Author Topic: Roll rates (Read 2685 times)

hitech · « **Reply #45 on:** May 27, 2003, 12:12:26 PM »

Yes on ail. reversal zigrat. It just falls out of the model from lift curves.

HiTech

icemaw · « **Reply #46 on:** May 27, 2003, 12:45:25 PM »

Here is a thought. Contact some pilots that have warbirds and ask them to run some tests.

joeblogs · « **Reply #47 on:** May 28, 2003, 01:17:24 PM »

This sounds like the right way to work a problem, manipulate that side of the equation where any uncertainties result in smaller effects. Engineers do this all the time, as do economists.

-blogs

Quote

Originally posted by Pyro
I follow you Zig. There's nothing tricky about the way the model works. It works as you would expect it to. The ailerons change the effective AoA of that portion of the wing and the resultant forces generate the effect. Looking at a typical roll rate chart for these planes, roll rate will increase with speed up to an apex and then fall off as stick force becomes a limiting factor. Once the data is plugged into the model, the plane should be closely match the part of the chart to the apex. If it doesn't then that tells me that something is off and I have to massage the variables to produce the desired output. Once you get past the optimal speed, roll rate is decreasing because maximum stick deflection can no longer be achieved. At that point, I'm looking at the backside of the roll rate curve and using that to input to the model that maximum lateral stick movement for that speed. In this case, the inputs for limiting stick deflection are solely based on the desired output. It is a lot of inputs to make and any mistakes made there are not readily apparent. It also means that if I later find something amiss on the front side of the curve and correct it, it throws everything off on the back side. One of the changes that I alluded to is that we're changing the input to the other side of the equation. Instead of stating what the maximum stick input is at a particular speed, the maximum roll rate will be used instead and the model will fill in the necessary stick input. That will give us a higher degree of accuracy and make errors less likely. We're also making changes to get rid of some limitations that introduce small errors that have to be overcome elsewhere. An example of this is that the current model assumes aileron travel is equal in both directions. That's not the case on many planes and modeling it that way could lead to some minor undesired characteristics.

As HT said, moments of inertia are nice to have but not something we really sweat. We had a good starting point and having an exact number is not needed because even a huge change can be barely perceptible. You really just need to get in the ballpark with it.

I don't know why some people feel there should be a tremendous amount of roll inertia in these planes. For me to impart a large amount on a typical single engine fighter, I would have to increase the MoI's an order or more of magnitude. It was something that we talked about prior to HT taking his P-51 flight because I thought that it should be present to a noticeable degree(I'm not talking about a big bellybutton lag, but rather the absense of crispness). Anyway, his flight laid those doubts about the model to rest. I guess the reason some people subscribe to that idea is because it seems to make sense intuitively. It's much easier to visualize mass than force.

Pyro · « **Reply #48 on:** May 28, 2003, 02:08:07 PM »

Zig, HT misunderstood your question. High speed roll reversal is not modeled.

Gripen, I don't believe I've ever seen that report. Do you have a copy perchance?

Zigrat · « **Reply #49 on:** May 29, 2003, 06:01:36 PM »

ah thanks pyro thats what i thought.. since you would need to model structural deformation too which isn't that fun!

MANDOBLE · « **Reply #50 on:** May 29, 2003, 06:28:04 PM »

Quote

Originally posted by Zigrat
I also agree that exact figures for roll inertia are probably not necessary. If you know it for a P-51 that number will probably be reasonably accurate for a Me-109 with a little massaging.

This has no sense at all for me, you are comparing something with big and heavy wings (fuel tanks and machineguns there) with something with small and "empty" wings. If exact figures for roll inertia are not necessary, then exact figures for acceleration/climb/speed etc are secondary ...

