Control of Vmc

[Wylbur]

I've done some searches and VMC is not found in EAA forums. I would assume that's because no one seems to want to tackle a twin (except for the Twin JAG).

For those of you who have aircraft engineering experience, or multi-engine piston a/c time, I have a question. How does one project what the Vmc would be?

And why is Vmc where it is for a particular a/c? Example: For a Seminole, Vmc is less than Vso. For another plane, Vmc is > 80KIAS and Vr is ~65KIAS (so pilots accelerate to Vsse and then rotate).

If you used pusher props with wing mounted engines, would you even have Vmc or would it be well below Vr?

Do jets have Vmc? I've never heard of such for a jet, just V1, where if you lose an engine by that point, you chop power and stop.

If your engines are mounted on the sides of the fuselage, does that prevent there being Vmc, and does the FAA consider this an inline thrust situation? I would bet that you would still need rudder to control yaw in that case.

Just someone with an inquiring mind trying to figure out a multi- design.

One last question: Does an E-AB have specific G loads to meet for the wings? Could one design around "transport" category (+2/-1 G I think is the requirement)? After all, if you design and build a plane for 6-8 people, would you really want to do 90 degree banks on purpose?

Regards,
Wylbur

[FlyingRon]

Vmc pretty much is an issue of the torque of asymmetrical thrust against the amount of control authority (mostly rudder). I don't think a pusher vs. tractor would probably make much of a difference. Jets with fuselage have less of a problem, but the FAA doesn't consider them centerline thrust as far as the multi rating (really only the 337 and a few of the military jets with through-the-fuselage jets qualify as such).

The FAA gives leeway in the experimental, though it would behoove you to meet the loading requirements of the intended use. Transport is +2.5/-1, Normal category is +3.8/-1.5. There's more than banking that's an issue. Lowering the G limits also greatly lowers maneuvering speed, for example.

One of the better books if you want to learn about things like this is "Aerodynamics of Light Aircraft" by John Lowry.

[martymayes]

How does one project what the Vmc would be?

As a function of thrust on the operating engine(s) vs. directional control. For certificated multiengine airplanes, regs permit use of bank in order to aid controllability when establishing a Vmc value.


And why is Vmc where it is for a particular a/c? Example: For a Seminole, Vmc is less than Vso. For another plane, Vmc is > 80KIAS and Vr is ~65KIAS (so pilots accelerate to Vsse and then rotate).
It usually ends up where it is after the design has been compromised with various tradeoffs, cost, performance, etc.


If you used pusher props with wing mounted engines, would you even have Vmc or would it be well below Vr?

Yes and the actual 1g minimum velocity where directional control can be maintained would depend on what the designer was trying to achieve. You can take a plane with low Vmc like the Seminole and increase it by increasing power, reducing rudder effectiveness (this is commonly done in multiengine flight training by artificially limiting rudder travel so a trainee can safely experience Vmc loss of control), etc. Likewise, you could take a plane with a high Vmc and decrease it by increasing directional control, reducing available power, etc. In both cases, redesign to change Vmc will likely cause another parameter to be compromised.


Do jets have Vmc?

Yes. They even have a Vmcg (Vmc on the ground, lol, no joke).

If your engines are mounted on the sides of the fuselage, does that prevent there being Vmc, and does the FAA consider this an inline thrust situation?

No and no. Vmc is typically not an issue on a jet because even during an engine out situation there is an enormous surplus of power so speed should never decay to the point where controllability is an issue.


Just someone with an inquiring mind trying to figure out a multi- design.

If you want a revolutionary, safe multi-engine design, it would have no Vmc handling/performance issues. One that would require no special skills by the pilot after failure of a powerplant. Oh wait, Rutan already did that. See his revolutionary twin, "Boomerang" or better yet, look at the evolutionary process he followed because basically it began as a Beach Baron.


One last question: Does an E-AB have specific G loads to meet for the wings? Could one design around "transport" category (+2/-1 G I think is the requirement)? After all, if you design and build a plane for 6-8 people, would you really want to do 90 degree banks on purpose?

No design g limits for E-AB. Not sure what airplane is designed for 90* banked turns since that is theoretically impossible. Standard category airplanes are designed for normal maneuvering and banks up to 60*.

[Matt Gonitzke]

No design g limits for E-AB. Not sure what airplane is designed for 90* banked turns since that is theoretically impossible. Standard category airplanes are designed for normal maneuvering and banks up to 60*.

How are 90-degree banks "theoretically impossible"? The fuselage provides the lift in knife-edge flight, i.e. during a 90-degree banked turn, and the g loading will be a function of the aircraft's speed and radius of turn. Nothing impossible about it in theory or practice... ;)


One last question: Does an E-AB have specific G loads to meet for the wings? Could one design around "transport" category (+2/-1 G I think is the requirement)? After all, if you design and build a plane for 6-8 people, would you really want to do 90 degree banks on purpose?

