gregW
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Falcon business jet horizontal designs - what are the advantages?

Fri Jun 28, 2019 1:11 am

Not being aeronautically versed, the horizontal stabilizer on recent Falcon's seem different compared to the other business jets with T tails - anyone know of the advantages or why they would be designed midway up the vertical stabilizer?

This seems like it brings more negatives that positives (at least to me)

1) shorter moment arm to Cg - requires a much larger area
2) is there a disadvantage being closer to the wing relative to stall characteristics?
3) is there a disadvantage of being closer to wing for deep stall recovery?
4) the rudder seems smaller
5) lots of structure around the horizontal compared to a T tail

What are the advantages or design benefits?

thanks
 
GalaxyFlyer
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Re: Falcon business jet horizontal designs - what are the advantages?

Fri Jun 28, 2019 1:04 pm

The mid-tail design probably saves weight by using the structure already required for the middle engine and reduced structure to manage the forces of a t-tail. The rudder is small and Vmcg on Falcons is high enough they can’t get all the advantages of their wing for take-off performance.

Don't discount appearance and fashion in the design. Falcon design is very part of their DNA since the original Mystere
 
mxaxai
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Re: Falcon business jet horizontal designs - what are the advantages?

Fri Jun 28, 2019 5:03 pm

T-tails require a strong vertical stabiliser to transfer the loads from the elevator to the fuselage. The mid-tail design saves you some weight there.

Regarding stall / deep stall, the higher up your tail is, the more of a problem you get. Yes, the mid-tail is closer to the wing (i. e. further to the front) but not by much compared to how much lower it is.
 
Max Q
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Re: Falcon business jet horizontal designs - what are the advantages?

Sat Jun 29, 2019 3:46 am

They are beautiful to look at without a doubt
The best contribution to safety is a competent Pilot.


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kitplane01
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Re: Falcon business jet horizontal designs - what are the advantages?

Sat Jun 29, 2019 4:36 am

GalaxyFlyer wrote:
The rudder is small and Vmcg on Falcons is high enough they can’t get all the advantages of their wing for take-off performance.



I think we're talking about the Falcon 7x/8x.

I'd like to learn more about this. Since it's three-engined, asymmetric thrust is relatively less than a twin. If they want to keep the blue-line for the Falcon the same as a twin, they could use a smaller rudder than a twin. And visually, that looks right.

First question ... I understand for Vmcg you cannot use brakes or nosewheel steering to maintain direction, but you do get the benefit of having wheels on the ground. On the ground, they have less asymmetric thrust, a smaller rudder, and the same landing gear as a twin. Why would this produce a worse Vmcg?

Second question .. how come you cannot use brakes or nosewheel steering to compute Vmcg? Especially since the Falcon 7x/8x is fly-by-wire and could activate these things is some very pilot-friendly way.

Third question ... is the blue line for the Falcon 7x/8x relatively high?

-Just trying to learn
 
GalaxyFlyer
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Re: Falcon business jet horizontal designs - what are the advantages?

Sat Jun 29, 2019 2:23 pm

There is no blue line on a Part 25 plane.

It’s not a “worse” Vmcg, it’s just not optimum for its wing. The take-off minimum V1 is restricted by Vmcg where it could lift off. For a counter example, the Global has a Vmcg of 86 KIAS, there’s never a weight light enough where Vmcg is a factor, Vi minimum is always restricted by Vsref. The Challenger 350 has a V1 min of 116knots, 5 knots more than the 300. The V1 speeds are 116 knots until about half fuel and passengers before Vs becomes limiting. IOW, it could probably lift off earlier having a shorter runway requirement.

Vmcg is “flying” limitation; at that speed plus 5% flying is the object, no brakes which would slow the acceleration and no NWS because at those speeds is pretty ineffective—little weight on it.

Vmcg is design specific, not exclusively on number or fitting of engines. A tri and tail-mounted twin are pretty much the same animal. The T-38 was called centerline but had pretty strong yaw with OEI. Have a few petals not open in afterburner and an F-100 would be uncontrollable at rotation.

GF
 
PerVG
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Re: Falcon business jet horizontal designs - what are the advantages?

Sat Jun 29, 2019 8:05 pm

The midway arrangement might be less susceptible to deep stall conditions. That's a possible advantage.
 
Max Q
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Re: Falcon business jet horizontal designs - what are the advantages?

Sat Jun 29, 2019 11:32 pm

Didn’t we use to call this a ‘cruciform tail’ ?
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kitplane01
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Re: Falcon business jet horizontal designs - what are the advantages?

Mon Jul 01, 2019 6:43 am

GalaxyFlyer wrote:
There is no blue line on a Part 25 plane.


My bad. What does a part 25 plane call the minimum speed one can fly with the worst engine out and maintain directional control?

GalaxyFlyer wrote:
It’s not a “worse” Vmcg, it’s just not optimum for its wing. The take-off minimum V1 is restricted by Vmcg where it could lift off.


I don't understand that. I would think the lowest Vmcg would be best. I understand that having excessively large control surfaces is no good, but if you can get a lower Vmcg at no cost you take it. There are zero advantages to a higher Vmcg.

GalaxyFlyer wrote:
Vmcg is design specific, not exclusively on number or fitting of engines. A tri and tail-mounted twin are pretty much the same animal. The T-38 was called centerline but had pretty strong yaw with OEI. Have a few petals not open in afterburner and an F-100 would be uncontrollable at rotation.
GF


Of course it's design specific. But one might image hypothetically changing a plane from 2-engine to 3-engine, and thinking about how that might effect Vmcg. I would think it would lower Vmcg.
 
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zeke
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Re: Falcon business jet horizontal designs - what are the advantages?

Mon Jul 01, 2019 12:15 pm

I think you are looking for Vmca, minimum control speed air, where Vmcg minimum control speed ground
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GalaxyFlyer
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Re: Falcon business jet horizontal designs - what are the advantages?

Mon Jul 01, 2019 1:35 pm

kitplane01 wrote:
GalaxyFlyer wrote:
There is no blue line on a Part 25 plane.


My bad. What does a part 25 plane call the minimum speed one can fly with the worst engine out and maintain directional control?

GalaxyFlyer wrote:
It’s not a “worse” Vmcg, it’s just not optimum for its wing. The take-off minimum V1 is restricted by Vmcg where it could lift off.


I don't understand that. I would think the lowest Vmcg would be best. I understand that having excessively large control surfaces is no good, but if you can get a lower Vmcg at no cost you take it. There are zero advantages to a higher Vmcg.

GalaxyFlyer wrote:
Vmcg is design specific, not exclusively on number or fitting of engines. A tri and tail-mounted twin are pretty much the same animal. The T-38 was called centerline but had pretty strong yaw with OEI. Have a few petals not open in afterburner and an F-100 would be uncontrollable at rotation.
GF


Of course it's design specific. But one might image hypothetically changing a plane from 2-engine to 3-engine, and thinking about how that might effect Vmcg. I would think it would lower Vmcg.


Blue Line is Vyse, best OEI rate of climb. Part 25 has V2 which is closer to best angle of climb. Blue Line is not Vmc in a Part 23 plane.

Lowering Vmcg requires other compromises in fin and rudder which adds weight.

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