Jet Fighter vs. Propeller Fighter Turning Characteristics

[HoHun]

Hi everyone,

Since we've had serveral threads on jet fighter vs. propeller fighter turning battles on this board in the recent past, I thought a comparison of their basically different charactistics might be interesting.

The attached graph is the result of a simplified calculation for the turn rate fo two generic fighters, the jet fighter patterned loosely after the Heinkel He 162, and the propeller fighter loosely after typical US single-engined fighters.

I have not attempted to match the historical parameters accurately to avoid opening several cans of worms at once.

The basic difference between jet fighter and propeller-drive fighter is the thrust-over-speed function, with propellers yielding good thrust at low speed but dropping off at higher speeds, while jets provide roughly constant thrust over the entire speed range. In WW2, the absolute thrust of jets was not very high, so at low speed they compared unfavourably against the traditional piston-engine/propeller combination.

Regards,

Henning (HoHun)

 

[Soren]

A very good Idea Hohun,

But did you use similar CL figures and the right wing area figures ?

An important thing to remember also is that the jets will hold their speed in tight turns allot better than any of the piston engined fighters.

 

[Soren]

Btw, the thrust available to the He-162 should be higher above 450 km/h, and for this reason so should the turn rate. The BMW 003 provided about 805 kgf of thrust. At a standstill the average piston engined fighter has around 1,800 - 2,000 kgf of available thrust, this would decrease considerably at 450 km/h though and the BMW 003 should have a clear edge from thereon.

 

[davparlr]

An important thing to remember also is that the jets will hold their speed in tight turns allot better than any of the piston engined fighters.

Could you expand on this some more? I would like to understand the principle behind this.

 

[Soren]

The jets are ALLOT less draggy Davparlr, and will therefore hold their speed in tight turns allot longer. This was a great advantage, one which is noted by both vet Me262 pilots as-well as British US test pilots who flew the a/c. The He-162 would've been no different, except it didn't possess the same power to weight ratio.

Me-262's Allied POH, put together by US UK pilot experience:
"(2) The airplane holds its speed in tight turn much longer than conventional types."

Luftwaffe test pilot, Hans fey:
"The Me-262 will turn much better at high than at low speeds, and due to its clean design, will keep its speed in tight turns much longer than the conventional type aircraft"

 

[HoHun]

Hi Davparlr,

>I would like to understand the principle behind this.

Here is the diagram again, indicating turns at constant specific excess power values (in 5 m/s increments) in addition to the turns sustained at Ps = 0.

The basic relationship between jet and propeller propulsion stays the same throughout the entire specific excess power range.

Regards,

Henning (HoHun)

 

[FLYBOYJ]

Ya know, I gotta agree with Soren's statement - in a jet you don't have scoops and cooling ducts, let alone that prop that although is producing thrust is also producing induced drag.

 

[HoHun]

Hi Flyboyj,

>in a jet you don't have scoops and cooling ducts, let alone that prop that although is producing thrust is also producing induced drag.

The dominating factors are the induced drag of the wing, and the higher equivalent power of the piston engine at low speeds.

A 8 kN jet provides 600 kW equivalent power at 75 m/s. A 1500 kW piston engine at 80% propeller efficiency provides 1200 kW.

However, at 200 m/s, the 8 kN jet provides 1600 kW equivalent power while the 1500 kW piston engine at reduced efficiency - say 75% - produces only 1125 kW.

That's the reason jets are better at high speeds, and propellers are better at low speeds.

For the generic example aircraft, the break-even between total thrust and total drag in a sustained turn occurs at about 580 km/h. Above that speed, the jet can turn at a higher rate than the propeller fighter, below that speed, the propeller fighter can turn at a higher rate than the jet.

Regards,

Henning (HoHun)

 

[Soren]

Hohun,

I have the thrust output figures for the different aircraft in Kgf, and from these the conclusion is that the average piston engined fighter in 1944 produced roughly 1,800 - 2,000 Kgf of thrust at a standstill. By comparison the BMW 003 produces a constant 815 Kgf throughout the speed range.

Knowing the above we can quite safely assume that at speeds above 450 km/h the He-162 will have a higher sustained turn rate than a piston engined fighter with a similar lift loading.

 

[FLYBOYJ]

Hi Flyboyj,

>in a jet you don't have scoops and cooling ducts, let alone that prop that although is producing thrust is also producing induced drag.

The dominating factors are the induced drag of the wing, and the higher equivalent power of the piston engine at low speeds.

A 8 kN jet provides 600 kW equivalent power at 75 m/s. A 1500 kW piston engine at 80% propeller efficiency provides 1200 kW.

