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Thread: What is the Best angle of climb VX on a PA-18 180hp gross 2000lb

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    What is the Best angle of climb VX on a PA-18 180hp gross 2000lb

    Hi all,

    I have read many contradictory things regarding best angle of climb speeds and what they actually are and the use of flaps etc.

    I am talking about a PA18, 180hp with 2,000 gross.

    The 1975 POH states VX is 45mph, but this I assume is at 1750lbs (could not find the reference to flaps)

    I could not find anything on the Wipair supplement about airspeeds.

    Not a habit I plan to get into, but if I am loaded up and looking for max angle, how do I work it out?

    (bold method mentioned half the percentage speed increase for the percentage of extra gross)

    Thanks

  2. #2
    stewartb's Avatar
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    Why would it be different?

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    nanook's Avatar
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    anything Piper printed was for a 1750lb, 150hp - Supercub. Your airplane will need to be flown to figure it out...I would be leery of formulas, more inclined to start flying it lightly loaded first, to see how steep a nose up angle to use and adjust from there...this would be done at or above 3000 agl. Every 180hp Cub is going to a little bit different, with all of the variations involved.
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    Vx in a high powered Cub is not a thing to be taken lightly. For the first couple hundred feet a power loss can easlily cost you your life.

    Full flaps, and on liftoff look for about a 35-40 degree deck angle. Airspeed will probably be unreliable, if not zero. Very uncomforable - maybe even scary.

    opinion.

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    Quote Originally Posted by damiens View Post
    Not a habit I plan to get into, but if I am loaded up and looking for max angle, how do I work it out?
    I suspect that careful flight test is the only answer.

    Have you ever verified that 45 mph IAS is Vx at 1,750 for your airplane, with your pitot static system, and your airspeed indicator? If the answer is no then extrapolation from 45 at 1,750 is not likely to give a good Vx value for 2,000 lb.

    Any extrapolation will probably assume the error in IAS is constant i.e. apply a factor to the 1,750 lb Vx and get 2,000 lb Vx. If the airspeed error is not constant then the extrapolation will be wrong.

    May not be of any significance but Top Cub at 2,300 lb has published Vx of 60 mph IAS. FX-3 at 2,000 lb has published Vx of 57 mph IAS. Top Cub and FX-3 both use fuselage static ports.

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    Like the others have said every plane is different. If you don't already have it go get a bunch of spin/stall training. Then go to altitude and see just how your plane performs. You can get a ASI that reads to 20 MPH and adjust the pitot tube for the high AOA or just leave it and note difference from GPS speed (calm day).
    DENNY

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    Quote Originally Posted by bob turner View Post
    Vx in a high powered Cub is not a thing to be taken lightly. For the first couple hundred feet a power loss can easlily cost you your life.

    Full flaps, and on liftoff look for about a 35-40 degree deck angle. Airspeed will probably be unreliable, if not zero. Very uncomforable - maybe even scary.

    opinion.
    Sounds like a typical glider winch launch except that most glider pilots are smart enough not to rotate to full climb pitch attitude until high enough to recover from a cable break or winch failure. If you are going into that corner it has to be instinctive to get the nose below the horizon when the cable breaks or the fan stops. The change in pitch attitude from full climb to that required to maintain airspeed is so great that most will not pitch down enough and will rapidly lose airspeed.

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    stewartb's Avatar
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    In my 180 I’ve added an approved gross weight increase and an approved engine-prop upgrade. Nothing in the docs say the Vx or Vy change and nothing in my flying before and after indicates they should change. All we’re talking about is a sustainable AOA.
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    Crash, Jr.'s Avatar
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    If you have a Garmin GPS they usually have a vertical speed indicator you can flip to and then just play around climbing at different speeds to see what gets you the biggest number for VY. VX is a little more difficult as you have to add distance into the equation and use a fixed course over the ground to figure it out. Something like flying from shoreline to shoreline on a lake and see what speed gets you the most altitude in that distance.

    But stewart is right, the airframe speeds normally don't change significantly with more power or more load on board. The wing is the same wing and will create the same amount of lift at a given speed. All the engine is doing is maintaining that speed at a higher deck angle and rate of climb. Weight as well. Do you remember any POH's having a performance chart that shows VX and VY changes across a weight or power range? Nope.

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    BC12D-4-85's Avatar
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    You might consider building the V speed curves discussed here to get some idea. Takes time (I did it once in my last PA-1 but it's a good way to learn your airplane's unique performance. Good time builder.

    Gary
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    Quote Originally Posted by Crash, Jr. View Post
    But stewart is right, the airframe speeds normally don't change significantly with more power or more load on board. The wing is the same wing and will create the same amount of lift at a given speed. All the engine is doing is maintaining that speed at a higher deck angle and rate of climb. Weight as well. Do you remember any POH's having a performance chart that shows VX and VY changes across a weight or power range? Nope.
    Then why do the CC-18 Top Cub and the A-1C Husky, at higher gross weight than their predecessors have published higher speeds? Power probably has little effect on climb angle, as you noted, same wing, but weight has a profound effect on the stall speed, and therefore climb angles. The Top Cub aerodynamically is identical for practical purposes to a PA-18.

    Manufacturers certificate airplanes based on "worst case" scenarios, in the case of climb performance, the certification standards call for performance at maximum certificated weight.

