Register here

Author Topic: Jet Engine Assisted Takeoff and Mixed Propulsion Differences  (Read 2358 times)

Offline Archibald

  • Senior Member
  • CLEARANCE: Top Secret
  • **
  • Posts: 2073
Re: Jet Engine Assisted Takeoff and Mixed Propulsion Differences
« Reply #30 on: August 03, 2018, 04:00:09 am »
Quote
. I doubt that they replaced or modified an avgas tank to kerosene, because it wouldnt make sense to reduce the range of the airplane by, lets say half an hour, to allow those wingtip jets to run maybe 3 minutes?

Well, there was a trade.
Let's suppose a Noratlas transport flying in Algeria, said in 1959. Temperature on the ground is around 45C.  Yet the Nortlas is heavily loaded - because this is the Algerian war.
At ordinary temperatures- let's say, 22C - a fully loaded Noratlas is already seriously underpowered.
 Now the hotter the temperature, the longer, and more dangerous, the lift-off, because very hot air degrades both engine power and wing lift.
 By this point, if the Noratlas crew is forced to liftoff fully loaded, and with such temperature, either their risk a crash, or either they light the wingtip jets to help liftoff, even at the expense of range.
in a nutshell: better to have a safe takeoff and a shorter range, rather than risking lives trying to get an overloaded, underpowered aircraft off the ground in a very hot temperature. Same for C-123s in Vietnam.

In fact a lot of very large, very heavy transport aircrafts that were piston-powered got jet pods to help liftoff. It is because turboprops were not ready yet.

When ones compare a Noratlas and a Transall, they are pretty similar in weight and overall dimensions and payload, more or less 10 mt. But their engine power is not the same. A Noratlas had two 2200 hp radials, so 4500 hp. A Transall has a pair of Tyne turboprops, 5500 hp each, total 11 000 hp, nearly three times more. The jet pods were there to help those big piston engine transport at lift-off. Then they vanished for the next generation of transports.
« Last Edit: August 03, 2018, 04:05:37 am by Archibald »
Conservatoire de l'Air et de l'Espace d'Aquitaine
http://www.caea.info/en/plan.php

Profanity: weaker mind trying to speak forcefully

Political correctness: just bury your head in the sand for the sake of appeasement and "peace for our time"
- https://fr.wikipedia.org/wiki/Serge_Dassault#Affaires_

Offline Jemiba

  • Global Moderator
  • CLEARANCE: Top Secret
  • ****
  • Posts: 7896
Re: Jet Engine Assisted Takeoff and Mixed Propulsion Differences
« Reply #31 on: August 03, 2018, 07:58:24 am »
... and nevertheless there were even thoughts about fitting auxiliary jet engines to the good old Trall :
https://www.secretprojects.co.uk/forum/index.php/topic,18205.msg173776.html#msg173776
It takes a long time, before all mistakes are made ...

Offline Stuka_Hunter

  • CLEARANCE: Confidential
  • *
  • Posts: 37
Re: Jet Engine Assisted Takeoff and Mixed Propulsion Differences
« Reply #32 on: August 03, 2018, 08:20:29 am »
Thx for explaining that with Noratlas. I was already aware of the problems with hot and high operations, but with the B727/JATO in particular.

And if anyone is confused right now, your sight isnt messed up. B727 with JATO instalation did exist.

Offline Archibald

  • Senior Member
  • CLEARANCE: Top Secret
  • **
  • Posts: 2073
Re: Jet Engine Assisted Takeoff and Mixed Propulsion Differences
« Reply #33 on: August 03, 2018, 09:52:42 am »
didn't knew about that ! Fair enough... according to many pilots Transall hard hard times flying over Afghanistan theater... their anemic rate of climb was not very reassuring when flying over mountains, and in summer, Kabul and other afghan airports had high temperature and high altitude, and Transalls hated that. The A400M should do a better job.
Conservatoire de l'Air et de l'Espace d'Aquitaine
http://www.caea.info/en/plan.php

Profanity: weaker mind trying to speak forcefully

Political correctness: just bury your head in the sand for the sake of appeasement and "peace for our time"
- https://fr.wikipedia.org/wiki/Serge_Dassault#Affaires_

Offline charleybarley

  • CLEARANCE: Confidential
  • *
  • Posts: 65
Re: Jet Engine Assisted Takeoff and Mixed Propulsion Differences
« Reply #34 on: September 22, 2018, 01:28:02 pm »


More generally, mixed piston-jet were a maintenance PITA for a simple reason. The piston engine burned avgas when the jet needed kerosene. So they needed separate tanks. Took me a while to realize this, I thought you could burn avgas into a jet.

