nwt

6 hours ago, olofscience said:

Agreed, I feel much safer in a turbine than a piston but there are still plenty of piston DZs around.

My home DZ is a single C182 operation and I am all too familiar with what that can mean for reliability. But realistically these DZs will be the last to switch to electric.

6 hours ago, olofscience said:

However, gearbox reliability IS a limiting issue - turbine power output is actually constrained by this, which is why Pratt&Whitney took more than 20 years to apply them to civil airliners via the GTF. Plenty of helicopters also have systems monitoring for metal chips in the oil system as gearbox failure there is the main killer (although admittedly, fixed-wing turbine safety record is much better than helicopters).

I'm not sure what you mean to get at here. I'm sure everyone agrees that turbines don't last forever. For the sake of this discussion, does it matter which particular component of a turbine power plant is the weak link?

9 hours ago, olofscience said:

Except for the inability to stop/start them frequently without shortening service life. And their weak point is still the gearbox which electrics can dispense with.

Yes, these are clear advantages for electric motors, but we were talking about safety. There's no reason not to feel perfectly safe in a twin otter. How often does a gearbox fail in flight?

9 hours ago, olofscience said:

Airbus is actually proposing hybrids which have electric motors powered by a turbine - having a fixed torque generator would get the best out of the turbine while overcoming the range and energy density issues of batteries.

I saw a presentation from a small company doing electric propulsion, and he said the real killer app for this tech is VTOL aircraft for transport within a large city, where noise is the limiting factor. The flat torque curve allows for large props spinning at lower RPMs, resulting in less noise. That makes a lot of sense regardless of energy storage (batteries vs. hybrid). However, that doesn't do nearly as much for us--most of the advantage for us is in battery storage.

I'm not saying there wouldn't be an advantages at all, but they would probably be relatively small and not very interesting to the jumper's POV, if they made it worth implementing at all.

13 hours ago, olofscience said:

One thing that's also great for electrics is potentially far better safety - carb icing, gearbox failures and most other mechanical failures would become a thing of the past. I'd be far more relaxed between takeoff roll and 1500 feet!

Compared to pistons for sure. Turbines seem pretty reliable, though.

14 hours ago, mdrejhon said:

Read again: $300-$400 of avgas in an electric Cessna Caravan only costs $6 to $30 of electricity.  10x to 50x fuel cost savings.  Donkey, meet carrot.  Carrot, meet donkey.

Which DZ wants to be first?

Everyone already understands that an electric plane could be much cheaper to run. Harping on that does nothing to support the feasibility.

What is the evidence that we will hit the required energy storage densities on your timeline?

You're advocating for DZOs to start planning now. What do you mean by that? What would you have them do, specifically?

7 minutes ago, kallend said:

Is that more or less than "a whole bunch"? "A shitload"?   "Heaps"?  "A bundle"? "A plethora"?

The exact number isn't so important, so long as we can agree that electing Biden will save lives.

12 minutes ago, JoeWeber said:

Thank you. I'll add that to the graph.

So you agree, then?

Just now, JoeWeber said:

That's good news. How many, then?

Probably a lot.

6 minutes ago, JoeWeber said:

The poster asked how many lives will be saved if Biden is elected. Like the cantaloupe harvest it's an obviously unknowable thing, the asking of it helps no one.

You state this as if it's obvious and uncontroversial but I don't think I agree and I doubt he does either.

11 hours ago, JoeWeber said:

How many cantaloupes will be harvested this year? How many lives would be saved if we all wore seatbelts, drove the speed limit, and never drove after drinking? You must know that is the lamest of the lame questions you could ask.

The thing about cantaloupes is a lame question, but I don't see the connection.

9 minutes ago, Westerly said:

But asymptomatic people dont normally get tested because they have no reason to go seek out a test in the first place. if you're asymptomatic than in your mind you're not infected as far as you know so there is no reason to get a test. 

Right. I am advocating for routine, asymptomatic testing. I feel I made this pretty clear.

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This plan would only work if there is large scale random, mandatory testing. Like employee drug testing basically. Even if tests could be found at every Walmart around, they were all free and all took 10 min, asymptomatic spread would still be rampid solely on the basis of people not being aware they should have taken a test in the first place.

This sort of incorrect thinking seems to be pretty common and it's been very frustrating for me. For this and other measures, it just isn't true that it won't be helpful with less than 100% compliance. There's not even any logical reason to think that. Certainly the more people who participate, the more effective the measure will be, and partial participation will be partially effective.

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Also I think you very seriously underestimate the number of people who flat out just dont give a shit. Either they think Covid is fake, or it's just the flu, or they just dont care either way--that's like 50% of the US population. Go outside and see how many people are not wearing a mask.

Why do you think that? Because of what you think you saw last time you went outside? You think the number of people you think you saw without a mask on your block today is representative of the entire country or world? Really?

