oggers:

You accusing me of holding out info and trying to guilt trip me has been bugging me. I shared all the resources I was using throughout this discussion. You and everyone else had access to them on here. I posted the whole AfNA document section on the propellers where it talks about induced power. The reason I hadn't picked up on this earlier and put the two together is because I hadn't read the article word for word and analyzed it. I also hadn't come across the article which you posted about Figure of Merit. Once you did post that, I picked up on it right away and I think it actually backfired on you! It added more evidence to the fact that you don't know what you're talking about. I highlighted that in my last post.

But I should ask the same of you: Have you been holding out this information knowingly as I constantly repeat myself in post after post, trying to explain THP? Have you been sitting on this information? Why didn't you speak up earlier about it? What proves that you haven't just been trolling the whole time, like someone else has mentioned in this thread, and knew all along about this?

I'm not responsible to teach you everything about aerodynamics. You need to take responsibility for your learning. You can't blame me for you not thinking about the efficiency of the prop in creating the thrust. I mentioned that there were other efficiencies that can be analyzed and I agreed that the propeller is doing work on the air. Did I really have to tell you that if anything is doing work, there is a way to analyze the efficiency of the object doing the work? I'm not responsible to think of everything for you.

You finally discovered something that would add to the discussion and then you blame me for not telling you earlier. You really are a piece of work. :ok:

That's right. You did make a mistake. But not just in that quote (which was actually two quotes anyway). No. You reiterated it across 4 pages:

etc etc.

Who are you trying to kid? You made it central to what you've been prattling on about. Now finally you've dropped it. :D

:{ Well, maybe it will help if you go back and read between the lines. I wasn't really accusing you of holding out info. I was suggesting that it hadn't occured to you.

But let's just be clear about one thing. After 4 pages you have now done a ma-housiv u-turn and decided that the aircraft can stay still after all and still output power from the prop.

blackhand:

Gets my vote :ok:

Flat Rated
 

Aerbat77. The Allison is not flat-rated and to clarify my comment on reaching the limiting T.I.T I was simply saying that, especially on the T56A-11 it was common to reach max permissable T.I.T before achieving limiting torque.

oggers:

We had never talked about a wind before and I had assumed that there would be no wind. Therefore, with no wind, TAS is the same as GS.

You hold onto it as if you 'won' or something. If you really were thinking about wind before, you would have made a comment about it the first time I mentioned a position relative to the ground. You also forgot to highlight the times when I referred to the same speed as "TAS". Of course you wouldn't want to provide an unbiased view because that would mean you don't really have a point in this case. I think its quite obvious to everyone that neither of us was thinking about wind.

I should also point out that out of the 4 quotes you posted, only 2 actually relate to what you were talking about. #2 and #3 mention earth or the ground. But #1 and #4 only mention motion.

You never addressed the recent 'issues' I brought up in my last 2 posts about what you said about induced power. It seems as though you actually can't admit being wrong at all. You would never be a good scientist or engineer.

I think the original point of discussion between us was if the aircraft velocity was zero, I said there would be zero THP and you obviously did not agree because you said this:

So, has this discussion changed your answer now, or do you still believe that the THP will equal the BHP because it's 'accelerating a mass of air rearwards'? After this discussion it's quite obvious that I have a better 'grip' on the fundamentals of aerodynamics than you do! :)

EDIT: Where did I say that the prop does not do work on the air when the aircraft is not moving? I never said that! So, to put it bluntly, you are lying.

And so when a PW4000 runs up to max on a terrestrial test bed, it is actually creating ~70,000 thrust HP? Is that what we surmise? :rolleyes:

Old Fella:Hmmm. Since the aircraft performance is dependent on the props being driven at some torque rating, I must ask how you determine your performance-limited TOGW??

barit1:

No. THP has to do with aircraft performance. When it's on a testbed, you wouldn't be analyzing THP. The engine would be producing thrust and you could compute how efficient the engine is in making thrust (ie: accelerating air).

EDIT: However, if you were to apply the concept of THP to the testbed scenario, the THP would be zero because the testbed/engine wouldn't be moving.

italia:

Not changed my pov. BHP x PE = THP. [notwithstanding Tx losses obviously]

Well, what I wrote and what you wrote underneath it aren't the same. But taking your little statement at face value, you are clearly accepting there that there is a power output from the prop even if the plane is stationary.

An equation for PE (that you have used yourself) is power output divided by power input. An equation for THP is BHP x PE. If you acknowledge there is a power output from the prop you must acknowledge there is THP.

