Author Topic: Couple of aerodynamics questions.... (Read 887 times)

Elysian · « **on:** March 07, 2002, 01:07:41 PM »

Wondering if any of you with a good understanding of the subject could help me here. Forgive me if the questions are somehwat "retarded"...

1. Could the speed a plane stalls at be a rough indicator of its potential to turn? In testing planes for a website on AH aircraft I'm making, I've noticed the better turning planes tend to have lower stall speeds. Plus I would think low stall speeds would equal more lift which in turn could translate to sharper turns.

2. Is climb a rough indicator of acceleration? This one seems like a more obvious yes, but due to my lack to understanding on the subject, I just want to double check.

Thanks for any help!

cmorris

FLS · « **Reply #1 on:** March 07, 2002, 03:27:50 PM »

1. Yes. A slower stall speed lets you turn at slower speeds which gives you a smaller turn circle. It doesn't mean you'll have a faster high speed turn though.

Check out this thread on Energy Maneuverability Charts.

http://www.hitechcreations.com/forums/showthread.php?s=&threadid=46911

2. Yes. A better thrust to weight ratio gives you better climb and better acceleration. A climb is an acceleration against gravity. In level acceleration one aircraft may have an initial advantage which is lost as increased speed results in increased drag.

--)-FLS----
Musketeers

funkedup · « **Reply #2 on:** March 07, 2002, 03:33:34 PM »

Additional info on question 1: A plane stalls at a given lift coefficient (CL). The CL is proportional to the lift divided by the square of true airspeed. And the amount of g you can pull in a turn is proportional to the lift you can generate. So if plane A has a 1g stall at a lower speed than plane B, plane A will also be able to pull more gees without stalling at higher speeds, assuming all other things are equal. Plane A will also have a lower corner speed.

More on CL and stalls:

http://wright.grc.nasa.gov/WWW/K-12/airplane/liftco.html
http://142.26.194.131/aerodynamics1/Lift/Page9.html
http://142.26.194.131/aerodynamics1/Lift/Page11.html

F4UDOA · « **Reply #3 on:** March 07, 2002, 04:03:23 PM »

Yes and Yes.

1. As far as 1Gstall speed (level flight) and maneuevering stall speed go, here is a pretty good indicator.

If an A/C stalls at 100MPH in level flight(1G) then at 3G's what would it stall at? Answer is 173. 20MPH.

Why? Because someone who understands this stuff better than I do showed me.

What I'm doing is multipling 100MPH times the square root of the G factor. In this case it is 1.73 which is the square root of 3 (3G's). The same applies to 4,5 and 6G turns. I don't know why it works but it is almost exactly correct if you look at the fligth envelope of the F4U, P-47, P51 or P-38 flight manuals.

HOWEVER!! One big draw back in AH is that many of the 1G stall speeds are off and do not adjust properly with weight changes. So AH stalls may not represent RL stalls acurately.

2. Is climb and acceleration directly linked. According to every expert on these boards they are one and the same. I however think that there are to many variables for such a linear relation.

In AH however they are directly linked indeed.

Zigrat · « **Reply #4 on:** March 07, 2002, 10:48:59 PM »

funked L=qCLs so it is proportional to airplane weight and indicated airspeed. true airspeed does not depend on density, but indicated airpseed does (ias is just a measurement of dynamic pressure using the ps tube)

acceleration is (nearly) directly proportional to climb capability AT A GIVEN FLIGHT SPEED ie if one plane outclimbs another at one speed it should also have a greater instantaneous acceleration at this flight speed. why? 1st law of thermo

Q-W=(v2^2-v1^2)*m/2+(u2-u1)+mg*(h2-h1)
assuming thre is no heat added and the internal energy of the system doesnt change cross out (u2-u1) and Q

(sign convention is that positive W is work out of system so since the engine is doing work on the airplane its -W)

you can see that for a given work (F*dx) you can either accelerate, climb, or do both in some proportion.

divide by time and you see that specific power governs climb and acceleration rates ( take into account taht W is net work and is therefore thrust - drag -- many other factors such as propeller efficiency (conversion from shp to thrust) and drag of the airplane need to be considered)

Elysian · « **Reply #5 on:** March 08, 2002, 12:10:22 AM »

Thanks for the great responses guys, very helpful!

After reading the responses to my 2nd question regarding accel equaling climb I feel better about some of the data I am collecting.

I have been timing acceleration of aircraft over different speed ranges at diferent altitudes. Example might be how long does it take a spit V to go from 150 to 250 mph at 9k. I was a little worried I might be just duplicating the climb charts on this site in a slightly different form.

Looks like this info could be wothwhile, as other variables come into play during level acceleration as opposed to a constant spped climb (obvious one being differing drag effects on planes as speed increases).

Maybe engines perform differently at different speeds in AH as well? Probably a lot of variables I am not aware of...

Thanks again guys,

cmorris

fdiron · « **Reply #6 on:** March 08, 2002, 02:29:22 AM »

One misconception is that wingloading is a directly proportional to turn radius. In the book 'Battle Of Britian', it compares the wing loading and turn radius of the Spitfire, Hurricane, and 109. I have forgotten which turned best, but it wasnt the plane with the lowest wingloading.

Sombra · « **Reply #7 on:** March 08, 2002, 02:52:29 AM »

about 2. , i ask:

Doesn't wing profile have much influence in this regard? I'm thinking on the A6M2. Or maybe wing profile only affects max speed?

Greetings

hitech · « **Reply #8 on:** March 08, 2002, 09:06:29 AM »

Zigrat it dosn't need to be considered to show Acc = climb,
because thrust does not change signifacantly if climbing or acceling for any given speed.

