Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Wmaker on July 30, 2009, 11:37:32 AM
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Hiya Pyro!
At first I would just like to say that you've done an awesome job on the Brewster's flight model. The lively ailerons, low control forces on higher speeds and all around great maneuverability, they are all there! There's just one aspect of the Brewster flight model that I've been wondering. That is the somewhat marginal (feels almost neutral at times) directional stability of the Brewster's current flight model. The first clue that doesn't nessasarily always point to only marginally positive directional stability of an aircraft is the excessive amount of left rudder needed the keep the aircraft coordinated in turns to both directions. On Brewster's case it seems like this is pretty much a direct symptom of only marginally positive directional stability.
Here's a quote regarding Brewster Model 239's directional stability from a finnish book Lentajan Nakokulma II (Pilot's View Point II) translated by Camo. The book is written by Mr.Jukka Raunio who's an aeronautical engineer (Dipl.Ins.) with a long career in Finnish aviation industry. The book covers origins, development, purchase to Finland, structures, systems and flying/handling qualities of all (non war booty) fighter aircraft of the Finnish Air Force that served during WWII. (Pyro has probabaly heard about the book more than he wants to but I just wanted to introduce it to rest of the readers. :))
"Directional stability was positive. When the plane was disturbed from level flight with the rudder, it returned to direct flight after a couple of sharp yaw and roll motions. The feel of the rudder was a bit poor in slow flight, but got better at speeds over 150 km/h. In a dive, at speeds over 500 km/h, the rudder was very sensitive. V-angle stability was especially good, due to the mid wing and 5.5 degree V-angle. Clean banked turns could be made with just the rudder or just the ailerons. In a straight side slip (clean, engine on idle) at 150 km/h the rudder was fully pressed and counter elevator was given, approximately half of the stick limit. The bank angle was approximately 15 degrees. Correspondingly, during landing the bank angle (Ville: roll or pitch??) was approximately 10 degrees."
AFAIK, the quote is based on Finnish flight testing which was done to determine the handling characteristics of the then new fighter.
If coordinated ("clean") turns could be done without touching the rudder, that is quite a big discrepancy when comparing it to the amout of left rudder needed in AH.
The other problem (they are probably related to each other) in addition to the rather big amout of slipping is the tendency for the plane to simply "depart on the yaw axis" with AoA of the fuselage nearing 90 degrees to the direction of the speed vector at low speeds. It *seems* as if the CoG is too far behind and "the tail simply wants to overtake the nose of the aircraft". I do realize that that was rather poor describtion but that's the way it feels to me. Wheather too far aft CoG is actually the reason to the phonemenon, I do not know. The only aircraft in AH that I've experienced to have similar characteristics in similar, though even bigger, magnitude is the Ta152H-1. On the other hand, FW190A-5/8 is maybe the closest AH-fighter that fits to this describtion: "Clean banked turns could be made with just the rudder or just the ailerons."
Sorry if I posted this a bit late for the possible correction to make into any of the possibly upcoming patches. The reason for this is that I wanted to testfly the plane properly and not make hasty conclusions based on limited initial experience. I do understand that ironing out the kinks of the graphic side of the new version is the primary concern for you right now but I hope you could look into this when you have the time.
P.S. I'll post couple of films of the "departing on the yaw axis"-phenomenon a bit later.
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Pyro,
Sorry it took so long for me to get off my butt and make use of my ISP's webpage. :( Anyway, here's couple of films which show the complete loss of directional stability quite well:
Just a short clip from a landing: Film1 (http://users.kymp.net/joonas.konttinen/Lossofdirectionalstability.ahf). As I noticed the condition was developing I didn't correct it with rudder and only corrected with ailerons and let it happen.
A couple minute long 1 vs 1 with Dicto: Film2 (http://users.kymp.net/joonas.konttinen/1vs1withDicto.ahf). Notice the trouble both of us have on several occations on controlling the yaw.
