downed CTs in the news

[GrassStripFlyBoy]

Correct in that gravity & increased G-force will effect the weight of both float and fuel equally, however the float level is not based on weight of fuel, but the density of the fluid it's within.  Density is constant (outside of temperature influences).  I see the dynamic being floats sink further.

[FlyingMonkey]

On 11/22/2023 at 9:57 PM, Anticept said:

Absolutely agree, but we also currently have edge case issues where morden and I have experienced power loss on takeoff (and I know of this event occuring with another carb flight designs) and I am pretty sure it's vapor.

I also noticed that the aircraft this is occuring on are those with e-props and unleaded car fuel. A theory that I have is that they already climb hard, but the e-props edge out the extra performance that the nose is lifted even higher for climb out, further reducing pressure feeding the engine fuel pump. Testing by climbing out at a higher than recommended airspeed showed that fuel pressure issues were far less, and haven't experienced the power loss again.

A fuel pump, even if were one of the lower pressure ones that only push a couple PSI max (the engine pump would boost from there to operating pressure), would make a difference. Just something that gets a little more positive pressure and flow going if vapor does form.

Interesting theory...maybe on the hot days I'll try for a little less than max climb.  I do tend to pull the nose until I see the climb and airspeed stabilize at best numbers for the conditions, even when not really necessary.

[FredG]

Guys, technically, the density of the float and the density of the fuel change the exact same amount due to a change in G-forces when density is defined as weight per unit volume (this assumes that neither the float nor the fuel is compressible).  If subjected to 2G, the float is twice as heavy and the fuel is twice as heavy per unit volume.  As a result, the amount of fuel that must be displaced for the float to achieve equilibrium (ie, its float level) is unchanged.

The physics of this is well established and you don't have to take my word for it.  There is sufficient credible discussion of this concept on the web to answer the question.  

[Anticept]

On 11/23/2023 at 9:54 AM, Madhatter said:

I guess you have it all figured out. Hope it works for you. There are hundreds of CT's flying in Europe with the E-Prop for the last 6 or seven years and so far no issues. All I'm trying to say is that fuel systems can be complex in ways that may elude someone initially.  After 51 years of changing things on aircraft I have found surprising reasons for something not working as planned. The main issue is if you are in the air when you find this out. I never ever think I have it all figured out until I can prove definitely that the issue is resolved and other professional interested parties agree with the results and are able to duplicate these results.

Different fuel standards.

Ohio has TERRIBLE fuel standards. By terrible, I mean there is literally no program or obligation to test the fuel in this state. We've had issues pumping fuel from one station that doesn't have issues at another, and even from the same station depending on time of the year.

The two sportcruisers have no issues as long as people are using the electric fuel pump, not even a peep about fuel pressure.

There are two other mechanics looked at the flight design with the e-prop to address fuel and stumbling issues and they can't find anything, on top of the fact it was Airtime Aviation that did all the work installing and testing.

Anyways, vapor issues plagued cars for *years* until it became standard to just put a friggin fuel pump in the fuel tank!

[Anticept]

On 11/24/2023 at 10:38 AM, FredG said:

Guys, technically, the density of the float and the density of the fuel change the exact same amount due to a change in G-forces when density is defined as weight per unit volume (this assumes that neither the float nor the fuel is compressible).  If subjected to 2G, the float is twice as heavy and the fuel is twice as heavy per unit volume.  As a result, the amount of fuel that must be displaced for the float to achieve equilibrium (ie, its float level) is unchanged.

The physics of this is well established and you don't have to take my word for it.  There is sufficient credible discussion of this concept on the web to answer the question.  

Backing up fred on this too.

To help illustrate, don't think of the fuel as a liquid or the floats as a solid, but rather a bunch of little points in space, held together in long chains by fields. Some chains flow past one another, some are interlocked in a big lattice, there are several fields that give rise to these various effects but I'm just summarizing with "fields".

