Cold weather flying

[opticsguy]

I had not flown my CT in anything colder than the standard atmosphere since I got it 7 years ago. Usually I am dealing with things like the 95F weather at 5000 feet in Arkansas last summer. Yesterday on the way to Chicago I was at 7500 with -7C on the OAT. I had to throttle back to keep the TAS under 125kt (per the Dynon but backed up by the GPS). Coming into Chicago I had to go under the clouds and into the bumps an it was -7C at 4500 ft. At this point I had to go under 5000

RPM to keep the speed down.

 

The Rotax really like cold air, I guess. I did have to cover half of my oil cooler with tape to keep my oil above 170F.

 

I was running Mogas with 10% ethanol and nothing seems to have frozen up. Has anyone else tried these kinds of temperatures with mogas?

[Roger Lee]

I have seen in the minus numbers with Auto fuel and ethanol. Many have and without any issues. Cold dry air is better for power than hot humid air.

[Runtoeat]

In the upper 49 states and lower Canada, -7 C is a warm temp for winter. I am in Michigan and use only 10% alcohol Mogas. No problems noted. At departure to Sebring last year temp at takeoff from KYIP was -6 F (about -22 C). The Rotax really likes cold weather - lotsa power.

[Ian]

Isn't the increased performance due more to the fact that Lift is dependent upon the density of the air which is greater when the air is colder, so for a given power input you get more lift in cold air than warm air?

[Brian]

Why did you have to "keep the speed down?" You would only need to do so if you were approaching Vne. Were you getting that fast? The LSA 120kt speed limit in the FAR's is a certification limit and applies at sea level. TAS at altitude would usually be faster. At least this is my understanding.

 

Brian

[Ed Cesnalis]

gbigs is right about cold intake air producing more power. I think the additional lift is offset by additional drag though.

 

I also believe there is no speed limit that we have to worry about. The exception would be if you get additional speed through a modification.

 

Early on we believed we couldn't legally optimize our prop pitch because we believed the un-optimized pitch was a method being utilized to slow the design down to 120kts. That concern went away, not sure why and an optimized CTSW can certainly be a bit fast.

 

I guess if you were flying a CubCrafters with a 180hp lyc that you would effectively have a speed limit (normal operating range top of green on airspeed indicator).

 

Clearly best speed in a LSA would come from a 914 that was limited to 120kts at sea level in some fashion. At 16-18,000' you would be legally realizing 145kts true.

[Ian]

.........I also believe there is no speed limit that we have to worry about. The exception would be if you get additional speed through a modification.........

 

I think the question Brian was asking related to the use of TAS in the OP

 

The speed we need to be considering in relation to the aircraft performance is IAS?

[Ed Cesnalis]

I think the question Brian was asking related to the use of TAS in the OP

 

The speed we need to be considering in relation to the aircraft performance is IAS?

 

The thread is talking about performance and questioning why Brian felt the needed to slow to 125kts and go below 5,000RPM to do it. Not sure what you mean 'in the OP' but IAS vs TAS doesn't matter because as long as there is no speed mod the 120kt US LSA speed limit is for certification and not the pilot to comply with, with exceptions like where the limitation is the green arc on the ASI. Actually the 120kt limit is CAS.

 

I agree IAS is to be considered in relation to performance ( adjusted for prop pitch and altitude and throttle setting )

[Brian]

I'm with Charlie Tango that "there is no speed limit that we have to worry about." Except for Vne which is redline on the ASI. And we should be careful flying in the yellow arc and only do so in smooth air. Groundspeed (and GPS) can be anything. Indicated airspeed= 100kts; in a 100 kts headwind = 0 kts groundspeed or you can do a 180 and go at 200 kts.

[Ed Cesnalis]

Vne is true therefore not on the ASI.

 

This is important when we descend in the 914 powered Europa and the concern is flutter. At 16,000' you can be in the green and exceeding Vne

[FastEddieB]

Vne is true therefore not on the ASI.

 

 

In general, I think Vne is IAS and is on the ASI (top of the yellow arc and red line).

 

http://en.wikipedia.org/wiki/V_speeds

 

 

Right?

[Ed Cesnalis]

In general, I think Vne is IAS and is on the ASI (top of the yellow arc and red line.

 

http://en.wikipedia.org/wiki/V_speeds

 

 

Right?

 

I'm relying on common sense here, if I am wrong show me, but: unless you are privy to the engineering and certification documentation for the aircraft, you can't really be sure why the manufacturer established the Vne.