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Pyro,
In the Brewster references sent to you you should have Report No.B.A.1689. (July, 1941), Handling tests on Buffalo (Brewster A.S.430) (RAE-Brewster.pdf). On page 6 there's the following mention:
"Banked turns with one control fixed: Excellent turns with very little sideslip can be done on ailerons alone. Sudden application of the ailerons increases the rate of entry and also the sideslip on going into a turn and during the recovery. More use of control is always needed to recover than enter."
This probably the source which Mr.Raunio used for his book.
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Thank you, Wmaker, for your extend write up.
I also notes the same lateral instability issues in Brewster. The Slip/Skid-Ball is swinging allover.
I just can't get a consistent 'feeling', how much I should add a feet-in different situations during the combat maneuvering.
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Pyro,
Something that just dawned to me after flying the rotary engined WWI fighters in AH...
Some of the handling quirks I seem to experience while flying the Brewster are very similar to the handling peculiarities of these WWI fighters. One of the effects which is clearly seen in all these planes is the quite large amount of side slip while pitching up or down from level flight. The amplitude of this effect is shorter and the frequency higher in the Brewster compared to the WWI fighters.
Of course, in general, these effects don't have nearly as big effect on the Brewster as they do on the WWI fighters but these effects seem to be partly the cause of the strange departures and other handling peculiarities of the Brewster.
So, could a too large propeller mass for instance be behind these effects which seem to be of gyroscopic nature?
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That's a pretty real possibility.
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P.S. I'm deep into another project at the moment already. I'll take a look at this once I am clear of this.
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P.S. I'm deep into another project at the moment already. I'll take a look at this once I am clear of this.
Totally understood! :salute
Thanks in advance! :)
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P.S. I'm deep into another project at the moment already. I'll take a look at this once I am clear of this.
Beaufighter? :)
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Sorry, existing plane remodel. :noid I'm all for a Beaufighter though.
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Sorry, existing plane remodel. :noid I'm all for a Beaufighter though.
Mossie? :P
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Sorry, existing plane remodel. :noid I'm all for a Beaufighter though.
beaufighter sounds fun
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I wonder which it will be?
P-40?
A6m?
C-47? (God I hope not)
Mossie?
Sure there is a few I am missing....
Strip
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Sorry, existing plane remodel. :noid I'm all for a Beaufighter though.
Yes, they are finally going to model the blender in the P-38s. About time.
ack-ack
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Sorry, existing plane remodel. :noid I'm all for a Beaufighter though.
HTC Spi hax!!! :noid
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Yes, they are finally going to model the blender in the P-38s. About time.
ack-ack
Is that where the props run away and shed the blades, thus blending the pilot? :P
(I should probably think about stones and glass houses on this one though.)
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Sorry, existing plane remodel. :noid I'm all for a Beaufighter though.
That leaves some hope for the Beau diehards at least :)
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Yes, they are finally going to model the blender in the P-38s. About time.
ack-ack
whats that?
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Back on topic...
Brewster's 9' Hamilton Standard prop and the spinner weighs 262lbs together. The blade type used is 6101A-12 and the hub is 3D40-235.
Is there anyone who knows some source which lists the weight of an individual blade?
I have no idea how AH's flight model handles the prop weight and its distribution but knowing the weight of an individual blade might help in getting the distribution just right. I wasn't able to find anything regarding the weight of one blade. That blade is still rather common. It is used in T-6 Texans and DHC-2 Beavers for example.
Sources:
1.) Brewster Aeronautical Corporation Report 350:
Detail Specification for Model 239 Airplane Class VF (Pyro: brewster-350.pdf)
2.) Brewster Aeronautical Corporation Report 353:
Pilot's Handbook for Model 239 Airplane
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Prop unit is a moment of inertia in slugs-ft^2. I glanced at it and it is high but I don't think that's the only thing that's going on there.