Every single one of these points in space experiences the exact same G forces, regardless of what chain they are apart of. They would add additional strain to those fields that keep the points from collapsing together into one super dense ball, but the key point is that they all experience it together, G forces don't just press on the top or bottom*, they affect *all* of those points at once, collectively. Because everything is affected together, changes are also equivocal. Whether they are floats or gasoline, it all comes down to those little points that are comprised of them for this discussion.

* Technically, it's not pressing, it's an effect of inertia but not relevant for this simplification.

[Madhatter]

[FlyingMonkey]

On 11/23/2023 at 6:19 AM, GrassStripFlyBoy said:

I've never had issue of power loss, the fuel line size (7.5mm or 5/16") and resulting GPM flow would be an interesting data point.  And sort of on that topic, this summer when I had a slight carb overflow issue I became aware of how sensitive the setting on carb float bowls are.

Andy, what fuel flow rate are you seeing at the gascolator?  Stab the drain port open, time for 6 minutes, then multiply the amount of fuel in jug by 10, is the technique I use.  My SW is typically around 2.25 -2.5 gallons or so / ~24 GPH.  Doing wing pull last weekend I had limited fuel in plane, around 7 gallons, and it was still solid 23+ gallons an hour flow.

Regarding carb floats, I recall you had a fuel overflow situation.  That is scary stuff and believe that was the carb float pin hole leaking on bottom and long corrected.  But that got me thinking if floats are set too low would a bit extra of "G force" on climb out tend to cause them to limit flow into bowl, one side carb more than other perhaps, and there's some roughness?  I'd double check float lever arm is nominal and not low tolerance or even too low.

Lastly on the fuel line size.  I'm just at point of 5 year rubber coming due, but have changed hoses within panel and lately the wing roots.  When I did the Y in cabin I selected 7.5mm as I didn't like anything less than a snug fit, and was in the camp of staying with 7.5mm.  When I did wing root, I took a section of both sizes and slipped on.  Both the pillar and the tank barb were snug on 5/16 hose, so opted to use Gates 5/16. That is a small section of hose, but the whole system being 5/16 would flow more, how much...  I see argument for either selection, but fuel flow could be the tiebreaker?  I'll likely go with 5/16 when replacing lines.

Hey Darrell...

I've considered a fuel line restriction, especially since I never had this problem until I switched over to all Teflon fuel lines.  I haven't measured the exact fuel flow, but I can say that from the gascolator the fuel flows now as fast or faster than it's ever been.  I have to be careful how much I let the gascolator valve open or it will overflow my fuel tester super quick.  I can't imagine that it's not flowing enough to feed the engine.  I have drained all the fuel out of the airplane and the flow rates stayed consistent all the way down to empty.  It might be worth another test, dumping 5gal or so into a jug just to make sure the flow rate stays good.  If there were an upstream restriction that should reveal it.

Another possibility I considered is hydrostatic drag.  It's possible the teflon lining of the hose generates more static and creates more drag than my old Gates hoses, which might affect things under specific conditions.  Another possibility is that the stainless outer sleeve of the hoses traps more heat inside the hoses and makes vapor bubbles more likely in hot conditions.  But I still have to believe that fuel blend is at least a part of the puzzle since I had zero issues in Georgia all summer and then this one outlier event in Florida -- where the airplane ran fine for a morning flight the same day, then sat heat-soaking on a hot ramp all day before having the incident on the return flight.

Before now I was convinced my issue was two problems together: an old fuel pump combined with a very obvious vapor trap hose run into the firewall and before the fuel valve.  Using Madhatter's setup, I added a 120° fitting on the top of the fuel valve and now the run is straight and level from the firewall to the fitting.  There's still the bend in the fitting that could be an issue I suppose.  Previously I had the vapor issue on two consecutive flights, when I fixed the hose run and replace the pump the issues vanished.

But now I have to admit my fuel system seems pretty marginal with ethanol fuel.  It seems fine with my local blends, but I don't think I'd trust other ethanol fuels elsewhere again.  Which is fine, when I'm refueling at other airports I'm always using 100LL anyway. 

[Madhatter]

2 hours ago, FlyingMonkey said:

Hey Darrell...