I look forward to reading the wriggling you will do to mitigate this conclusion. Not that I've entirely finished myself.

I think most people here, being aviation professionals, would feel the putting of an engine on a tested had quite a lot to do with aircraft performance. Unless you are a glider pilot.

oggers:

You're wrong. End of story.

There is no point explaining it anymore.

Good day!

Limiting MTOW
 

Barit1, I probably did not articulate my point as well as I might. All I was trying to convey was that the 19600"/lbs limiting torque was often not achievable before reaching the Max permissable Turbine Inlet Temperature, especially on the T56A-11 which from memory was limited to 971 degrees C at Take-off power and 927 degrees C Max Continuous. The T56A-15 however with a higher T.I.T limit could readily reach Max Torque below the limiting T.I.T of 1083 degrees C for Take-off. Of course, achievable Torque for the given ambient conditions and Runway length available, surface type etc all are considered to determine Max Take-off weight allowed. Hope this is clearer.

Prop Torque
 

Hi,

In my reply (post #24), On RR Tyne with 4 bladed variable pitch props, we had to have both Prop RPM and torque within limits before we called power was set. The engine and gearbox were bolted together so it was not possible to measure their individual torques separately.
Hence the torque sensor (which was bolted between an engine mount and the airframe) simply measured the reaction to the propeller torque - so it indicated actual Prop Torque.

In order to keep passenger comfort during stepped climbs, we kept the prop RPM constant, but varied the fuel flow by "trimming" the (guarded) HP fuel levers. Power changes were called for ("50% trim" etc) and measured by observing the torque.

Since we measured both torque (force) and prop RPM (velocity) one could calculate the propeller horse power being generated (with a known p.e.) - even when stationary on the runway

Since there is no torque meter fitted on a bypass jet engine, the most useful parameters are N1 / EPR which only give an indication of thrust, not power.

Edit for Capt Pit Bull's comments below.
Rolls-Royce Tyne - Wikipedia, the free encyclopedia
"RTy.1 Mk 506
3,259 kW or 4,985 e.s.h.p"

mmm mumble mumble walking away.
Is that because there is no electricity?

RPM & Torque
 

rudderrudderrat, the Allison T56 is essentially a constant speed engine/gearbox/propeller installation with propeller blade angle selectable only in the 'ground' range of throttle lever travel (except feathering or unfeathering of course). Once in the 'flight' range (flight idle to 90 degrees throttle angle) the throttle simply schedules fuel flow and the propeller is governed to 100% RPM (1021 RPM) + or - 2% in basic hydraulic governing mode. The obvious advantage in having such an engine is almost instantaneous response to throttle movement (limited to not moving the throttle from Flight Idle to Max in less than 1 second) Without getting the purists throwing stones the torque value indicated on the C130 is the load imposed on the engine torque shaft ( and thus the engine itself) by the reduction gearbox, accessories and propeller. This whole thread seems to have become the site for "ego boosting" by some, surely not what the original poster was looking for. I am sure you, barit1 and I each would understand that as a crew member our concern would be "Have I got the torque or power I expect within the RPM and T.I.T limitations applicable and, if not, why not?" I am familiar with the use of N1 and EPR (or IEPR on the RB211 engine) as a measure of thrust. Whether we call the motive force Horespower, Thrust, Torque or any other name doesn't really matter. Every aircraft I have operated has had a specific set of parameters which are to be met before and during flight.

Sorry I'm late.

Italia is correct (give or take the odd minor slip / assumed simplification e.g. zero wind etc).

Oggers, you don't understand efficiency.

An engine in a test bed does NOTHING except churn up air thereby producing heat. If the object an engine is attached to does not increase in the total of GPE +KE as a result of the efforts of the engine then no *useful* work is being done. Likewise a person straining on a wrench is just producing heat. In both cases energy is certainly used, but not usefully.

This is my one and only post on the topic. Oggers, you have a long ingrained 'alternative framework'. Unless you are at least prepared to consider that something you think you understand could be wrong I, like italia, see no point in spending any time over it.

pb

Oh yeah nice one Pitbull. I'll chalk that up to being 1st Apr :ok:

Capt Pit Bull:

The issue I’m disputing is the assertion by italia458 (note, he raised it, not me) that a plane at standstill can knock out 200 BHP and generate static thrust, yet there is zero THP.

That’s a very definite conclusion to draw from the rather subjective argument you offer.

‘Power out over power in’ is pretty uncontroversial.

