limit throttle or revs?

[pantdino]

The Titan T-51 with the GM LS3 430hp motor uses a 2.2 ratio Autoflight PSRU that is designed for up to 300hp at 6000 rpm.
The prop can spin up to 2550 rpm (so call it 2500,) so the engine can spin to 5500 rpm within that prop speed limit. (Redline on the engine is 6600 or so, so irrelevant.)

I am told the cure for this "engine makes too much power for the gearbox" situation is to limit the revs to less than the engine makes 300 hp, which is about 4000.
But the engine makes 400 ft-lbs of torque all across that rev range, so by spinning the engine slower you are exposing the gearbox to stronger impulses per rev, which is a bad thing, no?

Seems to me it is better to run the engine and gearbox faster and limit the horsepower being generated by limiting the throttle / MAP rather than using full throttle at a lower rpm.

What am I not understanding?

Jim

 

[Protech Racing]

You are correct. The lower speed has more piston pressure per revolution , and thus crank speed variance than the same power at higher revs.
Maybe use a smaller throttle body and flatter prop to run less tq at higher RPM. But I know nothing of the reduction drive. Maybe it has a resonance or heat issue resulting form RPM?
Using a 300 HP drive with a 400 HP engine is interesting logic. Is a larger drive not made?

 

[pantdino]

I think the logic of using that engine is that while the 250 hp 3.5 liter Honda engine previously used CAN spin at 4000 rpm, it was not designed to do that for hours on end. It is a minivan motor and will probably not last long at 4000 rpm continuous.

The LS3 is a crate race motor for which 4000 rpm is nothing, so it is way understressed compared to the Honda.
My plane has a TOW of 1850 lbs. Other owners get rate of climbs in the 2500-3000 fpm range with the revs limited to the equivalent of 250hp, so 400 hp is way more than anyone needs. Bigger gearbox = heavier gearbox = CG problems and more stress on the airframe.

Would it make sense to limit the MAP rather than revs, so say take off at 22" and more revs rather than 27" and less revs?

 

[rv7charlie]

I suspect that your concern about the Honda is without foundation; these days the big car makers seem to test their engines at levels of abuse far exceeding typical aircraft engines. That LS motor will make its full rated power for hundreds of hours on end, including radical treatments like dumping cold water into the engine while running at full operating temps, etc. I'd be very surprised if Honda was any different. A virtually identical engine to the Honda is used in their marine outboards, which run at full power all day long.

Your real problem is the engine packager/vendor knowingly installing an inadequate drive for the engine. While I might try the same trick, knowing I'd be the sole manipulator of the controls, I couldn't in good conscious place such a product on the market.

Torque is torque, which will determine the raw breaking strength of stuff in the box (ignoring heat/rpm, and obviously, resonance issues). Using a quick snip from Wikipedia, the Honda would be approximately:

• Power: 244 hp (182 kW) at 5750 rpm
• Torque: 240 lb⋅ft (325 N⋅m) at 4500 rpm

If we assume a flat torque curve for both engines, you're driving the gearbox at ~1.6 times rated torque. You *could* manage this by consulting a torque/MAP/HP/rpm table, and trying to stay away from the 'red zone' (some boosted a/c engines actually require this), but I know I'd be way too busy to do that while trying to fly the plane and manage the prop control. Could be done; I wouldn't want to do it.

FWIW....

 

[Protech Racing]

You can’t be the only one flying this combo. Copy the known good value /setup. Imho
A restrictor plate may solve it.
The Honda is. A Great engine also .

 

[Mad MAC]

Helicopters without torque gauges i.e. R22 use MAP as the torque limit method.

Flight Tested a R22 with a stain gauged output shaft, the output was somewhat steady with the appropriate smoothing harmonics applied until it hit the aspirational limit at which point it went so smooth I thought it was broke .

 

[wsimpso1]

The Titan T-51 with the GM LS3 430hp motor uses a 2.2 ratio Autoflight PSRU that is designed for up to 300hp at 6000 rpm.
The prop can spin up to 2550 rpm (so call it 2500,) so the engine can spin to 5500 rpm within that prop speed limit. (Redline on the engine is 6600 or so, so irrelevant.)

