Maneuvering Speeds versus Cruise Speeds

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Jungle Jim
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Maneuvering Speeds versus Cruise Speeds

#1 Post by Jungle Jim » Fri Feb 05, 2016 9:24 am

Guys,

I was looking at the specs on a number of aircraft and noticed that there is quite a range when comparing the maneuvering speeds versus the cruise speed for varous aircraft powered by the same engines. I was looking in particular planes with the same 160 hp engines. I noticed that in the case of some of the faster homebuilt planes with 160 hp engines, they all cruise about 160 mph at 65% power. The maneuvering speeds are quite different. See below:

RV9 - 160 mph cruise, 118 mph Va
RV6A - 160 mph cruise, 135 mph Va
Mustang II - 160 mph cruise, 140 Va
RV8 - 160 mph cruise, 142 mph Va
Thorp T18 - 160 mph cruise, 165 Va

For certified aircraft:
C172 - 120 mph cruise, 112 Va
Piper 161 - 120 cruise, 130 Va
C152 - 115 mph cruise, 110 mph

Interestly enough the Thorp T18 and Piper had design inputs from the same designer, John Thorp. I was surprised that the RV series of aircraft, being a popular as, they are have such a difference. In rough air you really have to slow down. The certified planes don't have much of a speed difference between cruise and maneuvering speeds.

Does anyone know if there have been issues with the RV series in rough air? I don't fly an RV. Frequently when I am in turbulance where I am using large control inputs the procedure is to just tighten the seat belt and throttle back a bit much like I used to do in the Stinson and C172.

Any one want to speak up as to what the situation is with the RV's?

Jim
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Re: Maneuvering Speeds versus Cruise Speeds

#2 Post by Justjohn » Fri Feb 05, 2016 10:43 am

The answer is in the spread from Vs to cruise and an understanding of what exactly Va, or manovering speed is.

Va is tied to Vs, so the newer, slipperier, higher Hp airplanes get farther from Vs in cruise.

Va is the speed at which the controls can be fully and abruptly displaced, seems most everyone understands that. But it is also the speed at which the airplane will stall when subjected to 3.8 G's (normal category).

For example , we all know that an airplane in a 60* bank is undergoing a constant 2 G's (presuming the bank angle is sustained). If that airplane got walloped by a big dose of turbulence, by the time the G's had risen to 3.8 the airplane would stall. Another case would be pulling out of a dive following some kind of inadvertent upset. 3.8 G's, the airplane stalls.

Why is a stall considered a designed for advantage? It provides for a kind of aerodynamic relief. When the wing stalls, it stops producing most of its lift. This way the spar doesn't break, and other parts of the structure, such as lift struts, engine mounts, etc, are protected.

Normal category airplanes are designed for at least 3.8 + and 2.5 - . Utility category airplanes have a larger spread, 4.4 + and 3 I believe.

Airplanes that are certified in both the normal and utility categories often have a reduced gross weight when operating in the utility catagory. This is not because it is suddenly 'stronger', just that a lighter weight accelerated to a higher G has a similar stress value as a heavier weight accelerated to a smaller G.

I hope I was clear and helpful.

Cheers,
John
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Re: Maneuvering Speeds versus Cruise Speeds

#3 Post by Justjohn » Fri Feb 05, 2016 10:50 am

I notice when I look at your list, some of the airplanes have quite high Va's, such as the Thorp T-18. My explanation of Va stands. I suspect that the quoted Va's are at a much higher, and designed for, G loading. Quite probably with a G loading while operating in regards to aerobatic level of G's.

Cheers,
John
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Re: Maneuvering Speeds versus Cruise Speeds

#4 Post by Jungle Jim » Fri Feb 05, 2016 12:13 pm

Hi John,

I get the expanation about the Va speeds and your expanation is good. I was just surprised that if say, all the planes were flying in the same turbulence the RV's would have to slow down quite a bit to remain within limits. I wonder if the RV guys are aware of this and adjust accordingly. I guess this is the price they pay for the lower wing loading. The wing loading of the planes are:

RV6 - 14.5 lb/sq ft
RV8 - 15.5 lb/sq ft
Mustang II - 16.5 lb/sq ft.
Thorp T18 - 18.6 lb/sq ft

Jim
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Re: Maneuvering Speeds versus Cruise Speeds

#5 Post by trampbike » Fri Feb 05, 2016 12:35 pm

Justjohn wrote:
Va is the speed at which the controls can be fully and abruptly displaced, seems most everyone understands that.
Yeah, most people understand it that way, but it's not a great news, because being at or below Va does NOT mean you won't break anything with abrubt movements of the controls.

For certified aircraft, reference FAA part 23. Also, you might want to google g-overshoots and asymmetrical load factors.
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Re: Maneuvering Speeds versus Cruise Speeds

#6 Post by photofly » Fri Feb 05, 2016 1:49 pm

Va is the speed below which the manufacturers certifies that a single full input of any control will not cause damage to the control surface attachment. It has nothing to do with stall speeds or breaking other parts of the airframe - just the control attachments.

