highspeed roll moment & aileron efficiency

So after the latest patch and the B-17G mod I am practicing a lot of high alt bomber intercept missions and I noticed in a high speed dive above 750 kph the roll moment to the right is pretty strong. Then I checked against documentation and realized two things.

 

First of all the instability the DCS K4 exhibits is not being described in the documents at speeds below ~ Mach 0.8 (900 kph TAS) and the ailerons are effective (or the pilot is strong enough to do corrections) even above Mach 0.8. After instability problems were seen for the small rudder (with horn) of the F and early G series, the rudder heigth was increased by 135 mm and the horn removed. This is the commonly known as the "tall" rudder which is also installed on the K4. As stated in the following picture the instability was gone and yaw axis movement could be compensated by rudder inputs.

 

 

Even with the old rudder typ instability onset was above 800 kph TAS (~Mach 0.72) as seen in the following text. The text mentions explicitly "Schieberollmoment", which is exactly what can not be compensated in DCS and is caused by insufficient yaw authority which in turn leads to a roll moment. The yaw axis is called Hochachse in german. 9 highspeed testflights were conducted with W.Nr. 9228 and the old type rudder.

 

 

In a second series of tests the same aircraft W.Nr 9228 was fitted with the "tall" type rudder and 14 highspeed tests were conducted. The tests done at max of Mach 0.78 showed no instability issues only for the last test at which almost Mach 0.81 was attained instability issues arose. The last test describes roll moment to the right and oscillations which both could be compensated by aileron input. The descriptions sounds to me like the test pilot encountered critical Mach speed somewhere between Mach 0.78 and 0.81. Aileron movement in the test even was restricted to 50% preventing aileron overcompensation at high speed which is assumed to have caused some accidents in the field.

 

 

 

So in essence the rudder & aileron conrol surface authority seems to be a tad too low at high speeds, which is likely due to too low "pilot srength" or too strong control stiffening. Furthermore the instability effects/ oscillations at ~ Mach 0.8 are not modelled and may be a nice detail addition.I would appreciate if ED takes a look at this as the tests are well documented and distinct in depiction for the most part. Thank you! :thumbup:

 

 

What I forgot to mention, the K4 also has a Flettner tab at the rudder which would further reduce rudder input strength. The tests shown had no rudder Flettner tab installed. Fun fact: the 109 tested was fitted with an ejection seat.

 

 

Last but not least I will attach the TAS and Mach diagrams of the test, so you get the whole picture.

 

 

 

The whole test can be found here for example:

 

deutscheluftwaffe.com/archiv/Dokumente/ABC/m/Messerschmitt/Me%20109/Hochgeschwindigkeits_Versuche_Me109.pdf

Edited July 15, 2016 by rel4y

I don't know about that more than the paper about dives says, but if you just let me point something out,

 

~ Mach 0.8 (900 kph TAS)

And what temperature that means? As I'm sure you know Mach number only depends on the temperature, how did you calculated that? Did you have all the atmospheric information? That is important as long as 0.8M/900Km/H TAS is well into transonic speeds and that means compressibility effects. Or not, provided the calculation isn't right. So all the surrounding information of your own test is necessary.

 

S!

I know that, thats why I gave you the tables for exact numbers. My text is just there for explanation and as you may see the numbers are always with a ~ before them. The tables are measured numbers and not calculated so temperature is factored in.

 

And this is what I wrote about compressibility in the text above:

 

The descriptions sounds to me like the test pilot encountered critical Mach speed somewhere between Mach 0.78 and 0.81.

Edited July 15, 2016 by rel4y

:thumbup:

I'll point ED at this and see what they say, I do know for a fact they had this document.

 

+1

 

And good find rel4y.

Thanks Sith! One of the graphs was apparantly already overwritten with some crap, but I also posted the link to the full test at the very bottom. :)

Thank you rel4y. I don't read German, but I know that our 109K-4 controls start to become stiff at a mere 400km/h near sea level. I've always thought that was a bit overdone, but that is my subjective impression based on what I've read about flying the aircraft.

Thank you rel4y. I don't read German, but I know that our 109K-4 controls start to become stiff at a mere 400km/h near sea level. I've always thought that was a bit overdone, but that is my subjective impression based on what I've read about flying the aircraft.

 

Real life sources say that the airplane becomes two handed at 550kph. Which means the airplane becomes so stiff you cannot use one hand to maneuver.

 

So I would imagine that at 400kph it would loose a bit of agillity due to rising stick forces. Especially on aileron control as the cockpit is so tight, it actually hinders stick movement to the sides.

Edited August 24, 2016 by Solty

Real life sources say that the airplane becomes two handed at 550kph.

 

I've read similar remarks about the P-51, which also has heavy controls. IIRC, its stick forces are reported as more than 20lbs per g. Supposedly this decreases after you reach about 5g.

 

Still, your 550km/h is a helluva lot faster than 400km/h.:smilewink:

I've read similar remarks about the P-51, which also has heavy controls. IIRC, its stick forces are reported as more than 20lbs per g. Supposedly this decreases after you reach about 5g.

 

Still, your 550km/h is a helluva lot faster than 400km/h.:smilewink:

 

Thats modern P-51's, that have their equipment and feuselage tank taken out which changes CG.

 

It was discussed before and there is an excerpt from P-51D manual that says the airplane has very low stick forces.. Page 67 "Caution" paragraf. "Instead of 6pounds per G of acceleration, you excert only 1 1⁄2 pounds, the stick forces reversing as acceleration exceeds 4G"

 

[ame=http://www.deutscheluftwaffe.de/archiv/Dokumente/ABC/a/Alliierte/P%2051%20Pilot%20manual.pdf]http://www.deutscheluftwaffe.de/archiv/Dokumente/ABC/a/Alliierte/P%2051%20Pilot%20manual.pdf[/ame]

Edited August 24, 2016 by Solty