I am told the cure for this "engine makes too much power for the gearbox" situation is to limit the revs to less than the engine makes 300 hp, which is about 4000.
But the engine makes 400 ft-lbs of torque all across that rev range, so by spinning the engine slower you are exposing the gearbox to stronger impulses per rev, which is a bad thing, no?

Seems to me it is better to run the engine and gearbox faster and limit the horsepower being generated by limiting the throttle / MAP rather than using full throttle at a lower rpm.

What am I not understanding?

Jim

We have no idea what the real limiting characteristics are for this gear box. There are a number of things that may draw the boundaries of any gearbox:

Heat is generated as a linear function of horsepower. It could be that this gearbox will overheat if horsepower is greater than 300. But a circulation pump moving gearbox oil to a cooler and back to the gearbox can extend things if only gearbox bulk temp is the limiting factor. A variety of internal parts may run hotter than the oil. Something has to. These parts may exceed their safe, durable temperatures based upon a horsepower limit. If horsepower alone is involved, then you could run 300 HP at any other rpm - for instance 400 lb-ft of torque at 3939 rpm is also 300 HP...

Torque can be a limiting issue because all stresses in the gearbox are linearly related to torque being carried by the system. 300 HP at 6000 rpm is 263 ft-lb of torque. Perhaps the maximum torque for durable safe operation of this gearbox is 263 ft-lb. If this is the limiting issue, then you will be limited to 263 lb-ft no matter the rpm. For instance at 4000 rpm, 263 lb-ft gives 200 HP, while at 5000 rpm that torque makes it 250 HP. All would be valid torques under this circumstance.

Then rpm can be the limiting issue for a couple reasons, with one being that gear faces and rolling element bearings are very sensitive to rpm. The gearbox may or may not give durable safe power at higher torques as long as the rpm is kept in bounds.

Other circumstances could apply, with the gearbox builder not knowing what the gearbox can actually stand. The only way to be "safe" is to keep max torque no higher than 263 lb-ft and rpm no higher than 6000.

In short, we do not know if it is power limited, rpm limited, or torque limited. And each would have its strategies. Sucks.

Now let's remember that as soon as we gain some altitude or pull back on the throttle, the engine torque vs rpm curve slides down, and it slips down more at high rpm than it does at low rpm. This is because torque is largely a function of manifold pressure with increasing rpm causing more frictional losses. In the case of your LS3 crate engine, it only makes 400 lb-ft across a big part of the rpm range when the throttle is left full.

I would not put a restrictor plate in, as that will choke the engine as you gain altitude. If I were running this engine, I would be inclined to find the manifold pressure vs rpm that makes 263 lb-ft, and then redline the rpm at 2550*2.2 = 5610 rpm. At least, i would do that until I know better about this application.

Billski

 

[pantdino]

You can’t be the only one flying this combo. Copy the known good value /setup. Imho
A restrictor plate may solve it.
The Honda is. A Great engine also .

A confounding factor here is that the Honda engines being used are from crashed cars which are then rebuilt / modified by Titan. Opinions vary regarding the result of that work. A new engine from the factory might be a different story.

 

[pantdino]

We have no idea what the real limiting characteristics are for this gear box. There are a number of things that may draw the boundaries of any gearbox:

Heat is generated as a linear function of horsepower. It could be that this gearbox will overheat if horsepower is greater than 300. But a circulation pump moving gearbox oil to a cooler and back to the gearbox can extend things if only gearbox bulk temp is the limiting factor. A variety of internal parts may run hotter than the oil. Something has to. These parts may exceed their safe, durable temperatures based upon a horsepower limit. If horsepower alone is involved, then you could run 300 HP at any other rpm - for instance 400 lb-ft of torque at 3939 rpm is also 300 HP...

Torque can be a limiting issue because all stresses in the gearbox are linearly related to torque being carried by the system. 300 HP at 6000 rpm is 263 ft-lb of torque. Perhaps the maximum torque for durable safe operation of this gearbox is 263 ft-lb. If this is the limiting issue, then you will be limited to 263 lb-ft no matter the rpm. For instance at 4000 rpm, 263 lb-ft gives 200 HP, while at 5000 rpm that torque makes it 250 HP. All would be valid torques under this circumstance.

