Dora roll rate and turning rate, true to real-life data?

[Crumpp]

But it is an issue Art-J.

 

It is not dramatic and will most effect instantaneous roll rate not sustained roll rate.

 

Instantaneous roll rate has the largest effect of the pilots perception of rolling ability.

 

BTW, I do not think there is a thing wrong with the DCS Dora's roll rate. The Dora was a very agile design even with the tail extension.

 

 

Tail off effects of tail length on Lateral Force coefficient, Yawing coefficient, and Rolling coefficient:

 

 

Tail on effects of tail length with a constant volume tail on Lateral Force coefficient, Yawing coefficient, and Rolling coefficient:

 

 

I admire the will to explain flight-dynamics issues in simple manner, but the explanation above might do more harm than good I'd say.

 

I am sorry man, not trying to be confusing. What seems obvious to me I guess is not so obvious to others. :cry:

 

So Yo-Yo is absolutely right in that the largest effect is longitudinal and directional stability.

 

Increasing the distance of the tail surface AC's is going to have an effect on our aircraft rolling ability because the stability margin is increased.

[Crumpp]

also don't get what the "aileron design" chapter and the two pages above have in common with our problem - difference between Anton and Dora rollrate? In both cases wing, horizontal and vertical stab geometry is the same,

 

It shows that the only difference is the tail length and breaks down what is important.

[sobek]

Please keep on topic.

[Bluedog]

Still can't see it. I think you're reading too much into the graphs, but thanks anyway.

 

Point taken, Sobek, I'll back out. Regret asking the question in the first place.

[Crumpp]

Still can't see it.

 

Read it then! It is spelled out in the NACA report I posted.

 

They thought the subject was worthy enough to spend considerable time and effort on the effects of length on lateral dynamics.

 

:thumbup:

 

FlightLab says:

Rolling moments come with yawing moments attached, and those yawing moments affect roll behavior.

 

See attached.

[Bluedog]

Moderator says desist - so I have!

[weta904]

Crumpp I think you are on "a hiding to nothing" re trying to explain the reasons for roll rate in less than 25 words ... so numbies like myself can understand. I found the following on the net and thought it was interesting.

 

Muscle Versus Roll Rate

Aileron systems are designed primarily in terms of the lateral control required at speeds near stall—a function of aileron size. At high speeds, roll rate is a function of the available aileron deflection. As mentioned earlier, for a given aileron deflection (thus roll helix angle), coordinated roll rate, p, varies directly with true airspeed.

 

Roll rate also depends on how big a gorilla is driving. In an aircraft without boosted or powered flight controls, the control force felt by the pilot increases as the square of the true airspeed. As a result, aileron forces go up faster than roll rates, and ultimately the force required for maximum deflection can exceed the pilot’s muscle power.

 

For example, a Spitfire had a maximum roll rate of around 105 degrees per second at about 175 knots EAS. A clipped-wing Spitfire made it to about 150 degrees per second at the same speed. A P-51B Mustang’s roll rate peaked at only about 90 degrees per second at around 260 knots EAS. When these airplanes went slower, maximum-performance roll rates decreased due to the slower TAS. When they went faster, roll rates decreased because the pilot couldn’t fully deflect the controls. The roll rate of the Japanese Zero went down drastically at high speed because of aileron reversal caused by wing twisting.

 

By way of comparison, if a contemporary, high performance aircraft designed for top aerobatic competition rolls less than 360 degrees per second, at maximum sustained level flight speed, it’s considered a slug.

Edited January 9, 2015 by weta904
formatting

[MiloMorai]

The BMW powered 190s were known to do a flick stall and spin (always to the right) when pushed hard. Some pilots even used it as an evasive maneuver.

 

Did the D-9 also have this problem?

 

The enlarged tailplane of the Spitfire was only an experiment. The last Spitfire IX/XVI had come off the production line a year earlier.

[Crumpp]

Crumpp I think you are on "a hiding to nothing" re trying to explain the reasons for roll rate in less than 25 words ...

 

Exactly.....I was trying to be helpful but anything said is just grist for the mill.

 

There is a lot of variables. That is why is so hard to find good data.

[Crumpp]

so numbies like myself can understand

 

You are not a "numbies" and I hope I do not come across as treating people as such.

 

I hope to see you online!

[Kwiatek]

The BMW powered 190s were known to do a flick stall and spin (always to the right) when pushed hard. Some pilots even used it as an evasive maneuver.

 

Did the D-9 also have this problem?

 

 

 

Yea i still wonder if DCS D-9 is not too forgive in spin charactersitic. There is really hard to push it in spin.

 

Other hand A version was known for harsh spin characteristic. It got quite strong wing drop during straight stall. In turn it could be even more nasty casuing flick roll.

 

Also i wonder if or how different was D-9 comparing to A regarding trim settings and high speed stick forces expecially in pitch.

[Crumpp]

Yea i still wonder if DCS D-9 is not too forgive in spin charactersitic.

 

It spins normally in DCS.

 

In the stall, none of the operating instructions mention this characteristic and the lift polars on the design show rather unremarkable, even gentle stall characteristics.

