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Reverse Thrust For Push-Back

I can remember riding the C-130 down to AUTEC and watching out the back as the pilot backed down half the runway to get to the turnoff. He didn't miss the turnoff, that is just how the runway was designed. I have heard about Boeing's idea of putting motors on the landing gear to enable the plane to push itself back, but I had the understanding that they were going to use hydraulic motors for this, not electric.:confused:
 
We used to back up P-3s at Adak all the time. Easiest way to get them into the parking spots. Otherwise we'd have to tow them. We'd cram everyone onboard into the cockpit to get enough weight on the nose wheel so it wouldn't slid on the ice.

Threadjack....

I did a windmill start on the runway in Perth once. CO said go for it so we did. After we taxied back in to load the rest of the crew, we got an urgent call from the squadron saying CTF-72 said not to do it (Skipper got cold feet and called them). We asked if they wanted us to shut it down and pretend we got the message in time. We figured it would give us a couple more days of liberty in Perth. The Skipper said to keep it running.

That was the ultimate good deal trip. We went from Dodge to Adelaide for a 3 day exercise. On the first flight we got a chips light. Australians would not give us an engine so we spent 2 weeks (mostly in Melbourne) waiting for one to be flown in from the States. Than we were to overnight in Perth on the way back to Dodge. Between Adelaide and Perth we got a prop leak (closer to Perth - big discussion to which was closer - the PPC and I won over the FE and Nav - hey, the P-3 flies perfectly fine on 3 engines) . We spent 3 weeks in Perth waiting for a prop from the States. Than we sheared the starter and CO said to windmill start it. Only thing that was bad was the ass chewing I got for destroying all the crypt early so we wouldn't have to mess with it everyday (this was just after the whole John Walker spy thing). % weeks of Australian liberty courtesy of the Navy. :D
 
A4sForever said:
.
One idiot did it on a 747-200 about 10-12 years or so ago --- he melted a couple of engines and took "early retirement". You gotta' love it!!!
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How can that harm the engine itself. Interference with exhaust gas pressures?
Beta is pretty basic but how can you goon it wth a clamshell type system?
r/
G
 
Schnugg said:
How can that harm the engine itself. Interference with exhaust gas pressures?
Beta is pretty basic but how can you goon it wth a clamshell type system?
r/
G
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Well, I'm not an engineer so I'll probably get myself into trouble here --- I don't build 'em ... I just fly 'em. But the Whale with Pratts has cascade-type vanes and a high-bypass engine --- thus recycling way too much super hot air back into the intakes (basically) while on the deck which overtemped two or three engines. Literally fried them, temperature-wise.

Again, I'm not aware of ANY wing-mounted engined A/C (pure jet -- not turboprop) that are certified for reverse thrust push back --- the main reasons (to my way of thinking) are:

1. Increased danger to ground personnel
2. Increased danger of FOD ingestion
3. HIGH EGT from ingested hot air .... not to be confused with my posts, however.

For example (this is NOT the incident) :

b747_ef.jpg


edit: BTW, Schnugg .... good to see your head is still above water. Happy New Year.
 
Mefesto said:
One of the contributing factors to Air Florida's Palm 90 (737). Tried to help the tug (which was sliding on the ice) by powering back from the gate. Sucked up ice/slush in both engines and iced over two EPR probes... combined with the other links in the chain that day... we all know what the outcome was.
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Gives a whole new meaning to, "Quit chewing your ice!"
 
A4sForever said:
Well, I'm not an engineer so I'll probably get myself into trouble here --- I don't build 'em ... I just fly 'em. But the Whale with Pratts has cascade-type vanes and a high-bypass engine --- thus recycling way too much super hot air back into the intakes (basically) while on the deck which overtemped two or three engines. Literally fried them, temperature-wise.

Again, I'm not aware of ANY wing-mounted engined A/C (pure jet -- not turboprop) that are certified for reverse thrust push back --- the main reasons (to my way of thinking) are:

1. Increased danger to ground personnel
2. Increased danger of FOD ingestion
3. HIGH EGT from ingested hot air .... not to be confused with my posts, however.
.
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A4s,
That makes good sense. I wasn't thinking hot gas ingestion as my expereince with reverse thrust is typically on the runway slowing down. Could easily overtemp or cause a stall if sitting still when you used it.
v/r
G
 
AllAmerican75 said:
Electric motors can generate an immense amount of torque and power! There are very few moving parts and what makes it work is the electro-magnetic force. Here, I'll let WikiPedia explain it to y'all, 'cuz it'll do a better job than I ever could. http://en.wikipedia.org/wiki/Electric_motors
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I appreciate your enthusiasm and the physics/EE lesson, but I did manage to eke out an engineering degree in college.
The link you provided didn't really answer my question, although I didn't quite word it accurately. While, I am sure it is possible to build an electrical motor that could turn the nose wheel of an airliner, it just doesn't seem very feasible. While I haven't even tried to crunch the numbers (may have the degree, but I ain't no engineer) it seems that the motor would be too big to be practically placed on a nose wheel landing gear OR the power requirements (especially during starting) would be far too high to be practical OR it would be too expensive.
 