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I don't think that's the only thing that's going on there.
That is my hunch aswell.
I still think there might be something going on with the vertical stab/rudder even though I do acknowledge the fact that, like you said in a pm, the moment arm between the CoG and the vertical tail area is shorter than in most AH fighters. However the total vertical tail area is quite large in relation to the plane (19.2sqft: report 350). The vert stab was enlargened quite substantially since the first prototype. Also like you said before, an uncalibrated slip indicator might be contributing to the sensation of directional unstability aswell. So overall it's most probably a combination of factors.
One thing that comes to mind is the quite high profile of the Brewster's rear fuselage. I have no certain idea about how much exactly that contributes to the directional stability or how you guys model the effects of the fuselage in terms of stability.
All in all, thank you in advance Pyro for looking into it! :) I hope you aren't thinking that I'm looking gift horse in the mouth as I really am grateful for the Brewster! :)
Anyways, take your time with your current project, this is worth the wait! :)
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Hi Pyro,
Are there any changes to be expected on this front in the upcoming version?
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Thank you so much for fixing the directional stability of the Brewster, Pyro/HTC!!
It flies a lot nicer now and is a lot better gun platform.
I noticed that the roll rate got slower at speeds below 250mph. Just a perception so far though, I haven't tested it as I didn't test it in the last version either. Probably a side product of the added stability?
Overall, it's great now! The only flight model related "issue" that I see is that the combat trim leaves it slighty tail heavy. This has nowhere as severe effect as the bug you squashed with the Me262 a while ago but is still noticeable.
Anyway, it's a lot more fun to fly now!
Thank you! :)
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Thank you so much for fixing the directional stability of the Brewster, Pyro/HTC!!
It flies a lot nicer now and is a lot better gun platform.
That's almost an understatement. Flew it first time tonight after the update and was highly impressed. While all the talk was about the new Mossie, the Brewster really made a leap forward... and a big one! :aok
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Yeah I sure hate fighting those things. I did before too, but they seem just a bit nastier now.
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Roll shouldn't be substantially different. The things I changed were that I found that it didn't have enough dihedral, too much prop mass, and I increased the effect of the rear fuselage on directional stability. Had to relearn combat trim of course.
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Roll shouldn't be substantially different. The things I changed were that I found that it didn't have enough dihedral, too much prop mass, and I increased the effect of the rear fuselage on directional stability. Had to relearn combat trim of course.
Increase in dihedral should = reduction in roll rate. How conspicuous that difference is can be debated.
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The structure of the sim should self correct for that.
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Roll shouldn't be substantially different. The things I changed were that I found that it didn't have enough dihedral, too much prop mass, and I increased the effect of the rear fuselage on directional stability.
Thanks for the details Pyro, they are always interesting to hear! The dihedral didn't come to my mind but the other issues were something I was suspecting.
Had to relearn combat trim of course.
I'm not sure if I understood this correctly or not. Do you mean that you had to "teach" the combat trim function to account for the changes you made to the model? Anyways, very interesting. :)
Regarding the rollrate,
Considering the limited data out there when it comes to the Brewster's roll rate, I don't really have any "issues" with the roll rate. I might cross test it with the previous version just to see if I'm imagining the change. :) Other than that the roll rate is fine, it could well be that it was "too good" previously and that it is now more realistic, who knows...or it might not be changed at all and that it's all in my head. :)
Stoney,
Just for interest, Brewster's dihedral is 5,5 degrees. No idea how much Pyro had to change it.
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I'm not sure if I understood this correctly or not. Do you mean that you had to "teach" the combat trim function to account for the changes you made to the model? Anyways, very interesting. Smiley
The combat trim table is simply filled by using the auto pilot to fly the plane at different speeds and storing the control settings at speed each increment.
HiTech
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The combat trim table is simply filled by using the auto pilot to fly the plane at different speeds and storing the control settings at speed each increment.
Thanks HT!