop is the fact that they are compromises , you can set them up for lotsa pull at low airspeeds or static with "fine pitch" ,but at high airbourne speed you need a "course pitch", which if tested statically, would give an inferior reading to the finer pitch.
What is very obvious to everyone is the fact that "horsepower" will get you moving faster initially , but "thrust" will generally have a better top speed , "all things being equal ".
Having said that, all my experiences with home made turbines have shown that if the "pure thrust" engine is put thru a freepower turbine and turned into "horsepower" you go faster at both ends of the spectrum .
It really depends on the "vehicle" size and shape , its drag coefficient especially in relation to the thrust output for frontal area,
A 4.5 lb thust engine could go 200 mph if it was the only thing moving, but in an aircraft with a lot of both structual and induced drag from creating lift , it wouldn't go anywhere near 200 mph.
A 100 lb thrust pure jet engine mounted in a kart will get the kart and driver up to maybe 70 mph before drag equals thrust , that same 100 lbs of thrust put thru another turbine and turned into horsepower will yield around 140 hp and could propel that kart and driver to double that 70 mph very easily, and with an enourmous advantage in acceleration.
Unfortunatelly ,xxbgxx, theres no simple answer to your query :-(( the pure jet thrust will sound more exciting though than boring shaft hp :-))
Cheers
John

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re: G8-2-130r

Post by Viv » Mon Aug 08, 2005 3:31 am

Well the oppinon of Conception GLC is that if you compare 1Lb of apples and 1Lb of oranges using a typical kitchen scale you will find that allthough they both waigh 1Lb they are in fact still two differant things ie apples and oranges.

You can play with the numbers or try using a metric system if you like but when it comes down to it you have to rememmber what it is you are comparing and what terms of referance you are using.

In the above experiment we found that the apples still tasted like apples and the oranges still tasted like oranges even though the scales said they were the same thing.

In our next experiment we will compare the thrust of our 4.7 litre V8 Dodge ram 1500 pickup truck to a Rolls Royce Derwent jet engine.

Viv:-)

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Re: re: G8-2-130r

Post by Mark » Mon Aug 08, 2005 4:04 am

Presentation is Everything

xxbgxx
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re: G8-2-130r

Post by xxbgxx » Sun Aug 14, 2005 9:20 pm

hi all
ok i dont know if it true or not in most sites i visted on the web the say every 1lbs of thurst=.5hp

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