 

It did occur so I tend to gravitate towards one of two explanations:

 

1 Aeroelasticity effects - occurred under high load conditions

 

2. Aileron Adjustment - very important to the types stall characteristics. Of all the good engineering in the design, the aileron adjustment system was not one of them. Out of adjustment aileron behavior is noted as aileron reversal just above the stall.

 

If you read Wright Patterson's evaluation of a Dora 9, despite the rigging and trim difficulties they experienced, the aircraft was noted as having a gentle stall. Power Off, the pilot had difficulty in getting the wing to break at the stall point which is actually a good characteristic and can be reproduced nicely in DCS.

 

Accelerated stalls came with little warning (probably in the form of aileron vibration) but no adverse behaviors were noted.

 

Most airplanes accelerated stall behavior depends on how coordinated the turn was at the time of the stall.

Edited January 9, 2015 by Crumpp
added Wright Patterson evaluation

[Kwiatek]

It looks that Dora was more stable then Anton regarding pitch manouvers and stall behaviour in straight flight. But still in hard manouvering it could stall with little symptoms. Also it looks that above 375 mph elevator became quite heavy.

 

Interesting is that report mentions heavy and excesive elevator forces at higher speeds. In DCS there is not feelable too much.

 

I wonder what Yo-Yo could say about it. I would like to see also Yo-Yo opinion about stability in flight both Fw 190 and 109 but also about stick forces and elevator effectivness at high speeds?

[Crumpp]

Interesting is that report mentions heavy and excesive elevator forces at higher speeds

 

Despite the unknown and lack of detail on the rigging issues at Wright Patterson, that ~375mph IAS is in perfect agreement with the trim shift data I have on the longitudinal stability and control of the FW-190A at 6km ALT.

 

"Excessive" is kind of subjective and not quantifiable. The increased tail length means the Dora's elevator can produce a larger moment about the CG than the Antons.

Edited January 9, 2015 by Crumpp

[Bluedog]

An important aspect of the Dora 9 vs Antons is the CG range is shifted forward on the Dora series.

 

The stability margin is larger and is reflected in the roll rate. That is why the Antons roll better.

 

Well, I've done a whole swag of reading trying to understand the issue (yes, I've read and re-read the posted reports etc) but I cannot find any specific reference to how movement of the CG (ie change in static margin) will affect roll rate. Not that we can assume that my failure to find a reference proves the theory wrong - I'm prepared to accept that. It's just that if it is the cause for a dramatic reduction in roll rate (I'm working on vague mention of 40 deg/sec at "cruising" airspeed), then surely it would be applicable to similar "stretching" situations and be a well known and well documented characteristic. The best I could find are a couple of references stating that CG movement has little or no effect on lateral stability but has an effect on directional stability in the usual sense. The weight of evidence.......?

 

 

The other factor is that comparing Anton to Dora may be akin to comparing apples with oranges. Now I admit I know practically nothing about either aircraft; I have no firm data on such things as CG ranges, mass distribution, fuselage/wing/horiz & vert stab/control surface sizes/areas, power & propeller characteristics/ and whatever else would be needed to make a worthy assessment of whether the two aircraft are or are not comparable (and even if I did have that info, the exercise would be well beyond my capabilities). But let's assume they are comparable:

 

 

The anti-rolling moment slowing the roll rate of the Dora must come from somewhere. The dominant factor is roll damping but I'm guessing the contributions will come from the usual culprits which might include prop wash: engine torque, yawing motions, mass distribution within each aircraft, and control system characteristics (and the all encompassing "etc"). I'm also guessing that if the static margin has an effect would only be a minor contributor, not the main cause.

 

 

But then again I might be wrong. I'm happy to be shown otherwise - I'm that kind of guy. A second opinion would be nice; is there a real aerodynamicist in the house?

 

 

What might be interesting is to compare roll rate under clinical conditions for a representative aircraft (doesn't have to be FW190) at a significant forward CG and then at a significant aft CG to find out if there is a dramatic change in rate of roll due to movement of the CG.

[Yo-Yo]

The BMW powered 190s were known to do a flick stall and spin (always to the right) when pushed hard. Some pilots even used it as an evasive maneuver.

 

Did the D-9 also have this problem?

 

The enlarged tailplane of the Spitfire was only an experiment. The last Spitfire IX/XVI had come off the production line a year earlier.

 

Anton flick rolled out of turn if the engine was set to idle before this maneover and there was no rudder input. Two sequental flick rolls turned defence to offence :). But the first roll must be at 450-500 kph.

[Crumpp]

That is interesting!

 

So no rudder input means the roll was uncoordinated making aero-elasticity effects the likely cause of it.

[Yo-Yo]

That is interesting!

 

So no rudder input means the roll was uncoordinated making aero-elasticity effects the likely cause of it.

 

Any slight deviation from coordinated turn (that always presents) leads to increasing yaw-roll because of negative damping at post-stall AoA, moreover, abrupt AoA increasing leads to instant rate of turn increasing and can cause instant out-of-coordination condition equivalent of out-of-turn pedal input.

