Vx and Vy AOA's

[surreal1221]

Alright, just a question.

Today after flying I got to thinking. This plane (172K) sure is a rocket when it is only me in it. Now granted I only add another 120-125lbs of weight to the bird, but christ.

The thing reaches Vr (60) very quickly, and by the time I actually have an AOA to leave the ground, my speed is well past Vx (68), and nearing Vy (82) before I'm even 200 feet off the ground, so I'll pitch for 68, and then pitch for 82. The problem comes when trying to maintain these airspeeds when it is only me in the plane.

I've been trying really hard to maintain Vx, then accelerate to Vy during climb outs. But for some reason, I just have a very uneasy feeling maintaining a 15-18º attitude during departure - you know, the whole power on stall deal, p-factor, etc etc. So I tend to bring the attitude down a bit to say 10-12º and allow the airspeed to build to say 90-95mph - and then I pitch for that. It works well, and I maintain about a 750fpm climb.

On past flights, where there have been two other people on the plane, I haven't had to have such a huge attitude to maintain Vx and eventually Vy, say 12-15 for Vx, 10 for Vy.

I don't know, I suppose I don't really have a technical question, but has anyone else ever encountered this revelation - and is there any further reading concerning Cessna Vx and Vy speeds at different weights?

Thanks for reading, and listening.

Cliff notes: I have trouble maintaining Vx and Vy when I am the only one in the aircraft, always fast, hesitant to maintain a high AOA on departure (15-20*). When another person, or two other people are in the plane, no problems at all maintaining Vx or Vy.

[Ian J]

Just a simple comment -

If you are maintaining Vx on climb out with full power, you won't stall. As for AOA - well, your AOA controls your airspeed, not vice-versa. If your POH says that X MPH is Vx, and you apply full power and adjust the pitch attitude to that speed, you will not exceed the critical AOA.

That being said, an excessively high pitch attitude can be uncomfortable. Use it to clear obstacles then maintain Vy or better yet, a cruise climb airspeed.

[tgrayson]

Originally Posted by surreal1221

I have trouble maintaining Vx and Vy when I am the only one in the aircraft, always fast, hesitant to maintain a high AOA on departure (15-20*).

Don't confuse your pitch attitude with your AOA. If you're flying at 68 MPH, your AOA is essentially the same in descent, level flight, and climbs at that airspeed.

[Ian J]

Originally Posted by tgrayson

Don't confuse your pitch attitude with your AOA. If you're flying at 68 MPH, your AOA is essentially the same in descent, level flight, and climbs at that airspeed.

That's what i was trying to say... you said it in a MUCH simpler way.

Sigh.

I'll get it one of these days.

[tgrayson]

Originally Posted by ChinookDriver

That's what i was trying to say... you said it in a MUCH simpler way.Sigh.I'll get it one of these days.

Sorry , I've seen the confusion *many* times among fairly experienced pilots. and it helps to laser in on the core issue. They often aren't aware of what they're confused about until it's pointed out to them.

[surreal1221]

Thanks gents. Now does Vx or Vy vary depending on weight?

[tgrayson]

Originally Posted by surreal1221

Thanks gents. Now does Vx or Vy vary depending on weight?

Yes, both will vary.

They both depend on minimizing drag and having lots of thrust or power. Obviously, thrust or power will not vary with weight, but they *do* vary with airspeed, and so does the drag on the airplane.

Your point of minimum drag will occur at a particular AOA for any given weight. Vx is set at max gross and occurs at a particular AOA. If you are not quite as heavy, then your AOA at that airspeed will be lower than optimal, because AOA and airspeed combined have to produce lift=weight. With less weight, you need less lift and if you hold airspeed constant, your AOA must be less. Got that? So if you increase your AOA to least drag, your airspeed will be lower than what it would be at max gross.

I can't tell you how much less though. Since Vx and Vy also depend on available thrust and power, the changes in airspeeds don't quite follow the rules that stall speed or best glide do.

[Ian J]

Originally Posted by surreal1221

Thanks gents. Now does Vx or Vy vary depending on weight?

Yup.

Vx decreases as weight decreases from max. Here's a little blurb for ya with some rules of thumb.

The Vx and Vy speed in your POH may be listed with only one airspeed for each performance condition. If so, they are for maximum gross weight (MGW) at sea level. Some manuals do list different airspeeds for distinct altitudes, and possibly other gross weights. Nevertheless, there are some rules of thumb for your non-turbocharged GA aerospace vehicle that can be applied to assist you in discovering the airspeed closest to the one required. For each 1000’ of increase in density altitude from sea level, reduce Vy climb speed by one percent. Or, make a reduction of one knot per 1000’ of density altitude gain and you’ll be close. For Vx, increase your climb speed one-half of one percent for each 1000’ increase in density altitude. Now you can apply the weight rule of thumb – both Vy and Vx speeds increase approximately one knot for each 100 pounds below MGW.