And drag only changes very slightly do to the increase in lifted needed when in level flight VS a stead state climb.

Now if you wish to compute climb rates or accels ,given HP and weight,from your equetions then prop eff and drag have huge effects.

F4UDOA:
Our stall speeds are correct, but care must be taken when looking at stall speeds in IAS of a plane do to the fact we do not have a correction factor in the IAS gauge do to change in AOA.
i.e. AH will show the same IAS no matter what your AOA,where a real plane will show a slower IAS do to increase in AOA and hence a smaller cross section of the tube being presented to the wind.

HiTech

Zigrat · « **Reply #9 on:** March 08, 2002, 09:50:42 AM »

quote hitech:

Now if you wish to compute climb rates or accels ,given HP and weight,from your equetions then prop eff and drag have huge effects.

of course this is what i meant

that equation above will work 100% for an elevator connected to a engine of certain power, but for airplanes conversion of power to thrust and the effects of drag must be considered.

hitech i have a question.. is reynolds number considered in your analysis? ie the skin friction compononent of zero lift drag decreases with increasing reynolds number. Also are scrubbing effects taken into account? This would have particular impact on the twins (especially in low speed operation) who would see higher local velocities behind the propeller disk and thus have more lift from that wing section but also more drag due to greater viscous losses.

hitech · « **Reply #10 on:** March 08, 2002, 10:49:44 AM »

Zigrat: On question 1. depends what your talking about "in the analisis".

On question 2. yes the prop slipstream effects are computed including lift and drag and also the slip stream angles over the airfoils.

Hamish · « **Reply #11 on:** March 08, 2002, 11:37:04 AM »

"uh-huh"
"cool"

"shoulda stayed in college"

Glad you guys understand all that. I can barely follow this conversation

F4UDOA · « **Reply #12 on:** March 08, 2002, 01:10:21 PM »

Errr, Ahhh, ehhh,

Hitech,

I am not actively lobbying to get the stall speeds changed however...

Exhibit A.

When tested at 25% fuel in the F4U-1 and F4U-1D

F4U-1 100% fuel
Weight =12,836LBS
F4U-1 25% fuel
Weight = 11,211LBS
Gallons of Fuel= 90.25gallons
Weight from fuel= 541.5LBS
Aces High Stall speed
Auto pilot on, power on, Clean condition= 101MPH
Auto pilot on, power on, Full flaps= 85MPH

F4U-1D 100%Fuel
Weight=12,039LBS
F4U-1D 25% Fuel
Weight= 10,972LBS
Gallons of fuel=59.25
Weight from fuel= 355LBS
Aces High Stall Speed
Auto pilot on, power on, Clean condition= 101MPH
Auto pilot on, power on, Full flaps= 85MPH

F4U-1D Flight Manual <==Flight manuals are conservative
Weight =11,300LBS
Stall Speed
Full flaps, power on= 66Knots or 76MPH
No Flaps, Power on = 84 Knots or 96MPH

This raises two questions
1. The F4U-1D stalls about 5MPH higher than the manual clean and 10MPH to high in landing condition and the AcesHigh F4U-1D weights 328LBS less than the manual gives for the same stalling condition so the numbers are more likely farther apart.

2. The F4U-1/1A hybrid in AH either does or does not have the stall strip on the Starboard wing to correct the assymetrical stall. However it lessoned Max Cl from 2.33 to 1.88. If so how come the F4U-1 and -1D stall at the same speeds? And if our -1 does have the spoiler strip why is the Assymetrical stall worse than in the -1D?

Exhibit B.

When AH planes stall under auto pilot they don't depart from level flight. They loose alt steadily and then stall. The F4U-1 looses 600FPM before stalling even at 18inches MAP as stated in the pilots manual.

When trying to hold the A/C in level flight manually it stalls at 125MPH in the clean condition and 100MPH full flaps. I'm sure this is not historically accurate.

One last point about the F4U-1 is that when it does stall it breaks the wrong way every time. It is historically supposed to break left. Instead it breaks right everytime.

Also the F4U-1D is modeled with pylons under each wing in the "Combat" condition regardless of loadout. This reduces top speed at sea level by 11MPH from 366MPH to 355MPH and from 417MPH to 409MPH at 20K (which our F4U-1D cannot reach anyway). The P-47 and F6F have similar pylons for carrying ordinace but their FM's are not affected when not carrying ordinance as well as the FW190A series.

Also the stall speeds have similar errors on the P-51 and F6F. There may be some error because of AOA and pitot tube config but it does not exist on all A/C uniformly.

funkedup · « **Reply #13 on:** March 08, 2002, 04:01:12 PM »

Zig:

Quote

funked L=qCLs so it is proportional to airplane weight and indicated airspeed. true airspeed does not depend on density, but indicated airpseed does (ias is just a measurement of dynamic pressure using the ps tube)

Weight's got nothing to do with it. I'm talking general case, not just 1g.

L=q*CL*S
CL=L/(q*S)
CL=L/(0.5*rho*V^2*S)
CL = (L/V^2)*(2/(rho*S))

Or in other words...

Quote

The CL is proportional to the lift divided by the square of true airspeed.

QED

Zigrat · « **Reply #14 on:** March 08, 2002, 05:57:31 PM »

ok it is proportional... but it is more proportional to indicated since that rho is your equatiuon is taken care of by the ias

IAS is more relevant when talking about stall since an airplane of a given mass flying at a given g load will stall at constant ias if viscocity is considered constant (visc of air wont vary that much in atmosphere)