Effect of engine on glide rate and speed retention

[Sled]

Most of us know that one of the ways you can entice an enemy into an over shoot, is to cut your engine. This is also the preferred method of speed management by some. Myself I have always cut my throttle but left my engine running, I don’t even have my engine kill switch programmed on my stick, But being I have a X45 it is easier to just cut the throttle. Recently I have wondered if there is any difference between the effect of cutting throttle, and cutting engine. I decided to run a test. I chose Aircraft that are popular in the MA, and tend to be the most used Planes. Where there were several versions of the same model I only chose 1 or 2. I would think that results (in THIS test) for other versions, of the same model, would be very similar.


All of the testing was conducted offline in the dueling arena; I flew out of base A11, and tested at 6K feet. Auto level was used through each test from start to finish. All Planes had 50 Gal. of fuel onboard at the time the test commenced. Fuel burn multiplier was set to 0.001, after 50gal was reached. Some planes were taken to 7K feet and put into a shallow dive to achieve the starting speed of 345mph; all aircraft were steady at 345mph when the test was started. Two tests were conducted upon reaching a steady speed of 345mph. Test one was to turn the engine OFF and let the plane glide down to 115mph on auto-level. Most planes were just getting into the stall horn at 115mph. Test two was to leave the engine ON and just cut the Throttle at 345mph, and let the plane glide down to 115mph.

I had to break the test up into two groups. Group A (test 1 & test 2) the Prop continues to spin even after the engine was cut off, through out the test length. Group B (test 1 & test 2) the Prop STOPS spinning after the engine was cut, and was stationary through out the test length. This obviously makes a HUGE difference in the amount of time the aircraft can maintain its energy after cutting engine off.


Group A --------------------Test 1-------------------Test 2
----------------------------Engine cut-------------Throttle cut
--------------------------345-115mph-----------345-115mph


190-A5---------------------53sec.------------------53sec.                        
190-D9---------------------46sec.------------------46sec.
109-G2---------------------42sec.------------------42sec.
109-G10--------------------46sec.------------------46sec.
La-7------------------------50sec.------------------52sec.
Yak-9U---------------------35sec.------------------35sec.
Spit-V----------------------36sec.------------------36sec.
F4U-1D---------------------49sec.------------------51sec.
F6-F------------------------54sec.------------------54sec.
P47-D40------------------1min 03sec.------------1min 03sec.
P51-D-----------------------54sec.-----------------56sec.
Typhoon---------------------37sec.-----------------38sec.


Group B                        


C.205--------------------1min. 55sec.-------------46sec.
110-G2------------------1min. 45sec.-------------47sec.
Ki-61---------------------1min. 54sec.-------------50sec.
N1K2---------------------1min. 50sec.-------------47sec.
P-38----------------------2min. 03sec.-------------39sec.



Conclusions:


WOW! Look at the difference between a prop that continues to turn after engine cut-off (group A) and a prop that stops turning after engine cut-off (group B). Notice the difference between engine cut-off and throttle cut-off on the P-38. Notice the P-47 in group A, it has the BEST glide time, and it is one of the Heaviest planes. While the Yak-9U, one of the smallest and lightest, has the shortest glide time. Now I realize that there are many factors that contribute to speed retention in a glide, everything from wing loading to drag coefficient. The purpose of this test for me was to determine how much effect engine shut-off has on bleeding E as apposed to just shutting off the throttle. It looks to me that depending on which plane you are flying, there is little to be gained by cutting your engine, and in some cases it my be a determent when it comes to bleeding E.

I also did this test to invite Constructive discussion on its content. One question I have is why do some aircraft props come to a stop, when others do not, given same speeds and conditions? I also wonder if this is a condition that existed in the real life aircraft.

[Wolfala]

I can't speak to the planes other then the multi engines, but cutting the throttle would turn the prop disc flat against the air stream, which would seem to correspond to your reduced glide time with power.  Cutting the engine entirely would, at least in the way its modeled here have lesser drag penalty b/c the props would be closer to a high pitch low rpm. Just a guess - I know the 110 and 38 don't have feathering props in here , but seems to add up.

Wolf

[rod64]

The other issue with cutting your engine (turning it off) is that the sound will give you away (or lack of sound in this case).

This is apparent also when waiting for someones engine to die when you know they have an oil hit, and are following them. When you hear that their engine stops, you can be alerted to a ditch or other manouver.