 

You can find at many places on the internet that if the flutter speed of an aircraft is close to the Vne than the manufacturer gives you the Vne for different altitudes, because at high altitudes your TAS will be higher than your IAS and this TAS could be higher then the critical flutter speed. That's why they give you lower Vne IAS speeds for higher altitudes.

 

So it means the flutter speed is a function of TAS.

 

If you are given a single Vne and you fly at altitude don't you have to change to TAS to avoid flutter considerations?.

 

Simply put Eddie, you are saying that I am free to violate Vne in terms of TAS yet flutter is a harmonic not a force consideration.

 

I agree this is not a huge consideration for a normally aspirated CT but with the aid of a mountain wave we can get to the flight levels and then it becomes a big concern.

[Roger Lee]

Hi Ed,

 

Our VNE is set more for the opening of the BRS. The real airframe VNE is higher. I'll leave it at that.

[FastEddieB]

Ed,

 

You made a lot of assumptions about what I was saying.

 

All I was saying was that V speeds are, in general, expressed in IAS, and some of them, including Vne, are shown on the ASI.

 

The subject of Vne may be more complicated than that, and the CT may be a special case - I'm just remembering what I was taught and what I was called upon to teach.

 

"V-speeds or Velocity-speeds are standard terms used to define airspeeds / performance speeds in a wide variety of operating conditions which are important or useful to the operation of aircraft.

The actual speeds represented by these designations are true airspeeds specific to a particular model of aircraft, and are expressed in terms of the aircraft’s indicated airspeed, so that pilots may use them directly, without having to apply correction factors."

Source: http://aviationglossary.com/v-speeds/

[FastEddieB]

Hi Ed,

 

Our VNE is set more for the opening of the BRS. The real airframe VNE is higher. I'll leave it at that.

 

That's interesting.

 

In the SR22, Vpd (maximum demonstrated parachute deployment speed) is 133 kias.

 

Yet Vne is 201kias.

 

At least in that plane, the lower chute speed does not affect Vne.

 

Maybe the CT is different in this respect.

[Ed Cesnalis]

Ed,

 

You made a lot of assumptions about what I was saying...

 

I don't see it, what assumptions did I make?

[FastEddieB]

I don't see it, what assumptions did I make?

 

"Simply put Eddie, you are saying that I am free to violate Vne in terms of TAS yet flutter is a harmonic not a force consideration."

[Ed Cesnalis]

If Vne is in fact TAS then your statement that "Vne is IAS" is not true.

 

I am not assuming it you said it.

 

And that one statement does not add up to " a lot of assumptions"

 

Bottom line is this question: Is Vne and therefore flutter a TAS or a IAS consideration? Given that High altitude designs reduce their Vne with altitude one is forced to assume it is a TAS consideration.

[FastEddieB]

If I have a chance I'll attempt to reeducate myself on the finer points of Vne.

 

My first stop will be Aerodynamics For Naval Aviators.

 

In light aircraft I still believe it is depicted on the ASI as an indicated airspeed. Beyond that, my knowledge of it is limited.

[Ed Cesnalis]

Some aircraft may have a flutter limit in TAS and will thus have a Vne that reduces with altitude.

Is our Vne simply stated due to the general low altitude usage?

 

We do not know the issues behind our Vne so it is only prudent to assume flutter. Flutter issues are complex so it is prudent to assume that things change when TAS exceeds Vne.

[mocfly]

I'm curious which production aircraft have flutter limit published?

 

[Ed Cesnalis]

I'm trying to understand the paste below, seems that TAS is a consideration but flutter limitations are given as IAS adjusted for altitude instead of in TAS, something about aerodynamic damping??

 

in either case my point stands, at altitude if you rely on your ASI for Vne it will be too fast and needs to be converted to TAS or some published value for IAS at your altitude.

 

 

VNE

 

The VNE , or the never exceed speed, of an aircraft is the V speed which refers to the velocity that should never be exceeded due to risk of structural failure, due to calculated factors such as wing or tail deformation or due to aeroelastic 'flutter' (unstable airframe or control oscillation). VNE is specified as a red line on many airspeed indicators. This speed is specific to the aircraft model, and represents the edge of its performance envelope in terms of speed. Well below the speed of sound, the VNE is read as Indicated Air Speed (IAS), since the pitot indication is a direct measure of the dynamic pressure for any given airspeed. At altitude, where TAS is higher than IAS, aerodynamic damping is weaker than at lower levels (damping is proportional to IAS) whereas inertia-induced disturbances are stronger (inertia grows with acceleration, which is the time derivative of TAS). This condition, if continued beyond tested limits, pre-disposes to unstable oscillations or 'flutter'. For instance, the TAS/IAS ratio at 40,000 ft on the ICAO ISA is 2:1, that is, TAS is approximately twice IAS.

http://x.palmspringspilots.com/content/view/118/167/

[mocfly]

Charlie Tango,

 

 

How are airspeed limits, especially Vne, determined?