2 minutes ago, Westerly said:

He is asking about vaccines, not testing. Testing doesent really help the person infected. It just helps the government keep numbers and charts. I'd say it helps others by allowing for contract tracing, but I'd just be blowing smoke as we all know no one really does contract tracing anymore. They never really did much of it to begin with and now people just flat out dont care anyway.

This is what he said:

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it's cheap and accurate testing with immediate results that will set us free.

Seems like he's saying cheap, accurate, fast testing is what we need, and he's right. It's not only about helping the person infected, it's also about reducing the spread from that person. If an asymptomatic person finds out early that he has it, that person can isolate and infect fewer people on average. That should have a great positive impact to the course of this pandemic, which is to a large degree driven by asymptomatic spread.

13 hours ago, JoeWeber said:

Awesome. So when do us corporals and privates get a jab? It's good news but the vaccine is a chimera, for sure. We want it, and hopefully you kids need to wait in line behind the deserving elderly, but it's cheap and accurate testing with immediate results that will set us free.  No one has any clue how long the immunity will last and, even after a few months when it finally trickles up to Canadiens, there will still be 40 million Americans who won't vaccinate. In the meantime, everyday I check for monoclonal anti-bodies on Amazon but still it's only books. 

The University of Illinois is doing great work on testing. Most undergrad students are required to test 2x weekly, or 3x for those identified as high risk. 1x/week for grad students and staff. Anyone can test more than that if they want at no direct cost to themselves and results have been same-day. A typical day is around 10,000 tests.

1 minute ago, olofscience said:

Yeah, I realised that after I posted

But the maintenance and fuel savings for electric ops will already be massive, so maybe regen is just a tiny bit of icing on the cake.

Yes, I think regen would just be a bit of icing. It might end up being useful, but I don't think the success of electric aircraft will depend on it.

17 hours ago, mdrejhon said:

Resistance on the propeller will cause some weird flight dynamics

Nah, pilots are already used to using propellers as speed brakes, so nothing changes much there.

16 hours ago, wolfriverjoe said:

First off, a variable pitch propeller would probably do best for regen at low pitch. The relative wind would spin it the fastest there. 

I think you might be missing the fact that during regeneration, you are going to need significant torque on the prop--it isn't going to be freewheeling. I'd expect the prop settings to be closer to that of a climb than anything else. To put it another way: intuitively, I'd expect the prop settings to be similar when you are trying to transfer the maximum amount of energy between the motor and the air, regardless of the direction of that transfer. Though we do care about efficiency, so maybe a cruise setting would be better... but anyway, this is a minor point as it doesn't affect feasibility--If we ever get to that point, we'll figure it out just fine either way.

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I don't think any exotic stuff would have to be done with the prop to get it to work better for regen. There might be some gains, but the 'cost to gain' would probably be not worth it. The primary function of the prop will still be moving the plane. Regen would be secondary.

Agreed. There should be no changes that affect powered flight in any way and no additional systems apart from the electronics required for harvesting the energy.

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The loss of weight (gravitational potential energy) would also not be a huge factor. 

The energy loss here will be exactly equal to the amount of weight lost in proportion to the total weight--no more, no less. For example, if an otter is fully loaded at max gross weight (12,500 lbs) and drops 23 jumpers at 200 lbs each (4,600 lbs), you've lost 37% of your gravitational potential energy. That's pretty significant, and my gut says jump otters don't operate anywhere near max gross.

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You aren't going to regen a significant amount of energy. Not in a 5-10 minute descent. But that's not the point. 

It's far more that some energy can be recovered. That would reduce the charge time on the ground. 

It would take a bit of experimenting to find out how much regen could be achieved and how beneficial it is. 

I agree wholeheartedly with this sentiment, except for the reference to time. Stop thinking about time and instead think about gravitational potential energy. Time isn't relevant in the way you're thinking, as I've explained previously.

16 hours ago, olofscience said:

Small propellers will have high drag losses for regen, so actually electric helicopter ops for skydiving will actually be more optimized - electric helicopter climbs to altitude, then does autorotation descent which will (hopefully) recover a significant amount of the energy used to climb (minus the skydivers' gravitational potential energy and drag losses of course).

Haha that's a really interesting point, but using a helicopter because it will recover energy better would be like cutting off your nose to spite your face. You are going to spend so much more energy to recover a couple more peanuts!

32 minutes ago, JerryBaumchen said:

 

PS)  I also would never put hip rings on a PEP rig.

 

What's a PEP rig?

On 10/18/2020 at 2:08 PM, mdrejhon said:

In reality, it scales a bit better than 8x+ power at double wind speed, because many blades tend to become more efficient at faster speeds, up to their sweet speed. 

Some ground wind turbines typically generate 9x better power approximately, at doubled wind speeds.  Basically the cubed bonus plus the higher-windspeed efficiency bonus (overcome inertia/friction/stiction/momentum), same reasons wind turbines can't spin anymore at half wind speeds -- blades don't spin until wind speed is fast enough.  Beyond a specific point, it tends to follow the cube, but there's a particularly large bonus-above-cubed speed magnitude above minimum windspeed to spin the blades.

