What PSRU Can we Really Trust???

Originally Posted by bmcj

I would guess that putting in a force to oppose another would simply increase the stresses on all parts involved.
No, I meant to enhance the rotation with electric power (not oppose). The motor would add power pulses in between combustion pulses to keep the flywheel speed smooth. Then a PSRU could be added with less problems.
Bruce

No, I meant to enhance the rotation with electric power (not oppose). The motor would add power pulses in between combustion pulses to keep the flywheel speed smooth. Then a PSRU could be added with less problems.

"If the prop is powered entirely by the electric motor and the engine only turns a generator, then there is no need."
That would be too heavy, I think.

Originally Posted by BBerson

No, I meant to enhance the rotation with electric power (not oppose). The motor would add power pulses in between combustion pulses to keep the flywheel speed smooth. Then a PSRU could be added with less problems.

You are right, but my bigger point is that there are portions of the drive train that get pulses from the electic motor, and portions that get pulses from the engine, so there will always be components that are in cyclic stress. Your idea does have merit as far as reducing elastic backlash from the prop into the redrive (by virtue of a flatter torque curve to the prop).

"If the prop is powered entirely by the electric motor and the engine only turns a generator, then there is no need."
That would be too heavy, I think.

I only added this because I was unsure of the configuration you were referring to.

Bruce

Well, when we were looking at integral starter generators (bellhousing mounted) as series hybrids, one of the discussions was running it as a electric damper, much like a tuned pendulum damper. Somebody did the calcs, and the electrical power required for the ISG to do this was much larger than the intended power for everything else.

In a noise cancelling headset, you just cancel the noise inside the headset. This idea is more like having a noise cancelling cabin...

I just put my memory to work, and the k factor curves are way too high to support the coupling point at cruise power settings. With reasonable sized torque converters, you are not likely to do much better than 75-80% efficiency. We have talked vibration absorption to death else where, including discussions of using torque converters and gear boxes...

Bill

OK, this is cooperating in some serious thread drift, but I understand...

More than one ratio between engine and prop: Why?

If you wanted 100% power for takeoff and initial climb or for emergency propulsion, you would run the engine at peak horsepower. That usually means near max rpm for that engine. And prop blades that are somewhere near best L/D and a certain Cl.

Now if you want best thrust and efficiencies, well, prop tip speed should be kept well below Mach. Practicalities of airplane design tend to drive us toward 0.7-0.8 Mach... And other efficiency issues drive us to larger discs and lower disc loadings, so that means slow that puppy way down, and make it as large as is practical.

So, let's say we have as big and slow turning prop as we can run with our fast turning engine, and we have a gear ratio that works great for take-off and climb. We are at a reasonable Cl and L/D for the blades. Now we want to convert to cruise, slow the engine down some, reduce the power pulse size too. In airplanes that are trying to go somewhere, the magic numbers work out to 55-75% power. If we reduce to 75% power, that only means dropping to 87% torque and 87% rpm (or 100% torque and 75% rpm or any other pair of settings that has a multiplicative product of 0.75). Going down to 55% power is 74% torque and 74% rpm... Now these are going in the right direction for prop efficiency (which actually goes up with reduced prop speed) and engine efficiency (which usually goes up when you pull speed and torque back a little from maximums). In fact, most of us would design for this condition, and accept being sub optimal at take-off and climb. So how do we handle 100% power if the prop and engine are "perfect" at 75%? Well, we let it run 15% faster where it has 33% more blade q to carry the 15% higher torque that the engine is running with at a slightly lower Cl, and accept a very modest efficiency loss until we can start pulling power back...

Or we can put in a gearbox to adjust the prop speed by what, 7% (for 75% cruise) or 16% (for a 55% cruise) , to buy back the small efficiency loss during take-off and initial climb. I suspect that the additional weight of a clutch, one-way clutch, additional gear set, cases, and cooling drag will cost you more fuel than it could ever save...

Oh, and folks have therorized about this since the 1920's. Not news.

So tell me why anyone is interested in hauling around the complexity and weight and additional sets of troublesome vibration modes and vibration inputs of a gearbox with more than one speed?