 

In FAR Part 23 section 23.335, Vd — the 'design diving speed' — is required to be not less than 1.4 times the design cruise speed for a normal category aircraft. To receive type certification, it must be demonstrated, possibly by analytical methods, that at Vd, the propeller, engine, engine mount, and airframe will be free from over-speeding, severe vibration, buffeting, control reversal and most importantly flutter and divergence. To provide some safety margin, Vne (the IAS that should never be exceeded in level flight, descent or manoeuvre) is set at 90% of the lower of Vd or Vdf. Vdf is a diving speed that has been worked up to by a test pilot and demonstrated to be without problem — though without pulling heavy airframe loads — and which must be lower than or equal to Vd. Vd is not required to be demonstrated in an in-flight test.

 

Vne is always specified in the pilot's documentation as an indicated airspeed and should be marked on the ASI (the red line). But unlike the performance airspeeds (also specified as indicated airspeeds and perhaps marked on the ASI), Vne is related to those structural characteristics and limitations associated with bending, twisting and flexibility, and which affect stability, control and even structural integrity. Limiting speeds are also associated with structural reaction to pilot-induced loads and to gust-induced loads. Limiting speeds could also be associated with other potential problems; for example, suction effects at particular speeds and attitudes might lead to canopy departure, or door or cowling security problems. Quite often, Vne is limited by the critical flutter speed.

 

 

Does the indicated speed for Vne stay the same no matter how high you fly?

 

The answer is affirmative because the airspeed indicated (Vis) is a reflection of one particular dynamic pressure (½roVis²) no matter what the aircraft's altitude might be. But there is a qualification in that one of the conditions that limits maximum safe airspeed is the onset of flutter, which is a function of the speed of the airflow, the rigidity of the airframe, the balance of the control surfaces and the condition of control hinges, torque tubes etc, rather than just the dynamic pressure. See 'Aerodynamic reactions to flight at excessive speed' but also read 'You can also induce structural damage at moderate speeds!'

 

For most sport and recreational aviation light aeroplanes only one Vne is specified in the Pilot's Operating Handbook or aircraft flight manual. That value is probably conservative and applicable for operations below 10 000 feet amsl. The designers of most piston-engined GA aircraft specify one fixed-value Vne for operations up to the aircraft's service ceiling; that value is represented by the fixed red line on the ASI. The flight testing program would have determined that the aircraft has no potential flutter problems at Vdf up to the service ceiling. However, a minority of GA aircraft have supplementary lower-value IAS Vne for operations in altitude bands above a stated altitude — perhaps above 10 000 or 15 000 feet. This approach to Vne specification is normal with sailplanes and powered sailplanes whose long, elastic, high aspect ratio wings are likely to develop flutter problems at high true airspeeds.

 

Where the aircraft designer selects a true air speed value as a limiting airspeed applicable when flying above a nominated altitude, FAR Part 23.1545 © requires that "If Vne varies with altitude, there must be means to indicate to the pilot the appropriate limitations throughout the operating altitude range". The 'means' is normally a placard next to the ASI. So, in such circumstances, designers must specify a series of Vne IAS values, corresponding with all possible operating altitude bands. For example, the RA-Aus registered Pipistrel Sinus has the altitude capability of 28 500 feet and can build speed rapidly even in a shallow descent. For their own reasons (most likely associated with the flutter potential of the sailplane-like, high aspect ratio wings) the Pipistrel designers have deemed it wise to limit maximum speed to a particular TAS at all altitudes. The following table reflects the Sinus flight manual and the ASI placard; the maximum true airspeed target is 122 knots. Note that density altitude rather than altimeter-indicated altitude, is specified — which is significant in Australian climatic conditions.

 

Density altitude/IAS for nominal Vne 122 knots IAS/CAS

Density altitude Vne knots IAS

0 122

3300 116

6500 111

10 000 105

13 000 100

16 500 95

19 700 90

23 000 85

26 300 80

 

 

If there is insufficient manufacturer's information available for the aircraft you fly — and you are uncertain about the appropriate Vne for an operating altitude — then multiply the density altitude, in thousands of feet, by a factor of 1.5 to get the percentage decrease to apply to the specified Vne to establish a safe Vne appropriate to the altitude. For example if density altitude is 8000 feet and specified Vne is 100 knots then 8[000] × 1.5 = 12%. Corrected Vne = 88% of 100 = 88 knots IAS/CAS.