However, at 200 m/s, the 8 kN jet provides 1600 kW equivalent power while the 1500 kW piston engine at reduced efficiency - say 75% - produces only 1125 kW.

That's the reason jets are better at high speeds, and propellers are better at low speeds.

For the generic example aircraft, the break-even between total thrust and total drag in a sustained turn occurs at about 580 km/h. Above that speed, the jet can turn at a higher rate than the propeller fighter, below that speed, the propeller fighter can turn at a higher rate than the jet.

Regards,

Henning (HoHun)

OK -

I could tell you that the jets I've flown are slow in building up airspeed at lower airspeeds (confirming what you say). At about 200 knots they seem be able to start to accelerate quicker as well as build up and retain airspeed. On the reverse end they don't want to slow down easily. On landings you're watching airspeed and anticipating that slow spool up should you have to go around and if you allowed too much airspeed to diminish. I'm talking T-33s, L-29s and L-39s.

As this was pointed out to me - the back end of the power curve.

The Jet Provost seems a little better on spool up.

In essence when flying recips and then jets, the jets "feel slicker," especially at higher speeds.

 

[Soren]

Remind me to come fly with you next I'm in the states FLYBOYJ! I've always wanted fly one of those L-39's

 

[FLYBOYJ]

Remind me to come fly with you next I'm in the states FLYBOYJ! I've always wanted fly one of those L-39's

If we plan it right we could do and L-29 and 39! 8)

 

[Soren]

Now that would be cool!

What kind of money do you guys take for a ride ?

 

[FLYBOYJ]

Now that would be cool!

What kind of money do you guys take for a ride ?

In the 29 it's about $2000 for about a 45 minute ride. That guy is in California. The 39 is a little more. I do work for a guy with an L-39 here in Colorado and he does unusual attitude and spin recovery training for new jet owners. I don't know what he charges but I'm guessing its got to be abut $4000 for a "lesson."

 

[Soren]

Roger that, gotta start saving up some money then

 

[buzzard]

Interesting thread...

On the topic of the cost of cool airplane flights, here's one on the other end of the scale.

A couple of years ago, I was at the Nova Scotia airshow, and got talking to a couple of guys with an An-2 biplane they'd brought over from somewhere in E. Europe. They were using it primarily as a skydiving plane, and told me that they'd take me up in it during the jump flights. A month or so later, I was at the airfield. The pilot told me that if I waited until the last jump, I'd get a longer ride.

Anyway, after the skydivers bailed, he let me fly it around for about 40 minutes. Quite a change from a Cessna 180...The cockpit looks like the inside of a tank, and is surprisingly cramped for such a big ole' beast. I had a blast, and all for the grand sum of...$45!

 

[davparlr]

An important thing to remember also is that the jets will hold their speed in tight turns allot better than any of the piston engined fighters.

I think just the opposite is true and that the generic WWII jet fighter would be wise to avoid a tight turning dogfight with the generic WWII propeller fighter. In a tight turning, high "g" dogfight, airspeed for both will decay quickly to below 580 km/hr (360 mph) and, according to HoHun,

For the generic example aircraft, the break-even between total thrust and total drag in a sustained turn occurs at about 580 km/h. Above that speed, the jet can turn at a higher rate than the propeller fighter, below that speed, the propeller fighter can turn at a higher rate than the jet.

(or below 450 km/hr (280 mph)) and therefore, the advantage of the jet will quickly evaporate and the propeller plane will be able to out turn the jet. A turning dogfight could easily turn into a death knell for the jet fighter as he would lose advantage in turning ability and in acceleration. At slower and slower speeds he would not be able to out turn nor out accelerate the propeller fighter and could maybe escape by diving, but he would have to endure a period while the propeller fighter had acceleration advantage before he was safe. I am sure no German jet fighter pilot decided to duke it out with a Mustang, Thunderbolt or Spitfire and probably had orders to avoid them and use their speed to attack the bombers and run, which would have been wise.

In fact, for equal conditions, it appears to me that a propeller aircraft will have an advantage over a jet aircraft in a turn. Let's take two aircraft of identical lift capabilities and same weight, one propeller driven and one jet propelled (P-51 and FJ-1, same wing?). Now if they are both flying at 650 km/hr and enter a 60 degree, 2 g, level turn with no adjustment to throttle, now, since they have similar lift characteristics and same weight, drag increases equally, both start to slow down. But since a propeller loses power quicker than a jet as airspeed increases (as discussed previously), then it must increase power quicker as airspeed decreases. The propeller aircraft should decrease airspeed at a slower rate than the jet, thus increasing energy level, and thus better turn performance. Now, lets assume both aircraft are at max performance, like 380 mph at 5k for the P-51B ('43) and the P-59A. If we make weight and lift coefficients equal, then the P-51B would be able to maintain a higher airspeed at a given "g" load or conversely pull a higher "g" load, tighter turn, at the same airspeed. It appears to me that the jet engine, by itself, does nothing to aid and possibly hinder turn performance. What does help is the greater thrust available, which, by the way, also makes them faster, except the P-59.