Then rpm can be the limiting issue for a couple reasons, with one being that gear faces and rolling element bearings are very sensitive to rpm. The gearbox may or may not give durable safe power at higher torques as long as the rpm is kept in bounds.

Other circumstances could apply, with the gearbox builder not knowing what the gearbox can actually stand. The only way to be "safe" is to keep max torque no higher than 263 lb-ft and rpm no higher than 6000.

In short, we do not know if it is power limited, rpm limited, or torque limited. And each would have its strategies. Sucks.

Now let's remember that as soon as we gain some altitude or pull back on the throttle, the engine torque vs rpm curve slides down, and it slips down more at high rpm than it does at low rpm. This is because torque is largely a function of manifold pressure with increasing rpm causing more frictional losses. In the case of your LS3 crate engine, it only makes 400 lb-ft across a big part of the rpm range when the throttle is left full.

I would not put a restrictor plate in, as that will choke the engine as you gain altitude. If I were running this engine, I would be inclined to find the manifold pressure vs rpm that makes 263 lb-ft, and then redline the rpm at 2550*2.2 = 5610 rpm. At least, i would do that until I know better about this application.

Billski

So what we really need here is a graph of what combinations of rpm and MAP yield 263 ft-lbs of torque?
The usual hp and torque figures are not useful because they only show the numbers at WOT / 29" or whatever, no?

Jim

Jim

 

[TFF]

There is the use of derated horsepower. So you can only use 300 out of 400 at a low altitude. As you climb, you loose power. If you could only make 300 at sea level, once you get to 8000 ft you are probably making 180 hp. What if you had a 100 extra in the bag? As you climb, you can eat that 100 and maintain 300 hp to a higher altitude before it drops off. You don’t use the power to make 400; you use the power to maintain 300 longer until the critical altitude is reached. You are still making more power as it drops off than the straight 300 hp engine. Ever fly on an jet airliner or fly in a helicopter? Those aircraft live on derated power. Extra power on the ground to the point of damage if they used it, just so they have the power capacity in the air.

 

[Hephaestus]

Mechanical detent - with an ability to bypass.

Think old school WEP - Think the p51s was a safety wire wasn't it?

Poop hits Fan... Mistakes made - hit canyon wall or buy a new PSRU. Yep, engine, PSRU might explode the instant you pass the stop, but if it doesn't, you might save your butt/aircraft.

 

[pantdino]

Mechanical detent - with an ability to bypass.

Think old school WEP - Think the p51s was a safety wire wasn't it?

Poop hits Fan... Mistakes made - hit canyon wall or buy a new PSRU. Yep, engine, PSRU might explode the instant you pass the stop, but if it doesn't, you might save your butt/aircraft.

That is my thinking, too. I saw a video and have read about planes that crashed in valleys because they couldn't climb over the terrain. Might have to buy a new PSRU and / or get the prop rebuilt, but I'll be alive to do it. The thought of flying in an underpowered airplane scares me.

 

[pylon500]

Just looking from a distance and I'd be interested to know how much modification the original designed airframe has gone through seeing it was initially designed to fly with around 115hp?
I just wonder if you're looking to push upwards of 450hp whether you should be looking at something like a Stewart or Thunder 51?
That said, are they still available?

 

[rv6ejguy]

I think the logic of using that engine is that while the 250 hp 3.5 liter Honda engine previously used CAN spin at 4000 rpm, it was not designed to do that for hours on end. It is a minivan motor and will probably not last long at 4000 rpm continuous.

The LS3 is a crate race motor for which 4000 rpm is nothing, so it is way understressed compared to the Honda.
My plane has a TOW of 1850 lbs. Other owners get rate of climbs in the 2500-3000 fpm range with the revs limited to the equivalent of 250hp, so 400 hp is way more than anyone needs. Bigger gearbox = heavier gearbox = CG problems and more stress on the airframe.

Would it make sense to limit the MAP rather than revs, so say take off at 22" and more revs rather than 27" and less revs?

The Honda and most other Japanese engines can spin at easily spin 4000 rpm all day long. I fly at 4500 with my Sube. The stroke of the J35 and LS3 are almost identical so piston speed is as well. The critical parameter on almost all stock automotive engines with cast pistons is piston crown and ring land temperatures. This fact means they need to be run rich (just like the factory does at WOT and high RPM) to have longevity.