THP = BHP x PE. If the thrust is not zero, PE cannot be zero, therefore THP is not zero. There is another equation out there based on airspeed for an arcraft in flight but it does not invalidate this one or vice versa. The trouble is Pit Bull, that italia458 is insisting it does, and you assert that he is correct. I would call that dogma and therefore I have to chuckle at the irony of your post.


It develops thrust.


Well, hopefullly the engine won't "increase in the total of GPE + KE" during the test because that wouldn't be terribly "useful work". It is on the test-bed for a reason. It is developing thrust and fulfilling a purpose other than moving an aircraft. You are entitled to your opinion - in a certain context I would agree with it - but it does not lead me to the conclusion that the propeller efficiency is zero, or that static thrust = zero THP.

Applying this to the helo scenario, if I want to maintain a steady hover, the work done on the air which provides the thrust to keep me there is 100% useful work. In no sense is it wasted.

oggers ´n italia : do you not have more important things to sort out in your life? :8

I squeeze it in between the pilates and AA meetings.

If you're not doing any useful work, you can't claim to be efficient. End of story.

As for your uncontroversial proof of understanding, it dictates that if the Joules leaving the system per time unit equals the Joules entering the system per time unit, efficiency is 100%. Any less than 100% efficiency, and less Joules leave the system than enter it.

http://www.htmlforums.com/images/smilies/Burning.gif

That's pretty amazing, and proves Pitbull's point rather nicely. :ok:

Had the dirt track car on the dyno yesterday. Watched very carefully and am sure the car did not move an inch.
Yet the dyno showed 327 rear wheel horsepower at 7200 RPM.
Lots of heat and noise, used 27 litres of fuel.

Blackhand: rolling road dyno - good example. Makes the point clearly.

oggers,
for once we agree!

That does make the point clearly. Power used, heat and noise generated, car remaining completely motionless - hence, no work done on the car. Efficiency zero (0).

Just as the aforementioned engine on a test stand, or non-moving aircraft during an engine runup. Not achieving anything, other than generating noise and heat.

Obviously not the point you've been failing for make for six pages of this thread, but I'm happy that you've seen the light at long last! :ok:

You sir, are being "purposely" obtuse.
Work is being done because the rear wheels are turning, as I tried to explain it is "relative".
The same as any stationary engine, test bed or otherwise.

The book "The Jet Engine" by Rolls Royce gives the following version of the equation for propulsive efficiency for a jet engine.

Propulsive Efficiency = (Work done on the aircraft) / (Work done on the aircraft + Work done on the exhaust)


We can modify this to apply for a propeller aircraft as follows.

Propulsive Efficiency = (Work done on the aircraft ) / (Work done on the aircraft + Work done on the propwash)



We can also modify it to apply to your car on the dynamometer as follows:

Propulsive Efficiency = (work done on the car) / (Work done on the car + Work done on the dynamometer)

The work done on the jet engine exhaust, the propwash and the dynomometer are all wasted work.

In each case if the aircraft or the car are not moving then the work done on them is zero and the propulsive efficiency is zero.

If we have zero propulsive efficiency this means that we are wasting all of the power that is being generated by our propulsion system. This means that we have no thrust horsepower.

Keith, all reasonable contributions are welcome.

The assumption is we have thrust:

I disagree with your conclusion. For a helo in the hover propulsive efficiency is zero. But “the power being generated by our propulsion system” is not being wasted.

Also, propulsive efficiency is not the same as propeller efficiency. If there is thrust, propeller efficiency absolutely cannot be zero. Whereas propulsive efficiency must be zero when the aircraft is stationary, regardless of thrust.

For that reason, any attempt to use propulsive efficiency to prove that THP must be zero in the static thrust scenario, is a logical fallacy. All that is proved is that propulsive efficiency is zero.

THP = BHP x Propeller Efficiency.

If there is thrust PE cannot be zero therefore THP cannot be zero.

“THP is the propeller output or power that is converted to useable thrust by the propeller”. That statement and the equation above are both taken from the document cited by italia458 after he floated this canard.

Before anyone jumps on the word “useable” to suggest the thrust is not useable unless it’s moving the aircraft, consider the hover again. The thrust is not only useable, it is being used.

Oggers,

My only purpose in contributing to this thread was an attempt to assist blackhand to understand why his car was not producing any THP. I can assure you that it was not in any way meant to convince you of your errors.

The past 6 pages of this thread have shown the following two things quite clearly:

1. You have a long held misunderstanding of the subjects being discussed in this thread.
2. No amount of logical argument by others will convince you that you are wrong.