 

[davparlr]

OK -

I could tell you that the jets I've flown are slow in building up airspeed at lower airspeeds (confirming what you say). At about 200 knots they seem be able to start to accelerate quicker as well as build up and retain airspeed. On the reverse end they don't want to slow down easily. On landings you're watching airspeed and anticipating that slow spool up should you have to go around and if you allowed too much airspeed to diminish. I'm talking T-33s, L-29s and L-39s.

As this was pointed out to me - the back end of the power curve.

The Jet Provost seems a little better on spool up.

In essence when flying recips and then jets, the jets "feel slicker," especially at higher speeds.

I think some of this slick feeling comes from the fact that a propeller is a thrust/drag device and a jet is basically all thrust. So when you retard the throttle on prop, not only takes away thrust but also adds drag, similar to extending the speed brake. It is similar to a car with a "freewheeling" overdrive (that shows you how old I am). If it is engaged, when you let up on the throttle, the wheels have no engine braking and, free wheel, and doesn't slow down very fast, this would be the jet. When it is not engaged, letting up on the throttle causes instant slow down due to engine braking, this would be the propeller plane. The car with engine braking is more sensitive to throttle movement. Actually, I am shooting from the hip, here.

 

[HoHun]

Hi Flyboyj,

>the back end of the power curve.

Hm, good point - I hadn't thought of it that way before, but the jet fighter's turn rate curve actually has a region that in fact is the back end of the power curve.

In a sustained turn, the curve shows that it can either turn at maximum turn rate at 435 km/h, or at minimum radius at 333 km/h.

In numerical terms:

333 km/h - 7.5 deg/s - 1.59 G - 702 m turn radius
435 km/h - 8.0 deg/s - 1.98 G - 870 m turn radius

The low-speed turn figure is achieved at the back end of the power curve, while the highest turn rate figure is achieved at the power curve's peak.

Compare to the generic propeller fighter:

364 km/h - 12.2 deg/s - 2.41 G - 475 m turn radius

(For the propeller-driven fighter, speed for best turn rate and speed for best turn radius coincede.)

Regards,

Henning (HoHun)

 

[Soren]

Davparlr,

A jet looses speed in maneuvers much much more slowly than a piston engined fighter, the reason being a Jet is a MUCH cleaner a/c, and at high speeds its also got more power available. This is simple fact, there's no point in trying to deny it Davparlr.

As both German, US British test pilots point out the Me-262 holds its speed in tight turns much better than any piston engined fighter, the reason being the much better aerodynamics of the a/c. It is also mentioned that the a/c turns allot better at high than at low speeds, and this is ofcourse in part because of the higher lift but also because of the better performance of the Jet engines.

And this is pretty much universal when comparing Jets to propeller a/c, as the prop whilst generating allot of thrust also generates allot of drag, and along with this there's the various cooling ducts which jets are without. At the same time the efficiency of the prop falls off quickly as speed increases, whilst the Jet keeps a constant amount of thrust during the entire speed regime.

So FLYBOYJ is completely right when he says that Jets are allot slicker than piston engined a/c. And like he says Jets are very reluctant to loose speed, which is one of the prime reasons you need reschooling before going from piston engined a/c to Jets. This very high energy retention is one of the major suprises to the German test pilots when they first flew the Me-262, the reluctance to loose speed during the turn in approach often forcing them to go around for a second try. (I'm sure FLYBOYJ has had a couple of these as-well when he first started flying jets)

As for the Me-262 vs Allied piston engined fighters;

The Me-262 is a far superior fighter to ANY Allied fighter of WW2, being much faster at all alts, featuring much better energy retention in maenuvers and a faster climb.

As for dogfighting, well as already mentioned the Me-262 will easily out-turn any piston engined fighter at high speeds, the energy retention being much better. As long as speed isn't dropped to below 450 km/h the Me-262 has a very marked advantage in acceleration.

These advantages is what allowed the Me-262 pilots to achieve their very high kill/loss ratio over Allied fighters, some pilots shooting down up to 10 P-51 Mustangs. (Hptm. Schall) More than proving the clear superiority of the Me-262 as fighter a/c.

90% of all Me-262 shot down were so when trying to take off or land.

There's a reason Galland wanted the Me-262 to operate as a fighter and have it replace ALL other German fighters, he knew its capability as a fighter was unrivalled.