I'd forget smaller throttle plates or restrictors. Simply limit MAP if you want to limit torque or HP for the gearbox. Most of us flying turbocharged engines do this on every takeoff and climb.

Titan has had a spotty record in assembling customer Suzuki and Honda engines in the past. They would be well served to take the LS3 out of the crate without touching the inside of it.

How much TV and bench run/ flight testing has gone into the LS3 gearbox? We haven't heard from Myron here for a while. Is his LS/T51 flying yet? How many hours on the high time LS powered one? Problems reported?

 

[wsimpso1]

So what we really need here is a graph of what combinations of rpm and MAP yield 263 ft-lbs of torque?
The usual hp and torque figures are not useful because they only show the numbers at WOT / 29" or whatever, no?

Jim

Plot works, table works. At sea level, you would have to set power to stay in range and adjust as you go up. At and above some altitude you could run WOT if it stays within rpm range.

The trick becomes how to get the data for your plot and/or table. I suspect that running three different "Eiffel clubs" during your break in runs can give you enough data to start flight ops. You will need a powerful fan to cool during those runs. I am suspecting several leaf blowers or the blowers for drying a wet floor carefully fitted to the inlets might do, or at least extend the run before temps stop each run.

I run a PA28-181 - fixed prop. It is optimized around cruise, and in cruise, you manage rpm with throttle until you get to around 8000 ft, where that includes WOT. The down side is that we only get somewhere around 50% power at the beginning of the takeoff run. RPM comes up as speed comes up, but we still climb at modest power. Simplifying this, we use full throttle and leave it there until leveling off. We lean as we go up, and then lean again once established in cruise.

Your situation with prop pitch fixed mechanically will be similar, but more complicated - you will have to manually set manifold pressure early in the takeoff run, and can either leave it there or even add throttle as air gets thinner with any eye on MP & RPM as you go. I assume the ECU takes care of mixture for you as you get into thinner air.

Even after you get back into prop governor operation, it sounds like you will be managing manifold pressure.

A few things to point out. Most gearboxes are not pure HP limited. A pure HP limit means gearbox temp limiting. IFF this is the only real limit, then you can probably increase power limits with increased cooling of the gearbox and lower gearbox temperatures. Also, a pure horsepower limit is unlikely in a simple box like this one. Likely torque limiting is also in play, while you already have an rpm limit imposed by the prop. Thing is, until someone explores the real torque limits, you do not know how much higher torque will be no problem. For that matter, you do not know if even at 263 lb-ft is actually too much or not.

One other comment - In an earlier post, you had talked about power pulses being more severe at lower rpm... Hmm, I suppose that depends upon your definitions.

First: At a given manifold pressure, mean torque is about the same across the rpm range - yeah, it does rise to a peak, then diminish as rpm rises, but if you keep the same manifold pressure across the speed, the range on torque is not huge. Mean torque is a time average of the smoothed sum of the cycle.

Second: Horsepower is a derived term. Power is the rate of work being performed - Force at a Speed. Torque is the motive force available to turn things. Torque (in ft-ln or N-m) times engine speed (rad/sec) is power in your torque measure per second. Torque in ft-lbs times RPM divided by 5252 is the much more convenient horsepower. To make a given horsepower at one RPM takes a certain torque at that RPM. To make that HP at a lower RPM takes more torque... No where in here are the sizes of the torque pulses considered...

Third: This PSRU does have an elastic element between the engine and gearbox, does it not? If so, then that soft element is put in to isolate the gearbox from the firing pulses of the engine. Generally, the spring rate is selected so that the natural frequency is less than one-half of the lowest idle speed the engine is expected to run. At idle, the isolation within the rotating parts should be pretty good, and as you get up into the powered flight speeds, the transmitted pulses will be tiny. The job of the elastic element is to smooth out the pulses. So, even if the transmitted pulses are a little larger at lower engine speeds, that is not likely to be a big actor in the gearbox. If that were so, the strict torque limit of the gearbox would have to be smaller at lower rpm than at higher rpm. Quite frankly, in 23 years of automotive transmission experience at Ford and Chrysler, I have never seen that.