I've considered a fuel line restriction, especially since I never had this problem until I switched over to all Teflon fuel lines.  I haven't measured the exact fuel flow, but I can say that from the gascolator the fuel flows now as fast or faster than it's ever been.  I have to be careful how much I let the gascolator valve open or it will overflow my fuel tester super quick.  I can't imagine that it's not flowing enough to feed the engine.  I have drained all the fuel out of the airplane and the flow rates stayed consistent all the way down to empty.  It might be worth another test, dumping 5gal or so into a jug just to make sure the flow rate stays good.  If there were an upstream restriction that should reveal it.

Another possibility I considered is hydrostatic drag.  It's possible the teflon lining of the hose generates more static and creates more drag than my old Gates hoses, which might affect things under specific conditions.  Another possibility is that the stainless outer sleeve of the hoses traps more heat inside the hoses and makes vapor bubbles more likely in hot conditions.  But I still have to believe that fuel blend is at least a part of the puzzle since I had zero issues in Georgia all summer and then this one outlier event in Florida -- where the airplane ran fine for a morning flight the same day, then sat heat-soaking on a hot ramp all day before having the incident on the return flight.

Before now I was convinced my issue was two problems together: an old fuel pump combined with a very obvious vapor trap hose run into the firewall and before the fuel valve.  Using Madhatter's setup, I added a 120° fitting on the top of the fuel valve and now the run is straight and level from the firewall to the fitting.  There's still the bend in the fitting that could be an issue I suppose.  Previously I had the vapor issue on two consecutive flights, when I fixed the hose run and replace the pump the issues vanished.

But now I have to admit my fuel system seems pretty marginal with ethanol fuel.  It seems fine with my local blends, but I don't think I'd trust other ethanol fuels elsewhere again.  Which is fine, when I'm refueling at other airports I'm always using 100LL anyway. 

Have you verified that the  fuel line return orifice is installed properly and is functioning properly. This is very important when is comes to help preventing vapor lock in the line before the carburetors. It will bleed fuel pressure after engine shutdown helping avoid vapor lock. I would also guess it would also bleed off vapor during engine operation. All my fuel lines are teflon and I have never had an issue.

[Anticept]

That line goes to the gasolator on flight designs, if they went to the wing like rotax recommends, they would do a much better job of getting vapor out of the system. With how they are designed right now, it just recycles vapors until either it re-condenses or purges via the carbs.

Their primary purpose is to prevent overpressure in that lines to the carburetors, though I don't think it's as critical these days with the current fuel pump design.

[FlyingMonkey]

On 11/26/2023 at 4:33 PM, Madhatter said:

Have you verified that the  fuel line return orifice is installed properly and is functioning properly. This is very important when is comes to help preventing vapor lock in the line before the carburetors. It will bleed fuel pressure after engine shutdown helping avoid vapor lock. I would also guess it would also bleed off vapor during engine operation. All my fuel lines are teflon and I have never had an issue.

The restricted banjo bolt?  Yes, it's properly in place.  I really do think I had the problem solved once I removed the p-trap situation and replaced the old fuel pump, and this was a (hopefully) one-off due to either "bad gas" or a Florida blend.  The airplane ran like a top all the way home on 100LL and continues to run fine over several flights on 93 octane ethanol fuel here in Georgia, as it has all Summer. 

I think I just need to be more cautious of ethanol fuel when traveling, and just use 100LL when out of the local area since I don't have experience with the local ethanol pump gas. 

[Bill3558]

On 11/21/2023 at 3:58 PM, FlyingMonkey said:

I'm a big BRS fan, but I have some mixed feelings on it.  It seems if you can get it on the ground without it like in this case, that's a good thing.  If there is a deployment problem like the chute wrapping around the tail (happened to a Cirrus) rocket failure, fire from rocket, etc the airplane might be unflyable.  But on the other hand statistically you're much more likely to walk away from a chute deployment than an off-airport landing.

I think it's really a game-time call based on the exact circumstances.

I’m a fan too. Saved my butt. 