You were right the first time. Gas turbines run on avgas.
 All that is required before allowing it is to run an endurance test (50 hours say) and checking the condition of certain components afterwards (eg fuel control) to make sure there is no unusual wear for example.

I've just read (The Handley Page Victor Vol1, Brooks) that the Victor wing structure was designed for the pressure required to stop avgas boiling at high altitude, ie a possible worse case than turbine fuel that could have occurred with engines cleared to run on avgas.

Offline charleybarley

  • CLEARANCE: Confidential
  • *
  • Posts: 65
Re: Jet Engine Assisted Takeoff and Mixed Propulsion Differences
« Reply #35 on: September 22, 2018, 03:57:03 pm »
More generally, mixed piston-jet were a maintenance PITA for a simple reason. The piston engine burned avgas when the jet needed kerosene. So they needed separate tanks. Took me awhile to realize this, I thought you could burn avgas into a jet.

Some of the early jet engines were cleared to burn Avgas.  "Aircraft Engines of the World" by P. Wilkinson, 1946 plus 1948 editions lists the I-16/J-31, J-33, J-35, and TG-100/T-31 all as being suitable for 100/130 gasoline.   

Gas wasn't the best for fuel pumps on the jet engines, and the tetraethyl lead in the gas was hard on the hot end parts, but they could burn gasoline.


Apologies for repeating, just seen this. Just to add, some of todays engines are cleared for limited running so they can refuel at remote strips where there is no jet fuel. eg small turboprops and business jets.

Offline charleybarley

  • CLEARANCE: Confidential
  • *
  • Posts: 65
Re: Jet Engine Assisted Takeoff and Mixed Propulsion Differences
« Reply #36 on: September 22, 2018, 04:51:47 pm »
Catapult isn't thrust. it's pull not push.

Chris.


Surely what makes the plane go forwards is known as thrust regardless of where it comes from? (Except when going downhill in which case it's usually called weight)

If we say pushing is from behind and pulling is from in front then the catapult deck shuttle pushes the bridle because the steam pressure comes from behind. The bridle, in turn,  pulls the towing hook from in front. So by these conventions the catapult mechanism has both push and pull to give thrust.

Offline charleybarley

  • CLEARANCE: Confidential
  • *
  • Posts: 65
Re: Jet Engine Assisted Takeoff - Examples
« Reply #37 on: September 22, 2018, 06:03:22 pm »
Quote
However, one question remains: How would you by definition separate the aircraft with mixed propulsion (example: Ryan Fireball) from the airplanes with auxiliary engines (example: C-123J), or does both mean the same thing?
The following statement will cause confusion and worse because your definition is different to mine.  The Skyhawk, Shackleton Mk3 and Gannet had thrust augmentation.
It's better if I say:
The thrust on the Skyhawk was insufficient to get it off the deck so it was augmented with a steam catapult.

I would not classify a catapult as thrust augmentation. To me (admittedly not being a native English speaker) thrust augmentation implies some sort of an additional propulsive system on board an aircraft (even if it may be separated after use), whereas a surface based launch aid such as a catapult in my view falls into a larger class of acceleration augmentation, which includes off board devices as well.

Martin

Thrust is what urges the plane forwards and augmentation just means making something bigger. The catapult does exactly that, gives more urge, augments the engine thrust.  Traditionally, for gas turbine engines the term has been used for things within the engine that do that. Therein lies the problem. Using just two words, thrust augmentation, because we all like trying to classify things, doesn't work well. You have pointed this out by using a few extra words to make a better job of communicating what you are actually thinking.

Here's an example of trying to use a definition or classification which touches on Chris' turboramjet 'can o' worms'. Kelly Johnson (in his autobiography Kelly More Than My Share of It All) said the J58 'flew as a ramjet". If he had left it at that, as do most other writers on this subject, we would have been left guessing. We would have comments like 'what definition for ramjet is he using?' 'He must mean it is because of this or because of that' It's up to us to guess or assume. The good news is, unlike other writers, he does go on to tell us why he used the term. 'With no machinery obstructing the airflow...' And it turns out to be incorrect as shown in published information, such as YF12 pilots manual, etc.