Last: In my experience, torque limits are always present and VERY real. Exceeding torque limits modestly generally shortens lives of things like gears and bearings. Exceeding torque limits by a lot breaks stuff quickly. RPM limits rarely do more than shorten lives of shafts, gears, and bearings. In rare cases, exceeding rpm limits gets into resonance of shafts and cases - failure happens quickly when resonance occurs. Exceeding horsepower limits without exceeding rpm limits usually falls into the area of exceeding torque limits - shortened life. While you could probably fly a bunch at higher torque than the 263 ft-lb, you would likely be the durability test lab for that. In flight? No thank you.

Billski

 

[pantdino]

Thank you very much , Billski

Please forgive me if I'm oversimplifying or misunderstanding, but these are my conclusions thus far:

1) The LS3 engine makes 350 ft-lb torque at 2000 rpm and 430 at 4500. So while Titan's suggested cure for the excess power issue is to never exceed 3600 rpm, it seems to me that if torque needs to be limited to 263, then at NO ENGINE SPEED IS IT SAFE TO USE WOT.

2) Since it takes more torque to produce the same hp at lower rpm than a higher one, this would seem to reinforce my perception that the gearbox will be less stressed if you spin the engine and gearbox faster at a lower throttle setting than slower at a higher throttle setting.

Do you have a guess what MP pressure an engine will be making only the amount of torque we can feed into it?
Does feeding 15" to an engine equate to "half throttle", and therefore half the output? I suspect the relationship is more complex than that.

I have reinstalled the governor on the new gearbox, and there are no visible fines in the oil after a couple of ground runs followed by oil changes before installing the governor. The minimal pitch adjustment has been made to provide 16.5" MAP at 2000 rpm, the idea being that if the governor fails you still have enough thrust to get home.

Thank you,
Jim

 

[Protech Racing]

What ecu are you using?
Most can be tuned to limit power on the ignition map .

 

[Toobuilder]

I'm guessing this "minimal pitch adjustment" was the low pitch stop on the prop (the mechanical stop on the piston dome), correct?

 

[wsimpso1]

[QUOTE="pantdino, post: 575696, member: 96632"\
1) The LS3 engine makes 350 ft-lb torque at 2000 rpm and 430 at 4500. So while Titan's suggested cure for the excess power issue is to never exceed 3600 rpm, it seems to me that if torque needs to be limited to 263, then at NO ENGINE SPEED IS IT SAFE TO USE WOT.[/QUOTE]

Let's be really clear on something. That torque and power curve you see, represents the engine output at exactly one air pressure, humidity, and temperature. By far, the most important is ambient air pressure. Usually the torque and thus power curve are corrected and published to 29.92 In Hg, 59 F, 50% humidity. Even if your dyno was in sight of the beach at Kitty Hawk NC, your engine would be a few feet above Mean Sea Level. We lose roughly 1" of manifold pressure for every 1000' altitude.

If the engine breathed exactly the same at all rpm, the torque curve would be absolutely flat. It is not flat because as rpm goes up, several things get worse:

• Friction losses from moving the engine internals get increase at the square of speed;
• Friction losses from moving air in the manifolds and past the valves increase at the square of speed;
• Inertia effects of the air (lag in response) in the manifolds increase.

We can not do much about the first two, but we can adjust valve timing, play with manifold runner lengths, etc to tune the system for fat torque curves and set the peak torque where we want it. Yeah, peak torque is most likely at the place where the breathing is closest to perfect.

The really big thing going on is that the air pressure in the manifold largely determines how much fresh air in the combustion chamber for each firing, and that largely sets power, yeah, just how the breathing is set up influences this some too, but usually having lower manifold pressure for whatever reason scales down the torque curve, with more loss at higher rpm than at low.

With a given engine, engine torque primarily changes with manifold pressure. As a first order approximation, if you were running 4500 rpm, you could expect 263 lb-ft at 29.92*263/450 = 18.3 in Hg. You can get this manifold pressure down low by closing the throttle, or when you get above 12000 feet. If you tried to climb with a fixed throttle position, that held 18.3" at sea level, and held that throttle position to 12,000 feet, you would have about 10.7" Hg manifold pressure when you got there and only about 160 lb-ft of torque. The throttle valve takes whatever ambient pressure is and reduces by some fraction based upon the opening of the valve.