As Ron alludes to, design for the operation category the aircraft is intended for. Transport category aircraft have much, much higher wing loading than a GA airplane; that is one of the reasons why the transport category have a lower limit load factor (+2.5/-1).

[martymayes]

How are 90-degree banks "theoretically impossible"? The fuselage provides the lift in knife-edge flight, i.e. during a 90-degree banked turn, and the g loading will be a function of the aircraft's speed and radius of turn. Nothing impossible about it in theory or practice.

Just begs the question: Why isn't this ever demonstrated?

A conventional steep turn with 90 degrees of bank is in fact "theoretically impossible" because load factor approaches infinity. Last time I checked, not many airplanes can handle that kind of in flight loading. Now, I'm sure in world of ducted thrust and other advancements it may be possible but that is well outside of the context of discussion here.

[tspear]

Martmayes

Two items, the load factor chart you posted assumes the bank is producing a turn. You do not have to turn when you are in a bank, also at this point the relative wind is perpendicular to the earth; so any turn is in reference to a normal pull up in terms of force.

To counter the force of gravity you have really three forces working in concert. First is the fuselage is acting like an airfoil, this produces some lift. Second the rudder is now almost horizontal and also provides some lift. Third is the engine, hard to see in a fighter, but obvious when watching acro. The nose will be pointed slightly up with the thrust from the engines pointing slightly down helping counter the effect of gravity.

This can easily be seen watching an acrobatic performance where the plane flies down the runway in a 90 degree bank.

Tim

[martymayes]

the load factor chart you posted assumes the bank is producing a turn.

Yes, that's what I wanted since I was referencing turning flight.

I think you are agreeing, it is theoretically impossible to fly a constant altitude turn with a bank angle of 90*. ;)

[Wylbur]

Thank you all for your answers. Really, I do have multi time. I have done Vmc demos in a Seminole. I know about the blocking the rudder and yoke.

I am trying to keep the design below 6000# MTOW. At that point, each wing must be designed to handle 13,700# for ~30% safety margin. That was why I asked what I did about Transport....

I know that I am limited to piston power. Given that I want to build a 6-8 seat twin, I figure I am looking at 300-350HP engines.

When you use pushers with the props behind the wing, you do not get a lift differential of some 200+# from the running engine compared to the "dead" engine on the other wing. So that stops that part of the roll force, right? Mounting the engines on the fuselage with a 7-10 degree angle toward the nose (for asymmetrical thrust issues) should cut down of the moment of the running engine vs the non-running engine because of the reduced ARM (Ok, only reduced by ~8").

So if I understand correctly, to lower Vmc, I need to make the rudder larger. But will I not also gain some lateral control by using winglets which should also lower Vmc?

Lastly, I'm not all that concerned about loss of an engine at pattern altitude or even climbing away from the airport. I'm concerned about loss of an engine just after rotation when you either climb or find a soft place to crash. I've seen what happens when you do a Vmc roll right after Vr (thanks to Youtube). So I want to design a plane with Vmc below Vr. [And you determine Vmc while doing all your test flights, so finding this out by having an engine die during a take-off is really not my idea of a good day at the office.]

Theoretically, at Vr using flaps (and of course the gear is down), Vmc should really be at or below Vr? So one should be sure that there is sufficient excess power with gear and flaps that one can climb at, at least, 250FPM at MTOW.

Given that Vmc demo is done with a decaying speed, nose high, full power, gear up , flaps up, and at least 3000 AGL, one can see in a Seminole that with flaps and gear you have a certain amount of stability (with too much drag ;-) but no ability to climb and possibly not be able to maintain altitude!!

Lastly, I've seen the Boomerang, and it is beyond my ability to build.

Regards,
Wylbur

[martymayes]

When you use pushers with the props behind the wing, you do not get a lift differential of some 200+# from the running engine compared to the "dead" engine on the other wing. So that stops that part of the roll force, right?

Have you looked at the Angel Aircraft Model 44?
http://www.angelaircraft.com/index.html

Seems to have good low speed performance and incorporates props behind the wings. But it also has a large vertical stabilizing surface. No doubt your goals can be achieve. Sounds like an interesting project.

[Wylbur]

Yes, I have looked at Angel Aircraft. Not the "prettiest" looking plane. Now I have to find reasonable compression ignition engines....

[WLIU]

Vmc is a factor of two forces, both of which act around the CG. You can best visualize and calculate the forces looking down at the aircraft.

First, the engines at 100% power (sea-level is the worst case) produce a specified thrust. That thrust works through the arm over to the CG. We locate the center of thrust slightly away from the center if the propeller to account for P-factor. This is why for Lyco's and Continental installations that are not counter-rotating, the left engine is critical. The thrust force acting at the end of the arm that originates at the CG creates a torque force around the CG. We can calculate this number.