When you roll, what you are moving are just the wings against the air flow, big wings offers more resistance to movement, heavy (big mass) wings too, heavy and big ones much more. I wonder how was the roll rate (if any) of P38 at speeds below 150 mph.

ergRTC · « **Reply #51 on:** May 30, 2003, 12:01:52 AM »

I cant believe how helpful this thread has been. I have been wondering about the 'insta' response in a role that you get with AH. I needed to hear it from somebody else cause intuitively I just could not believe that was accurate....... but when I was just on my honeymoon in Kauai, HI, I happened to have my hand out the window driving to some insanely nice golf course and started playing airfoil with it. The slightest turn or twist instantly wiped my heavy bellybutton hand in another direction. Imagining the force exerted by an aileron much larger on a rather aerodynamic 'turn oriented' device traveling through the air at 250 miles an hour made me realize that any small input by an aileron on a wing would be immense!

I salute pyro for digging for this info, and explaining the modeling. Wonderful stuff. I was going to squeak about how little inirtia you felt in AH roles previous to my honeymoon.....

gripen · « **Reply #52 on:** May 30, 2003, 03:47:06 AM »

Pyro,
I have the text part of the report, I'll try to scan it.

gripen

Neil Stirling1 · « **Reply #53 on:** May 30, 2003, 04:18:57 PM »

I have the whole document scanned and zipped, If I can be of any help?

Neil.

gripen · « **Reply #54 on:** May 30, 2003, 05:16:46 PM »

Neil,
You can help Pyro better than me! Another interesting RAE report at PRO is DSIR 23/13030. It's a quite long and theoretical report on aileron characters during maneuvers and probably something Pyro is looking for.

Somekind of study on flying characters of the Bf 109 (including aileron data) might come out before end of the year

gripen

AKIron · « **Reply #55 on:** May 30, 2003, 05:21:51 PM »

I've noticed something about forward slips that just doesn't seem right, though I have no numbers to back me up. I use them regularly when landing, saves time by slowing ya down quickly.

The problem is that when I come out of the slip I always seem to lose lift. I don't think this should be the case. The forward slip should not only increase drag but also reduce lift due to less of the wing being presented to the air stream and turbulence induced over the wing by the fuselage.

Wondering if maybe the fuselage is generating more lift in the slip than appropriate?

I couldn't find the thread about slips.

Neil Stirling1 · « **Reply #56 on:** May 30, 2003, 05:38:29 PM »

Gripen,
thanks for the reference, I look fwd to seeing the 109 flying characteristics study.
BTW would you like the parts of the report you are missing, if so post your E-mail.

Neil.

gripen · « **Reply #57 on:** May 30, 2003, 06:07:06 PM »

Neil,
Thanks! My e-mail is gripen39 at luukku dot com. I can mail you in return more PRO and other british archive references.

gripen

fats · « **Reply #58 on:** May 31, 2003, 08:16:08 AM »

Mandoble:

If one used P-51's inertia tensor for 109 and it made ~5% difference in roll acceleration - I say: "So what? Go with it!"

I don't think they are using the inertia tensors interchangeably anyway, it was just said that you can get the inertia tensor with in the ball park from one plane to another if you assume the structual part of a wing has same mass per volume unit and then use the actual dimensions of the plane in question.

// fats

Wilbus · « **Reply #59 on:** June 01, 2003, 08:48:55 AM »

Spitfire 1 compared to 109 E seems to roll way too fast. I have some test reports made by the RAF of the 109 E4, comparing it to the Spit 1. Roll rate at 400mph to 45 degree roll took 4 seconds in both planes. About the same as in AH. However bellow that speed in AH the Spit 1 greatly outrolls a 109 E. In no reports does it say that this should be the case, it actually says that the 109 E had great maneuverabilty in low speeds.

This is a part of the test repor made by the Brittish.

"Ailerons

Aileron control is very good at low speeds, there is positive feel and a definite resistance to stick movement.

As speed rises the ailerons gradually become heavier but retain excellent response, being at their best between 150 and 200mph. Between 200 and 300mph they become noticeably heavier and over 300mph become too heavy for comfortable manoeuvering. Over 400mph the pilot can exert up to one fifth aileron using all his strength."

It allso says that lowering the flaps will make the ailerons much heavier and slightly less effecitve cause the ailerons come down 11 degrees with the flaps.

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