Most airplane propellers are designed to be efficient at fast speeds, so the regen power (in theory) should scale better than cubed powers.   

Thusly, what regens at 50mph will be more than 64x regen power at 200mph in an airplane.  And, thusly, you can turn 10 kW regen to a megawatt regen simply by slightly more than quadrupling wind speed. 

A plane will have to respect approved airspeeds though, say, ~200mph, and the whole airplane has its own terminal velocity for idled dives too.  And regen will add slight more air resistance (a few percent less terminal velocity). 

So terminal velocity or maximum approved air speed may be reached before max regen capability depending on airframe design and propeller design.  Then by all means, the existing jump plane return dives, are already at optimal regen angle -- it is quite possible.

I think you are looking at this wrong. Wind turbine energy production scales in proportion to velocity cubed because the actual energy content of the wind scales that way. In our case, we have a fixed amount of energy available for recovery, in the form of gravitational potential. The more quickly we recover it, the more quickly it is depleted--you don't get more of it simply by burning through it faster.

Overall efficiency is going to be a summation of multiple factors. Propeller efficiency vs. speed as you mention is an important one that I don't have a good intuition for. Another one will be minimizing losses through the energy cost of flight, which would seem to favor getting the flight over with as soon as possible. Another one will be the cost of drag, which scales with the square of wind speed... though maybe this point is not distinct from my previous one.

On 10/17/2020 at 10:59 PM, mdrejhon said:

Beyond a certain descent rate, is just wasted gravity when regen exceeds motor/battery limits 

I think the amount of energy we recover is going to be sufficiently small compared to the energy we've spent, that this will not be a factor even though we are recovering it in a shorter period of time than we are spending it.

edit:

One last point: As people have mentioned, harvesting this energy will necessarily increase drag because energy is never free and it has to come from somewhere. However, this may actually work to our advantage by allowing an even faster descent. As has been mentioned, the descent is limited  by max airspeed. The consequence of that is the more drag you have, the steeper angle you can dive at to obtain that max speed, which results in a faster vertical rate.

edit 2:

Another realization: A good chunk of gravitational potential energy is going out the door on jump run.

3 hours ago, gowlerk said:

Maybe so, I'm not going to argue one way or the other over the legalities. But I would never send an aircraft up without adequate fuel to fly the load plus enough to get to an alternate strip in case something closes the field, plus a reserve on top of that.

I wouldn't argue against that.

1 hour ago, wolfriverjoe said:

Do you have any backup on that?

Because that's not what I was taught. 

"To the first intended point of landing" includes what ever flight is planned between takeoff & landing, even if it's at the same point.
 

It doesn't say that. So unless you have some other reference such as an FAA interpretation or something, I don't see why we should believe it means that.

For a typical skydiving flight, you're at the first intended point of landing for the entire flight. I could see that maybe being different for a sightseeing flight.

This hasn't been brought up in a long time, but I just wanted to add that IMO only 30 minutes total fuel is required if you are taking off and landing at the same airport--not 30 minutes in addition to the anticipated flight time.

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§ 91.151 Fuel requirements for flight in VFR conditions.
(a) No person may begin a flight in an airplane under VFR conditions unless (considering wind and forecast weather conditions) there is enough fuel to fly to the first point of intended landing and, assuming normal cruising speed -

(1) During the day, to fly after that for at least 30 minutes; or

(2) At night, to fly after that for at least 45 minutes.

 

https://www.law.cornell.edu/cfr/text/14/91.151

On 7/20/2020 at 6:58 PM, Zahnrad Kopf said:

Semi necro thread revival -

Has anyone here actually worn both a TFX & a G4 by now? I'm really like the chin bar of the TFX. Curious of user impressions comparing the two. I don't mind the extra cost of the TFX to have the pivoting chin bar, but not having one of their authorized dealers within casual travel distance is making me lean toward the G4.

Thanks.

I absolutely love my TFX. I tried to demo a G4 but couldn't make it work with my glasses.

On 9/12/2019 at 5:10 PM, sundevil777 said:

Quite right.

One would think that if the G4 was such an improvement, Cookie would be advertising about it, with graphs and videos from the tests and such. I would expect that skydivers being the tech wonks that we are, we would eat it up. Many more people would be replacing their G3 if there were solid data to back up the improvement. We are left to wonder if the numbers actually are impressive.  The standard to which the G4 complies might result in 50% less g loading, or 5% or who knows what? That standard may be no more ambitious than the old DOT motorcycle helmet, which was not at all ambitious.

 

I disagree. If the G3 performs poorly in the tests (and I expect it would), that could be pretty damaging to Cookie, even though they have something better now. I would say this even if the G3 were no longer being marketed, but guess what--they are still selling it!

"Look how bad our last product was" could be seen as a poor marketing strategy.