[Angus]

If the prop stops entirely,not to mention if its angle can be altered to coarse, the aircraft will glide much longer.
Many WW2 pilots witnessed that.

[bozon]

Very interesting SLED. thanks.


Bozon

[Pyro]

The difference between Group A and Group B is that B has fully feathering props.  However, certain members of group B(C205, Ki61, N1K2) should not have feathering props and that was simply a flag set incorrectly on those planes.  I've changed it for the next version.

[GRUNHERZ]

Any giuesses as to why the Fw190D9 keep its e so poorly compared to Fw190A5?

[Tarmac]

And why the 109g2 would crap out before its heavier, lumpier 109g10 brother?

[Pyro]

Originally posted by GRUNHERZ
Any giuesses as to why the Fw190D9 keep its e so poorly compared to Fw190A5?

Because it's really too broad of a test to make that conclusion.  Because of the speed range selected, you're really looking at two separate conditions and then summing them together.  You have high speed deceleration and low speed deceleration which are very different because parasitic drag is predominant at high speed and induced drag is predominant at low speed.  A few relations to keep in mind when you think about this:

1-Parasitic drag is proportional to the square of the velocity.
2-Induced drag is proportional to the square of the lift coefficient.
3-Lift is proportional to the square of the velocity.
4-Lift is proportional to the lift coefficeint.

So if you test the same plane at heavy and light weight conditions, you'll find the heavy condition retains speed better at high speeds, but doesn't at low speeds.  The reason being is that it has more mass (inertia), but does not pay much of a drag penalty for that at high speed.  At high speed it only takes a very slight difference in lift co to produce the extra lift required.  As the plane slows down below best glide speed, induced drag becomes the predominant drag as it is going to be inversely proportional to the square of the velocity.  So in this part of the envelope, the plane in the heavy condition is paying a huge penalty, whereas it wasn't on the other end of the envelope.

[Tilt]

I am surprised the La 7 holds unpowered speed so well compared to  eg  any 109.... I cant see any attribute the La7 has that would make it out glide these ac.

Its about the same weight

Its has greater frontal area

It has a deeper wing

It has a deeper fuselage

Its prop is much the same dia.


can any one spot one?


To my mind Lavochkins performance were very much a function of  applied power and some what energy inefficient............e loss when throttling back would have been significant.


We know for example (VVS state test 11 Oct 44)  that in dive the La7 would significantly out accelerate a G4 in the initial phase. After which the G4 would pull away if the dive continued.

To me this is a function of the above test......in dive the speed was eventually reached where the engine/prop capability was no longer the main acceleration function it being replaced by G and drag.

[Sled]

Good discussion guys. Pyro can you give a reason why the P-47 did so much better that the spit V or the Yak? It seems to me that the P-47 is so much heavier than the spit or yak, plus it has a Brick wall in the front for an engine. I understand that the greater weight of the 47 is going to help it at the higher speeds (body in motion tends to stay in motion) but that should be mostly offset by the radial engine (barn door)  that it has in the front. After all the amount of energy need to move air out of your way increases exponentially as speed increases. whereas the Spit has a very aerodynamic body, and although much lighter it should do very well at lower speeds. In my non-aircraft engineer thinking, I would think, at the least, it should be a tie between the spit and the 47, but the 47 stomps it by almost twice as long.

Thoughts anybody?

[bozon]

SLED,
radial engines do not contribute as much drag as you might expect. frontal area is a very bad estimate for drag.

from planes with inline engines you'd expect a much lower drag - and that might have been true if they didn't need to cool the engine. But they do, and they have to destroy almost every thing achived by the clean lines of the body by sticking a radiator to cool out in the breeze...

A radiator is an extremely draggy thing since you need it to mix the air for an efficiant heat trasfer, thus creating a lot of disturbance in the flow through it = drag.

with a good design of the radiator system, the effect of the so called "cooling drag" can be reduced. This is one of the secrets of the P51.

In high speeds, as pyro said, the drag is contributed mostly from parasitic drag that goes like Cd*V^2. So the deceleration depends on Cd/M where Cd is drag coef, and M is mass.
The jug is about twice as heavy as the spit but far from being twice as draggy.
The turning point in speed when induced drag takes over parasitic drag is about the speed of best climb rate ~160-180 mph in most AH planes. So in most of the speed range of the test you are in the high speed regime, but the low speed is not insignificant.

This is why you can throw a small rock to quite a distance, but you can't trow a peanut shell for more than a few feet.

Bozon