# Can thrust vectoring enable use of flaps on pure delta wing?

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#### Doggzilla

##### Well-Known Member
Isn’t this why leading edge slats exist?

I’ve heard from old school pilots that it made a significant improvement on early jet fighters.

#### Norman

##### Well-Known Member
BTW Thrust measurements on the workbench can be misleading. The thrust from a prop is not a constant. When the plane is standing still the prop will be partially stalled then as it picks up speed the prop will unstall and thrust increases then as you accelerate more thrust decreases again as the AoA of the blades decreases.

#### Thomas Marks

##### Member
I think your Y axis is mislabeled. We put a little 27.5 kilowatt engine in the U-2 and it cost over \$3,000
sorry, fixed the graph. The point is that with more power each pound of thrust is cheaper.
My calculation shows, that with just one-third cut of stall speed and Emrax engine the aircraft may become airborne in no time and just 20 meters https://docs.google.com/spreadsheets/d/12-COBaLsQV_yKPX3Hh-85AcpwiwuVfwU5tsN6IaySyM
I've put some reserve for power, and 10% drag loss for flaps.
Am I missing something?

#### Sockmonkey

##### Well-Known Member
If you did want some extra takeoff power and didn't mind the noise, a pulse jet providing auxiliary thrust would do it.

#### Thomas Marks

##### Member
Isn’t this why leading edge slats exist?
This is a must as well.
But using them will not get enough lift gain alone nor compensate for nose down pitch.

#### Topaz

##### Super Moderator
Staff member
Log Member
... Blown lift is interesting because it enables you to raise the vehicle CL beyond what is possible with flaps, while keeping the T/W ratio well below VTOL aircraft. See the Breuget 941 for an interesting example. This could allow you to get the short field performance you're looking for at a higher wing loading (which consequently allows you to cruise faster for a fixed amount of power).
Don't forget climb, though. And a decent gliding ability, so as to find a safe out-landing spot when the noisemaker goes silent.

#### Thomas Marks

##### Member
And a decent gliding ability, so as to find a safe out-landing spot when the noisemaker goes silent.
Having battery on-board and given the cost and weight of additional motor - engine redundancy seems to me a must have feature.
So, I can optimize the airfoil for cruise only.

#### Topaz

##### Super Moderator
Staff member
Log Member
Having battery on-board and given the cost and weight of additional motor - engine redundancy seems to me a must have feature.
So, I can optimize the airfoil for cruise only.
I wouldn't. Adding another motor and the drive-line to tie it into the power supply to the propeller so that it can operate when the main engine has failed is a huge increase in weight for something that, hopefully, will never be used. And your backup engine has its own failure modes, especially if it's never been used and is suddenly needed. That weight gain, given a particular airframe, comes directly off your payload capacity - people, baggage, fuel, whatever. You're carrying around a spare engine instead of the same weight of that stuff.

Whereas the ability to glide even reasonably well uses only components that are there for other reasons (wings, tails), in the way they were intended to be used. This adds no extra weight, and no additional failure modes. Compared to adding a "backup" motor, by gliding you get to have more payload capacity and more reliability, completely for free.

#### Johan Fleischer

##### Active Member
IMHO said:
Virus[/I] land in a short distance than try to get a delta-wing to do so.
Both Dyke Delta and Verhees Delta are surprisingly speedy for their installed engine power. And it looks like they don't need a high AOA to take off or land. They do so almost horizontally.

#### chris__88

##### New Member
Don't forget climb, though. And a decent gliding ability, so as to find a safe out-landing spot when the noisemaker goes silent.
Power-out glide capability is definitely a consideration (in any configuration); but if you're after really short field performance you'd probably have a low enough wing loading to meet part 23 power-out stall speed requirements.

Blown lift should be as good or better in climb than a comparably sized engine w/o distributed propulsion.

But using them will not get enough lift gain alone nor compensate for nose down pitch.
It would be useful to work out exactly what "enough" is for your design. A target CL would let you downselect some of these options more readily.

#### berridos

##### Well-Known Member
Reading his web i have a hard time understanding several issues. Maybe it would be nice to discuss some details that are particularly interesting.
Stability
Unlike conventional aircraft the longitudinal stability has to come from the wing itself.
This is not as difficult as it seems as long as the cord is large enough. The principle is the same as with any aircraft: the centre of gravity has to be ahead of the aerodynamic centre. So there must always be an aerodynamic moment keeping the tail down. When flying too fast due to an inadvertent descent, the aerodynamic forces will win over the mass forces and put the tail down and visa versa.
The aerodynamic centre with zero moment is with every section at ¼ cord. The delta has a tapered wing with sweepback, some of them have multiple sweepbacks. So finding the aerodynamic centre is not so easy. If you take the ¼ cord line and take in account every lift distribution along the span as equivalent of the cord, things can go wrong because the lift distribution is not proportional with the cord. There are parts of the wing that have a higher load than others, this depends on the planform.
Fortunately, because of the wide cord, the centre of gravity is not critical. However it is wise to start with a forward centre of gravity to be sure of positive stability.
Verhees states its difficult to find the aerodynamic centre of the wing. With fences at midspan would we get closer to an elliptical lift distribution?
Why does he talk bout several sweepbacks?