So if you set the governor to hold 4500 rpm, and your airport is at sea level, and then you set throttle to make 18.3", and commence climb, you could hold 263 lb-ft by nudging the throttle to keep the intake manifold at 18.3" as you climb. And you will be somewhere near 225 HP the whole time.

2) Since it takes more torque to produce the same hp at lower rpm than a higher one, this would seem to reinforce my perception that the gearbox will be less stressed if you spin the engine and gearbox faster at a lower throttle setting than slower at a higher throttle setting.

Do you have a guess what MP pressure an engine will be making only the amount of torque we can feed into it?
Does feeding 15" to an engine equate to "half throttle", and therefore half the output? I suspect the relationship is more complex than that.

Sort of... Yes, you could make 225 HP by running 263 lb-ft and 4500 rpm (and 18.3" Hg) or you could make the same 225 HP at 5252 rpm and 225 lb-ft and most likely a little more manifold pressure at that rpm.

For a first order approximation, you could look at WOT sea level torque at any particular rpm and scale torques to figure the fraction 29.92 in Hg you would need there...

Scaling is a pretty rough estimator, which is why I suggested a run or three, measuring torque, rpm, manifold pressures, and running up to some torque (263 lb-ft might be a good place to limit things) and find your limiting manifold pressure for each rpm you are flying. Each Eiffel Club size has its own torque vs rpm curve, so you can run each club over a range of manifold pressures, recording rpm at each setting, and get a pretty good idea of what your engine is doing...

The reason for all this annoying testing is that your intake and exhaust system is not the same as the one used on Chevrolet's dynos. So even if you could get a bunch of engine torque/ engine rpm/manifold pressure dyno operation points from Chevrolet, you would be guessing. Many folks running a V-8 in a Mustang look alike have short pipes, just like the Merlins and Allisons did, and they cost you some power. Hmm. Since you are ready to fly, perhaps you could instrument the engine mount to give you prop torque in real time, and divide by your PSRU gear ratio. Then you could very directly set throttle based upon prop torque.

I have reinstalled the governor on the new gearbox, and there are no visible fines in the oil after a couple of ground runs followed by oil changes before installing the governor. The minimal pitch adjustment has been made to provide 16.5" MAP at 2000 rpm, the idea being that if the governor fails you still have enough thrust to get home.

I hope that you have a flyable prop pitch there - you definitely want to fly at that power setting and establish your airpseed and sink rates, and find what range of airspeeds you can hold altitude. You may not be able to get home, but instead be able to pick where you will set it down. Having a glide cone 20 miles in radius is 16 times as much ground area for a landing than a glide cone that is only 5 miles in radius.

Being as you are not going to put in a dedicated inline oil filter for oil going to the governor, I suggest that you consider the partial flow filtering scheme we had mentioned. This will not prevent silting the hub and/or control valves nor prevent contaminating the bearings and gears, but it will reduce the rate at which these things happen, with the intent of giving you more time to catch the problem and fix it before having a major inflight issue.

Good luck.

Billski

 

[rv6ejguy]

Thank you very much , Billski

Please forgive me if I'm oversimplifying or misunderstanding, but these are my conclusions thus far:

1) The LS3 engine makes 350 ft-lb torque at 2000 rpm and 430 at 4500. So while Titan's suggested cure for the excess power issue is to never exceed 3600 rpm, it seems to me that if torque needs to be limited to 263, then at NO ENGINE SPEED IS IT SAFE TO USE WOT.

2) Since it takes more torque to produce the same hp at lower rpm than a higher one, this would seem to reinforce my perception that the gearbox will be less stressed if you spin the engine and gearbox faster at a lower throttle setting than slower at a higher throttle setting.

Do you have a guess what MP pressure an engine will be making only the amount of torque we can feed into it?
Does feeding 15" to an engine equate to "half throttle", and therefore half the output? I suspect the relationship is more complex than that.

I have reinstalled the governor on the new gearbox, and there are no visible fines in the oil after a couple of ground runs followed by oil changes before installing the governor. The minimal pitch adjustment has been made to provide 16.5" MAP at 2000 rpm, the idea being that if the governor fails you still have enough thrust to get home.

Thank you,
Jim

Yes, 15" MAP would give you roughly 1/2 power at a given rpm.

Titan needs to get a properly developed and tested PSRU on these things or they'll have a lot of pissed off customers.