 

[EricB]

I've heard worse controllers but never during an emergency.    I periodically think about when, in a controlled glide to an off airport landing, I'd pull the chute - can't make anything good but clear of trees - but you never know.  If I were talking to this controller, I might just pull it early and hope he found the right airplane.  

[FlyingMonkey]

16 hours ago, Bill3558 said:

I’m a fan too. Saved my butt. 

Thanks for that, it was very informational!  Great job keeping your wits and being able to talk the controller through your position...totally professional.

I'd really love to hear some more information about the chute deployment.  What was your speed and altitude AGL at deployment?  How loud and violent was the deployment?  How long did it take to get the canopy fully deployed and in a stable descent?  Inquiring minds want to know!

[Towner]

4 hours ago, FlyingMonkey said:

Thanks for that, it was very informational!  Great job keeping your wits and being able to talk the controller through your position...totally professional.

I'd really love to hear some more information about the chute deployment.  What was your speed and altitude AGL at deployment?  How loud and violent was the deployment?  How long did it take to get the canopy fully deployed and in a stable descent?  Inquiring minds want to know!

X2

[Mike Koerner]

FredG,

You are right. I was wrong. Please accept my apologies.

Archimedes said the weight of a floating object is equal to the weight of the fluid it displaces. But weight is a force. It’s not just the mass of the fluid displaced (volume times density), it’s the mass times the acceleration due to gravity.

The mass of fuel displaced doesn’t change under increased g loads, but its weight does. And as you said, that increase in fuel weight will be proportional to the increase in float weight.

So, under increased G loads, the floats will not sink further in the fuel bowl.

I’m going to go back now and add a correction of some kind to my previous statement, not to hide my error, but to warn future readers it is incorrect.

[FredG]

Mike, thanks for your note.  I am glad we got it worked out.  It is a bit mind bending that the fuel density (defined as weight per unit volume) goes up when subjected to 2g - not because it was compressed (and therefore occupied less volume) - but because it weighs more under the increased g-load (as you noted).  

Now we can move on to agree on other questions like what is the best motor oil, best propeller, and best way to land an airplane...

[Mike Koerner]

FredG,

(I hope no one else is still listening)

Moving on would be wonderful, but... Density is more properly defined as mass per volume, rather than weight per volume. The mass, and thus density, of the displaced fuel didn't go up with increased g-loads. But its weight, the force that mass exerts due to the g-loads applied to it, did go up.

[FredG]

Mike, I agree that the formal definition of density is mass per volume.  In my prior comment, I was simply noting that weight per volume is what changes under 2g loading in comparison to 1g loading (I don't really have another term for the metric of weight/volume).  

[Bill3558]

On 11/29/2023 at 9:04 AM, FlyingMonkey said:

Thanks for that, it was very informational!  Great job keeping your wits and being able to talk the controller through your position...totally professional.

I'd really love to hear some more information about the chute deployment.  What was your speed and altitude AGL at deployment?  How loud and violent was the deployment?  How long did it take to get the canopy fully deployed and in a stable descent?  Inquiring minds want to know!

When I heard 500 on my headset and the runway was still a mile away, I pulled. Pretty violent event. It was loud enough that people came out of their houses to see what it was. Went from 70 knots to zero in a second and immediately into the tree. Chute was fully open for only a second. Never got into a stable descent. One of the straps was not connected.  Hence the service bulletin from FD to check all straps. I was 200 feet from an electrical sub station. I was very lucky. 
 

[FlyingMonkey]

On 12/2/2023 at 7:33 AM, Bill3558 said:

When I heard 500 on my headset and the runway was still a mile away, I pulled. Pretty violent event. It was loud enough that people came out of their houses to see what it was. Went from 70 knots to zero in a second and immediately into the tree. Chute was fully open for only a second. Never got into a stable descent. One of the straps was not connected.  Hence the service bulletin from FD to check all straps. I was 200 feet from an electrical sub station. I was very lucky. 
 

Wow, sounds like you pulled at the last possible moment for your conditions.  Good to know you could still have a good outcome even with one bridle not connected.  I wonder how much worse it would have been if you'd hit the ground directly instead of having a tree to cushion the impact.  I'm very glad it worked out for you!