    MTV

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    Crash, Jr.'s Avatar
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    You're mixing in stall speed and climb angle with VX and VY speeds. Although related they are different things. Can you tell me why climb airspeed would change with a load? Sure the angle and rate of climb will change as the plane gets heavier (unless more power is added) but the speed the wing is flying through the air to make the most lift will not. If a wing makes the maximum amount of lift at a given speed then that will always be the speed to fly at to have maximum climb rate unless you modify the wing somehow.

    In my defense I haven't had my coffee yet so if my suppositions are off please let me know.

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    Quote Originally Posted by Crash, Jr. View Post
    If a wing makes the maximum amount of lift at a given speed then that will always be the speed to fly at to have maximum climb rate unless you modify the wing somehow.
    Are you confusing speed with alpha?

    Just as an aside for those that think mass (wing loading) does not impact performance speeds - why do you think racing sailplane pilots often load more water ballast than the empty mass of their sailplanes?

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    Vx is the point of maximum excess thrust. Propeller effeciency/design and engine output both change thrust. Propeller engines make more thrust at lower airspeeds.

    Click image for larger version. 

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    Yes, Vx will increase with weight. Why? Weight changes drag. More lift is required for level unaccelrated flight at a heavier weight. More lift creates more induced drag. This moves the curved total drag line in the above diagram to the right, and correspondingly the point at which the maximum difference between required thrust and available thrust exists also goes further to the right.

    Additionally, as stated earlier, the difference between indicated and calibrated airspeed in your aircraft can have a pretty substantial impact on the correct speed to fly for Vx.

    55 mph IAS is probably going to be in the ballpark. 45 mph seems low, but unless they specify weight and whether it's CAS or IAS it's hard to know. Again, prop makes a difference, not just engine. Any modifications to your aircraft which modify thrust, lift or drag will change Vx. Also I think that some manufacturers add a buffer to the true Vx out of concern for minimum maneuvering capability and stall margin.
    Last edited by Narwhal; 10-17-2021 at 08:12 PM.
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    [QUOTE=Narwhal;812760]This moves the curved total drag line in the above diagram to the right/QUOTE]

    To be complete - it moves it up and to the right. The family of curves for varying weight should all lie on the same tangent from the origin. The intersection of the tangent to the total drag curves is best glide speed which (despite the lack of info in AFM/POH) is also dependent on wing loading.
    Last edited by frequent_flyer; 10-17-2021 at 07:55 PM.
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    Bill Rusk's Avatar
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    For prop airplanes best angle of climb will be very close to stall speed (which goes up with weight). The old timers used to say “ hang it on the prop” and that is correct. Rate of climb will be very close to L/D max. This rule of thumb is NOT true for jets. There is a difference between power producers (props) and thrust producers (jets) so it is easy to get these things all mixed up.
    It is pretty well explained in “Aerodynamics for Naval Aviators” which is probably the best (over all) book on aerodynamics for pilots, (not necessarily so for engineers)

    Bill
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    Quote Originally Posted by frequent_flyer View Post
    Have you ever verified that 45 mph IAS is Vx at 1,750 for your airplane, with your pitot static system, and your airspeed indicator? If the answer is no then extrapolation from 45 at 1,750 is not likely to give a good Vx value for 2,000 lb.

    Any extrapolation will probably assume the error in IAS is constant i.e. apply a factor to the 1,750 lb Vx and get 2,000 lb Vx. If the airspeed error is not constant then the extrapolation will be wrong.
    If you have verified 45 mph IAS at 1,750 lb, and if the airspeed calibration does not change significantly between 45 and 50 mph IAS, then Vx at 2,000 lb will be 48 mph IAS.

    Speed increases by the square root of the ratio of the wing loadings i.e. speed_2=sqrt(mass_2/mass_1)*speed_1
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    BC12D-4-85's Avatar
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    Airspeed? There's too much mumbo jumbo error potential to trust and expect Vx is as Piper reported for each airplane. What was the CAS when Piper tested that? Try the tests in #10 to make sure your Vx IAS is actually Vx. It's a nice training concept to practice, but I've never required a Vx departure except during a check ride.

    Gary

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    skywagon8a's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    Airspeed? There's too much mumbo jumbo error potential to trust and expect Vx is as Piper reported for each airplane. What was the CAS when Piper tested that? Try the tests in #10 to make sure your Vx IAS is actually Vx. It's a nice training concept to practice, but I've never required a Vx departure except during a check ride.

    Gary
    Good article on sawtooth climbs with a major error. 10,000 feet! An altitude where you only get about 50% power! Totally useless data.

    These tests should be done at the lowest safe altitude for maximum performance/power.
    The procedure for stabilizing in the climb, on speed is to make tight turns at minimum safe altitude. Progressively tightening the turn until the desired speed is reached. Once the desired speed is reached, roll the wings level and begin the climb holding the desired speed.

    Edit: I should clarify.
    While making the tight turns at minimum safe altitude, the power will be retarded in order to expedite reducing the speed. Once the desired speed is reached, simultaneously roll wings level, commence climb and apply climb power. This will enable you to achieve a stabilized climb on speed at minimum, maximum performance altitude.
    Last edited by skywagon8a; 10-21-2021 at 12:28 PM.
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    Gordon Misch's Avatar
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    EAA has good flight test procedures to establish parameters for YOUR airplane.
    Gordon

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