I can give you one possible reason. If you were building a really long endurance airplane, that once at altitude was running 50% torque and 50% rpm (25% power), then maybe you would have a need to do this. But for really long endurance airplanes, solar cells and electric motors really make all of this redundent...

Billski

Like I said, I don't think any of the auto-manufacturer CVTs are suitable for drop-in use in an airplane PSRU. The auto manufacturers claim that they've got the service-life issues beaten.

Little late, but.. The CVT's from each and every OEM are very probmatic. Every single person I know that has a car with one, wishes he hasn't after having them replaced multiple times. CVTs just plain suck. They can say they have it 'beaten' all they want, but fact is that most are abandoning them in favor of high gear count, close ratio tranny's.

Another drift (but related)

Neil Hintz from Autoflight who produced the Titan gearbox discussed earlier has just posted information here... under suppliers\manufacturers

Re: What PSRU Can we Really Trust???

Hi ZK;

Yes, this is VERY related to my thread, especially since I am interested in the PSRU because of my replica Mustang activities.

This is the same manufacturer that supplies Titan with gearboxes (as you mentioned) and right up the proverbial alley.

Thanks, Tom.

No problems Tom. I'll have to admit after 6 pages I'd lost the original intent of the thread !

This months issue of Experimenter had an article titled Mythbusting Belt Drives. Thought you guys might like to see it.

EAA - EAA Experimenter - Mythbusting Belt Drives

Extemely informative, it makes sense. Stiffness does transmit regardless of the media (belts or gears). The old wives tales had me.

What PSRU Can we Really Trust???

If you ask, what PSRU Can't we trust? here is an example.

YouTube - Airplane take-off, emergency and landing in a Glasair

YouTube - Airplane take-off, emergency and landing in a Glasair
Blank screen? If you use an ad blocker, try disabling it then refresh this page. Still won't play? Try updating Adobe Flash Here

Seb

Now imagine that failure took place over a lake, or heavily treed area. As it was on the video, failure on takeoff is a bad time to have failure, but he survived.

This whole concept of trusting your life and your multi-year project to "some guy" who makes PSRU's but can't tell you if or why they'll survive for "X" hours is scary. I guess my approach will be to pick a few possible candidates, watch them for years, and hope they stay in business.

Tom.

That is one scary video. Those turns look very steep for low and slow. Wouldn't it be better not to rack it around QUITE so hard? He says he had power.

Give me a cheap puddle jumper that gets high enough to do a pattern by the time it gets to the end of the runway!

Originally Posted by lr27

That is one scary video. Those turns look very steep for low and slow. Wouldn't it be better not to rack it around QUITE so hard? He says he had power.

Better dive (for speed) and bank 60 degrees or more.

During initial mountain training my French instructor (10000 hours of mountain gliding) learned me to dive, even below the trees if you have to and then pull up slightly and bank up to 90 degrees if necessary.

That saved my butt when I had a cable break at 200 ft, with 1000 ft descending runway remaining, 50 ft high trees and mountains all around, not to mention the heavy turbulence...
Picture

I started to the left of the picture and had to bank to the left and then bank (very aggressively) to the right.

I think more people are harmed because they're afraid to get too low (and thus stall when pulling up) then by low banking.

Originally Posted by autoreply

Better dive (for speed) and bank 60 degrees or more [....] I think more people are harmed because they're afraid to get too low (and thus stall when pulling up) then by low banking.

I agree. I had one experience flying out of a mountain strip at night. I was in a plane that was a VERY poor climber and the strip was in a bowl surrounded by nearby hills. During my initial climb toward the north ridgeline, I entered the leeward downflow and found myself decending even though at my best climb rate airspeed/attitude. My only option at this point (in a very dark area) was to drop the nose well below level for airspeed and rack it over into a very steep bank (60+ degrees) so that I could clear my turn inside of the treeline (in the cleared area around the runway). I also lit up all of my landing lights, so that I could see the trees as I made my turn inside the line. Once I made my 180 and reestablished my climb toward the higher ridgeline to the south, I found that the winds that created the downwash on the north ridge were also creating ridge lift against the south ridge which helped lift me over the ridge.