 

I like flying my aircraft fast. If I stay below Vne, I won't have to worry about structural failure, right?

 

Vne is assessed at or near MTOW, with the cg at, or within, the fore and aft limits for the aircraft's specified category; it does not apply if weight, manoeuvring loads or cg positions are outside the specified limits. As a maximum airspeed it applies only in reasonably smooth atmospheric conditions for moderate, smoothly applied control movements and symmetrical aerodynamic loads. Even gusts associated with mild turbulence or sudden control surface movements greater than perhaps several degrees travel might lead to some unpleasant surprises, if operating close to but below Vne. Remember that dynamic pressure increases with the square of the true airspeed. At high speed the controls should be quite stiff and are very effective, with a probability of over-control applying extreme loads to the structures. Asymmetrical aerodynamic loads, such as combined rolling and pitching, reduce the maximum allowable airframe load by perhaps 30%. Take care because some aircraft control systems provide inadequate feedback of the load being exerted; i.e. a high load can be applied at high speed with a relatively low stick force, see 'The stick force gradient'.

 

(The effect of gust loads is expanded in the section on wind shear and turbulence.)

 

If an aircraft is operated within its specified manoeuvring and gust envelopes and weight and balance limits — observing the limiting accelerations and control movements, and maintaining airspeeds commensurate with atmospheric conditions — then the only possibilities of inflight structural failure relate to:

improper modification, repair or repainting of the structure

excessive free play in control surface hinges, torque tubes or control circuits

cumulative strain in ageing aircraft, eroding the designed safety margin, remembering that structural fatigue may not have been adequately assessed at the aircraft's design stage

failure to comply with the requirements of airworthiness notices and directives

poor care and maintenance of the airframe.

Flight at airspeeds outside the design manoeuvring flight envelope (or when applying inappropriate control loads in a high-speed descent or, indeed, at any time) is high risk and can lead to airframe failure.

 

Be aware: deliberately exceeding Vne is the realm of the test pilot — who always wears a parachute! The following text is an extract from an RA-Aus accident investigation report illustrating aileron flutter and wing divergence:

"(Witnesses) observed the aircraft in a steep dive at what appeared to be full power. The port wing appeared to detach from the aircraft …

 

That wing had the attach points intact but had pulled the mountings out of the top of the cockpit. This action would have released the door, which landed close to the wing. The wings were intact but the ailerons were detached. There was no delamination of the fibreglass structure. The ailerons were not mass-balanced.

 

The aircraft was a conventional design being a high-wing, monoplane of composite construction. While the fuselage was a proven design the pilot/builder had designed his own wing, including the aerofoil section. The workmanship was excellent and there is no evidence of any lack of structural integrity.

 

The eyewitnesses reported seeing "a sort of shimmying" from the aircraft. It is believed that this 'shimmying' was aileron flutter, which led to the detaching of both ailerons. This same flutter condition would account for the massive forces required to detach the wing from the aircraft in the manner that occurred. Flutter could have been triggered by the wing aerofoil design combined with the manoeuvre the pilot was conducting, or from the aileron control design …

 

The aircraft suffered a massive inflight structural failure almost certainly caused by severe aileron flutter and the aircraft speed in the dive. Any flutter would have been exacerbated by the lack of mass balancing."

 

 

 

 

[Ed Cesnalis]

Charlie Tango,

 

 

How are airspeed limits, especially Vne, determined?

 

 

Right from your post "Quite often, Vne is limited by the critical flutter speed." We don't really know because we are not privy to the documentation that determined Vne.

 

If flutter speed is the reason for the Vne or if flutter speed is close to Vne I think TAS should be used, given that we don't have graduated values for altitude.

[Ian]

Wow - I went to bed with just 4 posts on the topic and woke up to two pages - I love this forum!!

 

For Charlie Tango "OP" which I referred to is terrible forum speak for "Original Post" or "Original Poster"

 

I'm still just wondering about the statement in the original post that "....I had to throttle back to keep the TAS under 125kt ....." - I'm not sure that I would ever be concerned, at the altitudes and speeds at which we fly, with anything other than IAS - now when they bring out the pressurised CTLS and supercharged Rotax all bets are off