Stall
Due to the sweepback the delta wing tends to have a higher load at half span. The air going to the half span section is sucked upwards by the air that is already flowing over the nose of the centre part. This causes a higher angle of attack here.
In a stall this can be a nusance when the wings stalls first half span. Of course this happens on one side first and the result is a spin.
The Verhees Deltas have counteracted this by making the elevons very wide at this point. When defected up, necessary for a high angle of attack, the wing load will be diminished at this section and a stall is prevented.
This shape of elevon helps also to make the lift distribution more like an ellips, giving a better efficiency.
Dutch roll
The directional stability of a Delta is in fact the same as with conventional aircraft, with the exception that roll stabilty tends to be better as directional stability. This is due to the fact that the aircraft is short and has sweepback.
This can cause dutch roll: when the aircraft is rolled, say to the right it also slides to the right. The relative wind will “see” a longer right wing and the aircraft starts rolling left before the tail is pushed to the left, so the aircraft rolls left and slides right. This movement will repeat at the other side and the aircraft will make a funny oscillation.
In airliners this is compensated with a yaw damper, we have to do it in a natural way. The Verhees Deltas have done it by giving the wing anhedral, so on purpose diminishing the roll stability. The positive effect is also that a windgust will push the aircraft with the wing into the wind instead of the other way like with a conventional aircraft.
Could this duth roll behaviour beeing prevented with a mix of aileron action and airbrakes (the normal ones, not on the belly) like in modern gliders (ASW20 i believe)?

Efficiency
Another pro is that the propwash will give a benefit. With a normal tractor propeller setup the fuselage will fly in faster air than the wings, this causes extra drag. With a delta the faster airflow over the wing causes extra lift and diminishes the induced drag, so it is a benefit.
A delta can be efficient but it not a certainty. The wing area has to be bigger than for a conventional airplane (because you can’t use flaps, on the contrary for more lift you even have to put the elevons up), the span is shorter, giving more induced drag.
The advantage must come from having no fuselage and a lighter structure. Flying wings with fuselage are per definition less efficient than conventional aircraft.
Furthermore, because of the high induced drag, the delta has to be flown fast for good efficiency. The D2 at 100 km/h takes the same power as at 220 km/h. Because it is a fast aircraft a relative big engine is necessary, the big engine and the light weight compensate for the higher induced drag when flying slow for climbing.
Because the aircraft needs relatively much power for flying slow, there is less left for climbing. On a hot day or at high altitude the climb performance is relatively more decreased than with fi. a motorglider that needs little power to fly and has the rest to climb.
Shouldnt sliding weights be used to adjust rearwards the cog in cruise mode? for example sliding the battery backwards?

Centre of gravity
The further the centre of gravity is to the rear the less up elevon is needed to keep the nose up, so the wing will give more lift. The D1 has the same performance with almost empty tank and without luggage than with full tanks and luggage due to the tanks and luggage being after the centre of gravity. Of course, a centre of gravity too far to the rear gives an instable aircraft, although the stability is improved with the D1 and D2 by the fact that the elevons are not balanced: flying too slow and the weight of the elevon will win and push the nose down and flying too fast and the aerodynamic trim will win and the nose goes up, kind of artificial stability instead of those computers big aircraft have and are not allowed for us.
Cant understand from a practical perspective the statement of unbalanced ailerons? Are the ailerons so heavy to have that effect?

#### Thomas Marks

##### Member
leading edge devices add about 0,5 lift. That never would make for 40kmh.
I'm totally fine with all the spec of delta in first column, except the stall speed and take-off / landing roll as a result.
I would be more than happy with landmark (value in red) ultralight category limit of 24 knot, while anything under urban area speed limit should also work (varies by country from 30mph to 60 kmh)
@chris__88, unfortunately I cannot edit anymore the initial post to stack all gathered information in a single place, as here https://aviation.stackexchange.com/questions/71694/can-thrust-vectoring-be-used-to-enable-the-use-of-flaps-on-canardless-tailless-d, but I'll keep updating the reference file https://docs.google.com/spreadsheets/d/1SGK_is-R1lgW5lgIff3CNpK6R52SmUzpb6J944VdoS4

#### Thomas Marks

##### Member
Shouldnt sliding weights be used to adjust rearwards the cog in cruise mode? for example sliding the battery backwards?
This is my ultimate goal avoiding all complication with thrust vectoring.
Cant understand from a practical perspective the statement of unbalanced ailerons? Are the ailerons so heavy to have that effect?
It's not about the ailerons being heavy, at low load (pilot only) the nose-heavy state is balanced with ailerons a little up, eating some lift.
The cabin inside is 2m deep, you could possibly seat third person in the second row. And this extra load behind the pilot wouldn't make D2 tail-heavy because of unbalanced aileron reserve.