FAR23 requires the control attachments to be strong enough that Va is not less than Vso times the square root of the load limit. Which is why pilots wrongly associate Va with the stall. If the control attachments are beefy enough Va can, at the manufacturers option, be certified at higher than the the square root of the load limit times Vso, and in some airframes it is so.

The stall-before-break limiting sped - if it gives you a warm feeling to know it - is easily calculated as the square root of the load limit time Vso. That can be any speed up to Va.

Please don't misunderstand what Va is, and isn't.
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Re: Maneuvering Speeds versus Cruise Speeds

#7 Post by Justjohn » Fri Feb 05, 2016 1:57 pm

Jim

Notice that the airplanes with the lowest wing loadings have the lowest Va's? Bet they have the lowest stall speeds too. As Va is tied to the bottom end of the speed envelope, any airplane with a lower Vs is going to have a lower Va too, assuming Va is based on the same load limit.

It is also why Va decreases with weight. Little airplanes generally have 1 published Va number, but as you dig into the POH, you can usually find a note regarding Va decreasing with a lighter weight because the whole airplane is still only certified to the same load limit.

Cheers,
John
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Re: Maneuvering Speeds versus Cruise Speeds

#8 Post by Justjohn » Fri Feb 05, 2016 2:09 pm

Photofly:

While the far you quote is correct, it is NOT the whole story regarding Va. It IS directly related to stall speed. That is why even a lowly old C 172 publishes a lower Va at lighter weights.
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Flying is better than walking. Walking is better than running. Running is better than crawling. All of these however, are better than extraction by a Med-Evac, even if this is technically a form of flying.

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Re: Maneuvering Speeds versus Cruise Speeds

#9 Post by Justjohn » Fri Feb 05, 2016 2:15 pm

Photo fly, notice the decreasing speed of Va with weight and therefore stall speed? I don't imagine the beefiness of the hinges changed much with the weight of the airplane. If the manufacture could just publish 1 number and stand by it, I think that they would have.

Regardless, I'll enjoy my warm feeling knowing what I think I know.

Cheers,
John
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Re: Maneuvering Speeds versus Cruise Speeds

#10 Post by photofly » Fri Feb 05, 2016 3:50 pm

14CFR23.335:

(c) Design maneuvering speed VA. For VA, the following applies:
(1) VA may not be less than VS√n where—
(i) VS is a computed stalling speed with flaps retracted at the design weight, normally based on the maximum airplane normal force coefficients, CNA; and
(ii) n is the limit maneuvering load factor used in design
(2) The value of VA need not exceed the value of VC used in design.



The thing to remember is that VA (or Va) is a single speed nominated by the manufacturer during the certification process. The speed at which the airframe will reach its load limit at the critical angle of attack is calculated from Vso and the load limits for the category in which the aircraft is to be certified. The manufacturer has no choice about that except to pick a different certification category.
If the manufacture could just publish 1 number and stand by it, I think that they would have.
They do have just one number. What they publish in the POH is not the certification value of Va, it's more like puppy feed for pilots who can't think.

The drafters of the SR22 AFM understood that they should not try to adopt informal and incorrect "definitions" of Va, and instead call the same puppy-feed value for pilots Vo, "Operating Manoeuvering Speed":
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Re: Maneuvering Speeds versus Cruise Speeds

#11 Post by Justjohn » Fri Feb 05, 2016 4:25 pm

Well Photofoy, you're still not right. The work airplane, built by bombardier, also has a sliding scale for Va based on weight and stall speed.

Now that you've made it a 'thing' , I'll guess I'll dig out the books and publish the numbers. Before I do that, just wanna say, after 10,000hrs, 30 yrs of professional flying, and a longish time as a 705 skipper, I'm a long way from puppy mill flying skills and knowledge .
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Re: Maneuvering Speeds versus Cruise Speeds

#12 Post by Justjohn » Fri Feb 05, 2016 4:30 pm

BTW Photo fly, while I do my leg work, how about you look up some V speed definitions 'cause Va and Vo are both regulatory speeds and are Not the same thing.
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Re: Maneuvering Speeds versus Cruise Speeds

#13 Post by photofly » Fri Feb 05, 2016 4:53 pm

Happy to make it a "thing".

Vo is a (relatively new -1990?) operating limitation defined to replace unthinking pilots' previously incorrect use of Va as some kind of "stay below" speed in flight:

http://rgl.faa.gov/Regulatory_and_Guida ... enDocument

Va is a certification speed (one of a series, Va, Vb, Vc and Vd) and is not relevant to pilot manoeuvring. It's not an operating limitation and need not be published.

Please note that Vo must be not *more* than the square root of the load limit time Vsol and the pilot must use it as a limiting speed under the relevant circumstances. Va must be not *less* than the same number, and the designer must warrant the airframe is strong enough to meet the relevant criteria at that speed a speed which they themselves pick.

Vo - for pilots. Va - for aircraft designers.

(I don't give a @#$! if you're old as Methusalah and fly the space shuttle.)
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Re: Maneuvering Speeds versus Cruise Speeds

#14 Post by Justjohn » Fri Feb 05, 2016 5:10 pm

Devastated and gut shot, I humbly walk away.

Cheers folks
John
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