[Crumpp]

Any slight deviation from coordinated turn (that always presents) leads to increasing yaw-roll because of negative damping at post-stall AoA, moreover, abrupt AoA increasing leads to instant rate of turn increasing and can cause instant out-of-coordination condition equivalent of out-of-turn pedal input.

 

I understand Yo-Yo and agree that the dynamics of an uncoordinated turn certainly contribute in no small way to this interesting phenomenon in the FW190.

 

Those effect are universal and in my humble opinion does not fully explain the violence in this behavior compared to the aircraft's lift polar or other aircraft in the same conditions of flight.

 

The Germans investigated this in "Die Entwicklung der Tragwerkkonstuktion FW190" Bericht 176 der Lillenthal-Gesellschaft, 2 Teil, January 1944.

 

From that Report, which I do not have a copy of but have being meaning to get on my next trip to the Paul Gerber Facility, it was concluded that aero-elasticity was removing the washout of the wing causing the entire wing to stall lending an even larger moment and more abrupt AoA change for the instant the wing is deformed.

 

The entry velocity requirement of 450-500kph and uncoordinated entry certainly set up the conditions for this to occur.

[bongodriver]

The Fw190 seems to have quite an unusual main wing spar, it is cranked longitudinally and is situated quite far forward compared to typical designs, particularly on the outboard section of the wing, to me it would seem the wing would twist with increased washout under aerodynamic loads but maybe g loads could have pulled the trailing edge down?

[Hummingbird]

What I don't understand is why the Dora-9 doesn't out turn the P-51 ingame, as I believe this was very much the case in reality, albeit they were close.

 

A Fw190 Jabo was tested against P-51B in Britain and their turn performance was found to be pretty much at a wash, despite the Anton's higher wing loading. The explanation for this IMO is the airfoils used, the 190 using an airfoil with a higher CLmax and better suited for operation at high AoA.

 

Therefore I'd expect the cleaner and more powerful Dora to outturn the heavier P-51D in the sustained turn fight, however this is not the case ingame atm and IMHO this needs to be corrected.

 

There must be some reliable modern data on the airfoils in question that could be used.

 

On a final note we will probably not get a definitive answer until the two aircraft are pitted against each other in real life :)

[Yo-Yo]

What I don't understand is why the Dora-9 doesn't out turn the P-51 ingame, as I believe this was very much the case in reality, albeit they were close.

 

A Fw190 Jabo was tested against P-51B in Britain and their turn performance was found to be pretty much at a wash, despite the Anton's higher wing loading. The explanation for this IMO is the airfoils used, the 190 using an airfoil with a higher CLmax and better suited for operation at high AoA.

 

Therefore I'd expect the cleaner and more powerful Dora to outturn the heavier P-51D in the sustained turn fight, however this is not the case ingame atm and IMHO this needs to be corrected.

 

There must be some reliable modern data on the airfoils in question that could be used.

 

On a final note we will probably not get a definitive answer until the two aircraft are pitted against each other in real life :)

 

This fairy tales about "pour CL of P-51 airfoil" and "cleaner Dora" are not more than city legends. There are exact measurements of lift for the P-51 at all range of M numbers so for 23xxx airfoil Dora has.

 

Dora can not be "cleaner" than P-51 - the most clean amongst WWII fighters - having less than 0.02 minimal CD.

 

What is the reason for P-51 to be heavier than 4300 kg Dora? They are very close in weight but P-51 has less wing load due to wing area difference.

Edited April 1, 2015 by Yo-Yo

[GrapeJam]

Therefore I'd expect the cleaner and more powerful Dora

A plane that achieves 625km/h at SL with a 2250hp engine is cleaner than a plane that achieves the same speed but with a 1850hp engine....

 

Yeah.....

 

Hope that was your April's fool contribution.

[Hummingbird]

*ahem* I was refering to the Dora being cleaner than the Anton...

[Hummingbird]

This fairy tales about "pour CL of P-51 airfoil" and "cleaner Dora" are not more than city legends. There are exact measurements of lift for the P-51 at all range of M numbers so for 23xxx airfoil Dora has.

 

Dora can not be "cleaner" than P-51 - the most clean amongst WWII fighters - having less than 0.02 minimal CD.

 

What is the reason for P-51 to be heavier than 4300 kg Dora? They are very close in weight but P-51 has less wing load due to wing area difference.

 

Fairy tale? First of all I was refering to the Dora being cleaner than the Anton, not the P-51. Secondly I don't think that the low CL of the laminar flow airfoil is a fairy tale, all we have is one document from the 40's on scale models and a rather low reynolds number, where'as in the field the P-51's wing couldn't maintain the laminar flow it could in the wind tunnel tests.

 

By sharpening the leading edge of an airfoil and moving back the point of max thickness you will reduce drag significantly, but you WILL also reduce the critical AoA and Clmax. You can't have the best of both worlds in this area.

 

The evidence to back this up are the hard figures we have on the aircraft, the Dora stalls at a lower speed and lands & takes off at lower speeds than the P-51 as well.

 

Hence why the British found no difference in the turning performance between that of an Fw-190 Jabo and P-51B mustang.

Edited April 2, 2015 by Hummingbird