For these reasons I (and clearly many other members) do not intend to engage in further pointless debate with you.

I suspect that you will come to recognise your mistakes only if you manage to discover them for yourself.

you have to split two things here : propulsive efficiency/ power / useful work of the pure engine vs the aircraft as a whole package ( or a car or whatever elese) .

since all we know that any useful work needs a motion by physics a tied up plane or a car on a dyno does not any useful work since it does not move- efficency zero.

BUT

the engine of the car or the plane moves ( spins) , produces a given torque at a given rpm, the prop translates this torque to thrust etc. so the engine by itself of course makes useful work , produces power and has a given efficiency.

the useful work of the pure engine is used to run the dyno,to stress the brakes when they hold the aircraft, to keep our helicopter in hover against gravity.

long story short conclusion : the aircraft as the whole package does not provide any useful work, its engines do with the moment as they start to spin.

cheers !

Keith, I'm not interested in your purpose for contributing. You did contribute.

If you believe that propeller efficiency cannot be other than zero when the aircraft is stationary just ask yourself next time you start the take off run where the aircraft finds the thrust and power to even begin moving. :ok:

You must be a mechanic :ok:

Induced Power
 

Hi oggers,

Is this what you are searching for?

"Induced Power
Induced Power is that portion of the power required to produce lift. It is the power required to overcome the portion of rotor drag which is caused by the induced flow tilting the total reaction rearwards.
Induced power is the force required to move a mass of air through the disk at the induced velocity.
If T is the rotor thrust (in a hover equal to weight (Mass * G)), which is a force, and this force moves the air at a velocity Vi , Pi = TVi .

Helicopter Power Required

RRT,

Thanks that's a nice clear reference.

Pi = T x Vi

THP = T x V

The principle of THP extended to the hover. No need for the aircraft to move.

A few thoughts:

At zero airspeed, how much induced drag does the aeroplane see? How much profile drag?

How much horsepower does it then take to (ahem) "move" it at zero airspeed?

There may be a clue here... :uhoh:

Perhaps it would help to remember that aircraft propulsion systems are not "lossless". Not all the power goes into moving the aircraft so you can't simply say the power generated by the engine is

Power = force x velocity

You have to write something like...

Power = (force x velocity) + "losses"

You can define:

force = thrust
velocity = aircraft airspeed

but best leave the definition of "losses" vague for the moment as there are too many things to list and they are NOT constant. Some of the losses are also dependant on velocity.

Now it's clear from the modified equation that the power output is not necessarily zero when the aircraft is stationary. In such a case "Losses" includes throwing a lot of air backwards for no purpose.

In the case of the car on the dyno, "losses" include the load the dyno places on the wheels (eg heating up the brake unit).

Interesting to consider what happens at take off. Lets say you have brakes on, run up the engines and then release the brakes... as the aircraft starts moving through the air the sum of all the "losses" must reduce to balance the equation. (I'll ignore the fact that some engines produce more power when moving).

Hi barit1,
That depends on the acceleration which the power unit is giving to the aircraft at that very instant.

True, but I believe the unstated assumption here is a steady-state (unaccelerated) vehicle.

Are you only considering a shed then?

The conditions I propose:

o Parked, brakes locked (zero GS)

o Zero wind (thus zero IAS)

The questions: How much induced AND profile drag?

and...

From a THP standpoint, how is this any different from running the engine on a test bench?

Keeping in mind the definition of one horsepower (i.e. 33,000 foot-pounds of work per minute), is there any useful work being done?

Sure, no power is required if the aircraft is at standstill but that does not mean no power is available. OTOH power is required to accelerate the mass of the vehicle from standstill with or without aerodynamic drag. No power available = no acceleration. THP is the power available. If the aircraft has to move before you get THP then you are in a chicken and egg situation - you can't accelerate the aircraft without power but you can't have power until you move the aircraft.

The question is: if the engine is producing 200 BHP and the prop is providing thrust can we say that the prop is providing THP, even whilst the aircraft is stationary?

THP = BHP x PE

If there is thrust PE is not zero. If PE is not zero THP cannot be zero. THP is not the same as power required, though it may have the same value.

-------------------

RRT,

As an aside, the power required curve in the link you provided should be very familiar to helicopter pilots: the high power required to hover and the ‘hump’ near zero IAS as one transitions to forward flight are important characteristics for rotorheads to consider.

Nope. THP is the power DELIVERED TO THE VEHICLE.

See above.