Do You Know What Vmc Is?

[time builder]

The E90 in our hangar does show a 7-knot increase in Vmc with the prop windmilling...that's why autofeather has to be installed and operating per modification supplement. This 7-knot increase, by the way, increases Vmc ABOVE the "decision speed" AND rotation speed that our performance charts are predicated on.

David

7 Kts is all? The BE99 I flew had a Vmc of 91, but without autofeather, they said you should plan on more like 130 for Vmc, although I never saw it in writing.

At low altitude (say below 10,000), I don't know that you're going to have a significant difference in your normal Vmc with a turboprop. Remember that one of the key factors in Vmc is engine power output. Unless you're temp limited or something you should be able to get full takeoff power out of your turboprop at significantly higher altitudes than a normally aspriated recip.

With normally aspirated engines, however, you lose manifold pressure with altitude right from the beginning, and thus, your drop in Vmc with altitude due to the loss of engine power.

 

[Alchemy]

On RJ's and most other jets with fuselage mounted engines, Vmc is so far below stall speed that it is never really published. There is no pupblished "VMC speed" for the WSCoD I fly. I asked about this during systems class and that was the answer I was given.

Even on jets with wing mounted engines, I believe the rudders are large enough and the stall speeds high enough that Vmc is not a relevant speed for single engine performance. Not having the P-factor, accelerated slipstream, spirlaing slipstream, and torque issues also helps push Vmc down to a fairly low airspeed.

Now for a 707 or a DC8, I'd imagine that Vmc might be a factor if you lose both engines on one side....can't say for certain though.

 

[MauleSkinner]

On RJ's and most other jets with fuselage mounted engines, Vmc is so far below stall speed that it is never really published. There is no pupblished "VMC speed" for the WSCoD I fly. I asked about this during systems class and that was the answer I was given.

All the jets I've flown with fuselage mounted engines have published Vmc speeds...a couple of them have rudder boost or rudder bias to reduce Vmc, and it needs to be operative as well.

Fly safe!

David

 

[Alchemy]

All the jets I've flown with fuselage mounted engines have published Vmc speeds...a couple of them have rudder boost or rudder bias to reduce Vmc, and it needs to be operative as well.

Fly safe!

David

Interesting, do you recall what the approximate airspeeds were? I assumed it would be 100 knots or less for most types with fuselage mounted engines, which is probably below stall speed for everything except a few citations.

 

[wmuflyguy]

I have been told Vmcg on the RJ is 90kias, I have not come across what Vmca is.

We are jsut told do not fly slower than min-speed appropriate for the aircraft configuration (They are on a plastic card in the cockpit)

 

[MauleSkinner]

Interesting, do you recall what the approximate airspeeds were? I assumed it would be 100 knots or less for most types with fuselage mounted engines, which is probably below stall speed for everything except a few citations.

The only info I've got handy is for the Hawker 800...Vmca is pusblished at 115, Vmcg at112. Vref at nearly max landing weight (lower than MGW, so Vmc should theoretically increase) is 130, an additional 15 knots for 0 flaps makes 145, which should be approximately 1.3 Vs1, putting Vs1 somewhere around 112.

But the plot thickens...at lighter weights, our flap 15 takeoff (same Vmc numbers as flap 0, according to the manual) V1/Vr increase at colder temperatures...from 116 at the high-temp to as high as 122 at ISA. My guess is they're pushed up by Vmc issues, unless you can see another reason to increase runway requirements at lower temperatures.

Fly safe!

David

 

[kevdog]

The only info I've got handy is for the Hawker 800...Vmca is pusblished at 115, Vmcg at112. Vref at nearly max landing weight (lower than MGW, so Vmc should theoretically increase) is 130, an additional 15 knots for 0 flaps makes 145, which should be approximately 1.3 Vs1, putting Vs1 somewhere around 112.

But the plot thickens...at lighter weights, our flap 15 takeoff (same Vmc numbers as flap 0, according to the manual) V1/Vr increase at colder temperatures...from 116 at the high-temp to as high as 122 at ISA. My guess is they're pushed up by Vmc issues, unless you can see another reason to increase runway requirements at lower temperatures.

Fly safe!

David

Then you get into issues when you upper or lower rudder is inop. Changes Vmca.

 

[tomgoodman]

Speed is Life

Now for a 707 or a DC8, I'd imagine that Vmc might be a factor if you lose both engines on one side....can't say for certain though.

Very much so. Here's data from a very old Air Force C-135 manual:

(Sea level, 15 degrees C, two engines out on the same side, other two at full power, full rudder hydraulic power available) --- 124 kias

(Same, but only one engine out, rudder power inoperative) --- 155 kias

Vmca speeds decrease with higher temperatures and pressure altitudes.

For directional control purposes, losing rudder authority is worse than losing a second engine, as you can see. They could only estimate the Vmca for two engines out AND loss of full rudder hydraulic power --- 200 kias.

 

[Thresh]

I'd rather screw my way across the country, then blow my way.

(props vs jets)

 

[MauleSkinner]

Then you get into issues when you upper or lower rudder is inop. Changes Vmca.

Good thing I've only got one rudder

 

[siucavflight]

I'd rather screw my way across the country, then blow my way.

(props vs jets)

Is that your pic in your avatar?

 

[Midnight Flyer]

Yes, I know what VMC is. Visual meteorological conditions.

 

[siucavflight]

Thresh is going the way of iflyvfrifr

 

[Flyerjosh]

I'd rather screw my way across the country, then blow my way.

(props vs jets)

Have it how you like it... but just remember that I'll be drinking beers and sitting on the beaches hours (or even days) ahead of you...

 

[Flyerjosh]

Vmca (Minimum controllable airspeed) in the Citation is 86 KIAS. Since I don't really plan on flying below about 100 KIAS (lightest Vref is 91 KIAS and lightest rotate speed is 87KIAS for a sea level 50C OAT takeoff), I don't worry about it.

In the event of an engine failure, we pitch initially to V2 up to 400' AGL and then clean up and climb out at Venr which is the factor established single engine enroute climb speed (160 KIAS).