[nosehair]

Originally Posted by surreal1221

does Vx or Vy vary depending on weight?

Yes. All speeds vary with weight. The speeds in light airplanes are at gross weight. As weight goes down so does the speed when you keep that same AoA.

Think of going down a sliding board. It is pitched down at a 30* angle. You are sitting at the top and start sliding down. At your light weight, you will achieve speed XX. If you could increase your weight, your sliding speed would increase but the angle stays the same.. Same thing with approach speeds in big airplanes. The speed difference in light airplanes is not enough to be concerned about, but in big airplanes where weights might differ in thousands of pounds, the speed is computed according to weight.

[surreal1221]

both Vy and Vx speeds increase approximately one knot for each 100 pounds below MGW.

NOW that was what I was looking for.

Excellent! Thanks for all the insight guys. I just have a very uncomfortable feeling when I'm the only one in the plane and I'm launching off the ground like a damn Saturn V looking at the stars.

[tgrayson]

Originally Posted by surreal1221

both Vy and Vx speeds increase approximately one knot for each 100 pounds below MGW.

NOW that was what I was looking for.

Argh, this cannot be correct. Vy and Vx will decrease with a decrease in weight, not increase. Where did this formula come from?

[Ian J]

Originally Posted by tgrayson

Argh, this cannot be correct. Vy and Vx will decrease with a decrease in weight, not increase. Where did this formula come from?

Right you are... most likely a typo I'm guessing.

[surreal1221]

Ah hell, I didn't even notice it said increase. . . read it as decrease.

[tgrayson]

Originally Posted by surreal1221

Ah hell, I didn't even notice it said increase. . . read it as decrease.

Take these rules of thumb with a grain of salt. (Two metaphors in one sentence?)

I've seen some that were flagrantly in conflict with the POH. In this case, when written correctly, the formula is just scaling Vx and Vy with Sqrt(W1/W0), in the same way that we scale the stall speed. This will keep you safe from stalling, but it doesn't take into account the thrust available and power available, both of which vary with airspeed, or the secondary effect of weight on climb angle and rate [actually, this only affects the actual angle or rate, not the speed at which the maximum occurs.]. In other words, not accurate, but probably better than nothing.

[surreal1221]

Right, makes sense.

My problem is, with only being an additional 120lbs above GW, it can be somewhat uncomfortable pitching at such a high attitude, and still gaining speed - but like I said, I'm still keeping a very good VS though the initial climb.

I just hope my examiner weighs a good 180-200lbs to help me maintaing Vx / Vy without making the departure uncomfortable - for both of us. lol

[tgrayson]

Originally Posted by surreal1221

My problem is, with only being an additional 120lbs above GW, it can be somewhat uncomfortable pitching at such a high attitude, and still gaining speed - but like I said, I'm still keeping a very good VS though the initial climb.

Hopefully, you're not 120 lbs *above* GW.

Anyway, the more thrust you have, the steeper that picture is going to be. As you move up into more powerful airplanes, it's only going to get worse. Or better, depending on how you look at it.

[surreal1221]

empty weight - I'll get use to it I'm sure.

[gredenko]

Excellent thread. As a CFI, we usually only emphasize that Va changes with weight, and leave it at that. I forget that all of those V speeds are calculated with (help me if I'm wrong) max gross, standard conditions, at sea level and most unfavorable CG. Maybe that's just for the POHs I've seen; I know they vary.

[Pilot2087]

Originally Posted by surreal1221

The thing reaches Vr (60) very quickly, and by the time I actually have an AOA to leave the ground, my speed is well past Vx (68), and nearing Vy (82) before I'm even 200 feet off the ground, so I'll pitch for 68, and then pitch for 82. The problem comes when trying to maintain these airspeeds when it is only me in the plane.



Cliff notes: I have trouble maintaining Vx and Vy when I am the only one in the aircraft, always fast, hesitant to maintain a high AOA on departure (15-20*). When another person, or two other people are in the plane, no problems at all maintaining Vx or Vy.

Why are you pitching for an airspeed? What you need to do is find the pitch altitude for Vx and Vy and get that in your head. Once you rotate, pitch up for a Vx pitch attitude and the airspeed WILL eventually go to Vx airspeed, That way your not chasing the airspeed needle, and not doing crazy pitch attitudes.

[tgrayson]

Originally Posted by Pilot2087

Once you rotate, pitch up for a Vx pitch attitude and the airspeed WILL eventually go to Vx airspeed,

That's only true if the density altitude [and weight] never changes. At high altitudes on a hot day, the pitch attitude is much lower, due to lower thrust and lower power.