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View Full Version : Thrust (HP) vs. Weight



Dash_C.
12-12-2006, 02:44 PM
With the addition of the Lerche, I was wondering what aircraft have thrust-to-weight ratios approaching unity and how to convert engine horsepower into thrust. Do any prop a/c in the sim (besides the Lerche) come close to 1:1? How is this determined?<div class="ev_tpc_signature">

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DomJScott
12-12-2006, 05:56 PM
It's not a straight conversion of HP to Thrust. It's very dependant on the propeller attached to the engine. As all WWII aircraft cannot go ballistic - none have enough thrust generated by their propellers to overcome the weight of the aircraft.

Helicopters manage it of course but have to use massive propellers to achieve it. The only aircraft with a propeller ( all be it a turboprop) I know of ( barring oddities like hovercars) that can go (technically at least) ballistic is the CV-22 although that's obvously modern aircraft. Note not sure if it CAN go ballistic in a conventional sense but of course it's an aircraft that can use Helicopter technology to do VTOL (which is implicitly ballistic flight ).

To achieve this the CV-22 has a combine effective Horsepower of around 12,000HP, way above WWII aircraft levels.

Dash_C.
12-12-2006, 07:25 PM
Originally posted by DomJScott:
[...]To achieve this the CV-22 has a combine effective Horsepower of around 12,000HP, way above WWII aircraft levels.

However, a Bf-109G has a higher HP to weight ratio than a V-22. I know there are a lot more factors at work here (disc area, etc.), but this is why I'm curious.

I suppose I'm just looking for an explanation of why I can't hang a 109 on its prop.<div class="ev_tpc_signature">

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NekoReaperman
12-12-2006, 07:54 PM
because the 109 requires its wings to assist the engine in lifting the airplane off the ground, that takes a LOT of load off of the engine, you're asking for thrust to counter both drag and weight, and thats a lot to ask... it would take massive amounts of thrust to allow this in level flight, much less vertical


EDIT: BI-1 can do it http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif<div class="ev_tpc_signature">

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WB_Outlaw
12-12-2006, 08:51 PM
Not all of an engine's power is converted to thrust. Besides gearbox losses, props are not 100% effecient. IIRC, a good constant speed prop is going to get about 85% effeciency. I don't know the equation to get static thrust from horsepower offhand but you can google it easily.

--Outlaw.

Dash_C.
12-12-2006, 09:12 PM
After some analysis, this problem is more complex than I thought. Too bad my aero prof. left for break. I don't think thin airfoil theory can solve this one... http://forums.ubi.com/groupee_common/emoticons/icon_confused.gif

Anybody got a DB605 and a test stand?<div class="ev_tpc_signature">

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WB_Outlaw
12-13-2006, 05:42 AM
You can't hang a 109 on the prop b/c the T/W ratio is not even close to 1. P/W means nothing in and of itself. A 2000 HP engine without a prop attached produces nothing more than exhaust thrust (which is about 400lbs IIRC).

If the short form of the answer to your question isn't satisfying, keep in mind that calculating thrust to a high degree of precision by hand is difficult at the very least as it is very dependent on blade design. Approximating it is fairly easy though b/c unless the prop is very badly designed or is very special (like bent blade tips), there isn't that much difference.

You can also approximate drag using known performance values and BOOM, there's your thrust.

--Outlaw.

Dash_C.
12-13-2006, 10:47 AM
Originally posted by WB_Outlaw:
You can't hang a 109 on the prop b/c the T/W ratio is not even close to 1. P/W means nothing in and of itself. A 2000 HP engine without a prop attached produces nothing more than exhaust thrust (which is about 400lbs IIRC).

If the short form of the answer to your question isn't satisfying, keep in mind that calculating thrust to a high degree of precision by hand is difficult at the very least as it is very dependent on blade design. Approximating it is fairly easy though b/c unless the prop is very badly designed or is very special (like bent blade tips), there isn't that much difference.

You can also approximate drag using known performance values and BOOM, there's your thrust.

--Outlaw.

I considered calculating drag at full speed, but this would not yield static thrust.

I think the best way would be to find lift and drag at power-on stall conditions. This would best approximate static thrust.

400 lbs of thrust just from exhaust? That seems really high!<div class="ev_tpc_signature">

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TX-EcoDragon
12-13-2006, 01:38 PM
http://www.waynehandley.com/archive.html<div class="ev_tpc_signature">

S!
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StellarRat
12-13-2006, 03:07 PM
Originally posted by Dash_C.:
I considered calculating drag at full speed, but this would not yield static thrust.

I think the best way would be to find lift and drag at power-on stall conditions. This would best approximate static thrust.

400 lbs of thrust just from exhaust? That seems really high! You also need to consider the drag and torque from the propeller. This is another reason fighters don't have giant propellers like a helicopter. There is a balance between max. forward speed and max. thrust/lift. Another reason jet engines are superior.

Sergio_101
12-14-2006, 04:54 PM
The USAF Museum says that thrust to HP is about
equal at 400mph, as you drop below that a lower HP delivers
increasing thrust.
Above that the HP goes way up for the same speed.

It's riddled with variables, but in essence
they are saying if it took 1200 HP to go 400mph
then it will take 1200 lbs of thrust to do the
same in the same aircraft.
Very simplistic but it's a reasonable example.

400lbs in usable thrust from exhaust gasses seems
a bit high.
Allison found when they turbo compounded their
V-1710 that it was easy to recover 700HP.
But that's in a Power Recovery Turbine.

Curtiss Wright got similar results with PRTs
with the R-3350TCW turbo compound radials.

Sergio<div class="ev_tpc_signature">

P-51s may not have won the war, but they did not loose it.
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RocketRobin__
12-14-2006, 08:32 PM
The Lerche "is" not a conventional propeller plane.
The Lerche "uses" a contra-rotating ducted fan to produce thrust. Ducted fans can be fairly effective at producing high static thrust, because the duct effectively eliminates losses due to blade tip vortices. The larger the duct, the greater the available static thrust/power ratio. On the downside, interference drag between the duct and impeller(s) vs torque limit the duct size, as does the frontal area of the duct which produces far more drag than a typical a conventional propeller.
In fact, the Lerche (as originally concieved) probably never could have left the ground using reciprocating engine technology.
Modern ducted fans are almost always attached to a very powerful torque producing engine, the turbojet.<div class="ev_tpc_signature">

Rocket in my pocket, finger in the socket.

Dash_C.
12-14-2006, 09:48 PM
Originally posted by RocketRobin__:
The Lerche "is" not a conventional propeller plane.
The Lerche "uses" a contra-rotating ducted fan to produce thrust. Ducted fans can be fairly effective at producing high static thrust, because the duct effectively eliminates losses due to blade tip vortices. The larger the duct, the greater the available static thrust/power ratio. On the downside, interference drag between the duct and impeller(s) vs torque limit the duct size, as does the frontal area of the duct which produces far more drag than a typical a conventional propeller.
In fact, the Lerche (as originally concieved) probably never could have left the ground using reciprocating engine technology.
Modern ducted fans are almost always attached to a very powerful torque producing engine, the turbojet.

Cool! Didn't know that.<div class="ev_tpc_signature">

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