HH-60H said:
I appreciate your enthusiasm and the physics/EE lesson, but I did manage to eke out an engineering degree in college.
The link you provided didn't really answer my question, although I didn't quite word it accurately. While, I am sure it is possible to build an electrical motor that could turn the nose wheel of an airliner, it just doesn't seem very feasible. While I haven't even tried to crunch the numbers (may have the degree, but I ain't no engineer) it seems that the motor would be too big to be practically placed on a nose wheel landing gear OR the power requirements (especially during starting) would be far too high to be practical OR it would be too expensive.
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Yeah, I'm with you and think the whole idea is bunk, but since we're all engaged in conjecture, what about a hydraulic motor powered by the existing hyd system? I still say that you'd need an enormous transmission in order to drive a beast that big, making it both cost and size prohibitive. I suspect that the cost of keeping a small fleet of tugs around is by far cheaper and more efficient then modifying the entire fleet of aircraft with a system of questionable efficacy.

Brett
 
Reverse Thrust

Well I am an Engineer so here are my 2 cents;

Basically reverse thrust on Hi-Bypass engines are used ONLY for braking action during the landing. Airflow through the core engine is relatively cool in comparison to that part that is used for combustion. Turbine blades (not the rotating vanes) are internally cooled by that so called recoupe air. I am talking now about the 25% percent of air which goes through the core engine and not about the 75% (which is really cool air) going through the Bypass duct.

When reverse thrust is given while the engine has a certain speed, the RAM AIR Effect will provide enough air to feed the internal cooling system (LPC 1.6 Off Take on P&W Engines). If an Engine has no forward speed at all (standstill) there will be a lack of cooling air for the TBC (Turbine Blade Cooling) system and a thermal damage to the turbine section will occur.
The cascades on the Reverser halves are also pointed in different directions, also to create turbulence on purpose to those wingareas which normally create Lift. With this in mind several vectors of reverse thrust in different directions won't back the aircraft up, so it will only thermally damage internal turbine section.

During Testrun session we are only allowed to rev the engines up till 1.4 EPR for the max of 20 seconds. Considering the amount of thrust needed to back up a widebody, 1.2 to 1.7 EPR will be required which equals Take Off Power.
Ramp Rats won't be amused when containers and stuff like that follow them into their holes......

On the previous engine generation, internal airflow management was not such as critical as on nowadays generation. Also heavier materials were used at that time who could withstand higher temps but were less economical in inspection intervals.

There are experiments on the market for electrically driven tow trucks which clamps on of the main gear wheels and provide movement from the parking stand. The aircraft NWS (nose wheel steering) will provide directional control, but as in aviation every lbs is one too much, it's hard to believe that an operator will carry such an amount of weight on daily flights.


Happy New Year to everyone!
 
An electric motor would be a poor choice because of the high current draw associated with starting the motion of the plane. This is going to cause either: bigger generators or bigger motors. Either way, its heavy and expensive. A better bet would be hydraulic motors. You already have high pressure high capacity hydraulics for the flight controls, and the motor would be much smaller and lighter. The best example I can think of is that you don't see electric flaps or gear on an airliner, but its common on something small like a T-34. My two cents as an engineer.
 
luckechance said:
....you don't see electric flaps ...on an airliner....
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Actually, .... Yeah, you do see 'em .... the backup leading/trailing edge flap system(s) on the Boeing 747 -- and others -- utilizes electric motors. Slow and inefficient, but electric motors, nevertheless ... It may somtimes take @ 20 minutes using these alternate systems to configure for a landing.
luckechance said:
My two cents as an engineer.
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God deliver Aviators from engineers .... my $20 as an Aviator ... :icon_wink
 
I'm an engineer too!:o
I agree with the weight comment, every lb you add onto a plane is a lb too much (you can call me a communist, but I prefer metric, so Kg would be more appropriate). It seems to me that the extra weight, and cost as previously mentioned could not outwigh the cheap use of tugs. There is a cost associated with flying that extra weight around all of the time.

Agree as well about the induction load on the generators that power the motor as well. There would need to be a lot of coils on those motors. that means lots of weight.

I'm surprised that fly hasn't chimed in on this Engineering BS.
 
Below is a setup of the proposed electric drive. Its been awhile since I did motor design but electric motors are capable of tremendous output torque. There are numuerous electrical control methods to keep current draw low so an aircraft apu could power it. The manufacturers website goes into the technical details of how this is done and is typically what I see in high power electric motor applications. As for added weight I saw no mention of it but is probably a couple hundered pounds per gear.
Running on a hydraulic drive may actually be less efficient as you incur losses from the hydraulic pump and the the hydraulic drive instead of just the motor.

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