Second, the vertical stab and rudder create a lift force in the horizontal plane. At full rudder deflection we can calculate this lift force vs airspeed. Its a wing turned sideways so you can figure out its coefficients of lift, drag, etc. You can spend a few hours with a program like XFLR5, OpenFOAM, etc figuring out lift vs airspeed graph for the airfoil shaped by the vertical stab and rudder at full deflection.

The vertical surface generates lift force that works around the CG just like the engine thrust does. So you can convert the graph of horizontal lift vs speed to a graph of the torque provided by the force of the vertical surface acting on the arm over to the CG.

Once you have this info, you just take the torque around the CG that you calculated the engine creates, and move across the graph you just plotted for the vertical stab/rudder until you find the same value. From that point go to the airspeed axis of that graph and you have your calculated Vmc.

Flight test will confirm your calculation.

The above is the Cliff Notes version of the calculation. There are more steps that add to the accuracy/correctness of the calculated result.

Best of luck,

Wes

[martymayes]

Yes, I have looked at Angel Aircraft. Not the "prettiest" looking plane.

I bet one of those paint jobs with "swoopy lines" would change your mind......

[Matt Gonitzke]

Airplane Performance, Stability, and Control by Perkins and Hage should have the required calculations. It has been out of print for awhile, but used copies can be found. Anyone designing an airplane should have one.

[Ron Blum]

There are many Vmc numbers. Yes, an airplane that stalls before you lose control would be good. Vmcg is used when the airplane is on the ground (the yaw pivot axis is at the main wheels). Vmca is used when airborne (and may be different for each flap/gear configuration). Reading the regulations, available free online at www.faa.gov (http://www.faa.gov), (14CFR23, subpart B, Flight) will tell you the relationship between all the takeoff speeds. Be safe and remember to start on the fast side ... especially with Vmcg.

[Wylbur]

Sorry for taking a while to get back to this. I've been a bit busy... Until I can find compression ignition engines that meet my design specs, this project is on hold.

[Frank Giger]

So long as you're kicking around the ideas and looking for what fits and what doesn't, the project isn't on hold.

It's just in pre-planning stages.

:)

[Byron J. Covey]

[Q
UOTE=FlyingRon;40910]The FAA gives leeway in the experimental, though it would behoove you to meet the loading requirements of the intended use. Transport is +2.5/-1, Normal category is +3.8/-1.5. There's more than banking that's an issue. Lowering the G limits also greatly lowers maneuvering speed, for example.[/QUOTE]

Both Transport and Normal category aircraft have been tested to the load factors. They also have consistent quality control to ensure that they actually achieve the intended load capability. That may or may not exist in homebuilts - especially composite structure homebuilts.

[CDS]

I've done some searches and VMC is not found in EAA forums. I would assume that's because no one seems to want to tackle a twin (except for the Twin JAG).

For those of you who have aircraft engineering experience, or multi-engine piston a/c time, I have a question. How does one project what the Vmc would be?

And why is Vmc where it is for a particular a/c? Example: For a Seminole, Vmc is less than Vso. For another plane, Vmc is > 80KIAS and Vr is ~65KIAS (so pilots accelerate to Vsse and then rotate).

If you used pusher props with wing mounted engines, would you even have Vmc or would it be well below Vr?

Do jets have Vmc? I've never heard of such for a jet, just V1, where if you lose an engine by that point, you chop power and stop.

If your engines are mounted on the sides of the fuselage, does that prevent there being Vmc, and does the FAA consider this an inline thrust situation? I would bet that you would still need rudder to control yaw in that case.

Just someone with an inquiring mind trying to figure out a multi- design.

One last question: Does an E-AB have specific G loads to meet for the wings? Could one design around "transport" category (+2/-1 G I think is the requirement)? After all, if you design and build a plane for 6-8 people, would you really want to do 90 degree banks on purpose?

Regards,
Wylbur

Wylbur - based on your posts in this thread, it occurred to me that a "design study" might be helpful to you. That is, compare existing aircraft that are comparable to your requirements and you'll find a range of solutions; wing area, aspect ration, power, power loading, empty and max weights, tail volume (square footage of the vertical stabilizer and rudder multiplied by the distance from the aft edge of the C.G. range). Again, based on your posts, it seems that your thoughts are akin to the Navajo series and Cessna 402s.

Once you've digested that information, when you design your own twin, if you come up with answers that are significantly different than the design study aircraft, you'd best rethink those answers. That is, there's a reason existing aircraft are set up the way they are (and in the end, you might discover that it's cheaper to buy a twin than to build one - some twins are relatively cheap right now (who wants to pay the fuel and insurance?)).