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#### Doggzilla

##### Well-Known Member
But using them will not get enough lift gain alone nor compensate for nose down pitch.
They definitely allow a much lower angle of attack, and much better lift at higher angle of attack.

#### pictsidhe

##### Well-Known Member
This is my ultimate goal avoiding all complication with thrust vectoring.

It's not about the ailerons being heavy, at low load (pilot only) the nose-heavy state is balanced with ailerons a little up, eating some lift.
The cabin inside is 2m deep, you could possibly seat third person in the second row. And this extra load behind the pilot wouldn't make D2 tail-heavy because of unbalanced aileron reserve.
You need to keep the cg in certain range, or the aircraft will become unstable. Too far aft, it won't matter if the ailerons have enough authority to counter the aero effect, it will be unflyable by us mere mortals. Cue smoking hole in the ground, unless you add artificial stability.

I strongly recommend that you stick to design practices that are known to produce good results. Without a lot of education and experience, you are extremely unlikely to come up with something that a hundred years of engineers and experimenters have not. You are, however, very likely to repeat some of the errors made on the way to 2019. Looking at some of the dead ends that have been pursued with a lot of effort by very clever people wil give an insight at how hard it will be to break new ground.
Start by looking at what has worked for STOL. You may also notice that modern airliners are built for economy cruise but need a very high CL for landing with the 'small' wings used to optimise cruise. You could do worse by looking at some of the tricks they use to achieve that.

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#### Thomas Marks

##### Member
unless you add artificial stability.
Thanks, that was one of my questions in the first post.
The reason to break a new ground now are new technologies became available.
As @Norman mentioned for example power was expensive. But not any more. 1:1 thrust to weight ratio peak is affordable. Electric engines are more flexible as to were to put them. Fly-by-wire is making its way to GA. And many more.

It looks like both thrust vectoring and weight balancing would need computer input, but what is better?

#### Aesquire

##### Well-Known Member
The idea of moving the center of gravity in flight to improve performance has been around for decades. It's probably easiest to find in the report of the last test flight of a prototype. If you search this site you will find multiple posts warning about the dangers of moving mass in flight, and having it jam in an unsafe rear position.

The technology of moving the CG in flight is most often found in supersonic bombers. ( we aren't counting hang gliders & other weight shift craft here ) In big bombers like the B-1 or B-58, they pump fuel back and forth to ballast tanks. That only occasionally fails, losing plane & crew.

So I generally caution against such schemes, that introduce both otherwise useless weight, and the possibility of unrecoverable loss of control.

Weight is the enemy to STOL performance. It means more induced drag, and more inertia to overcome to accelerate the plane. There is no upside.

Using that mass to use a more powerful engine is better than ballast, by far. ( assuming the plane can safely take more power. ) Better yet is a lighter plane.

As to thrust as a way to balance a plane? Nyet. When, not if, the fan stops, the balance is gone.

And finally,a personal opinion. I see a lot of designs where in pursuit of higher top speed, the designer makes the wings and/or control surfaces "just big enough" for the calculated weight. In the real world projects get heavier, and more drag is present once antenna, brackets holding flaps, etc. are added. Sometimes "well meaning" builders just can't resist the heated seats out of a Lexus and full leather, sound deadening insulation, and end up with stall speeds over 100 knots. A luxurious, but dangerous waste of time and money.

It's better, in my not so humble opinion to put a nice big wing on in the first place, and hope that history will praise you for the foresight that made the "clipped wing Your Name Here" a plane mentioned in sentences like. "Oh, The Your Name Here was a great Bush plane, but did you see that air show pilot with the Clipped Wing version? I want one of those! "

#### pictsidhe

##### Well-Known Member
Thanks, that was one of my questions in the first post.
The reason to break a new ground now are new technologies became available.
As @Norman mentioned for example power was expensive. But not any more. 1:1 thrust to weight ratio peak is affordable. Electric engines are more flexible as to were to put them. Fly-by-wire is making its way to GA. And many more.

It looks like both thrust vectoring and weight balancing would need computer input, but what is better?
Thanks, that was one of my questions in the first post.
The reason to break a new ground now are new technologies became available.
As @Norman mentioned for example power was expensive. But not any more. 1:1 thrust to weight ratio peak is affordable. Electric engines are more flexible as to were to put them. Fly-by-wire is making its way to GA. And many more.

It looks like both thrust vectoring and weight balancing would need computer input, but what is better?
When the big military fly by wire projects have a glitch, the pilot often manages to eject. The plane is always a smoking hole in the ground. Maybe you can do as well as gigabuck funded teams of experienced engineers, but I hope you won't be testing over my house. Perhaps a few in a million people design and build a decent conventional plane. Shooting higher at your first attempt seems a trifle optimistic.