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Archer shares detailed overview after its Midnight aircraft undergoes six-week wind tunnel campaign

Like many eVTOL OEMs in this space, Archer is con­tin­u­ing to ramp up its efforts with its own air­craft, Mid­night. The com­pa­ny has announced sig­nif­i­cant mile­stones in recent months and its mis­sion is to launch com­mer­cial oper­a­tions in 2024.

In its lat­est blog on its web­site, Archer’s team has shared more details about a six-week long wind tun­nel test cam­paign car­ried out at the RUAG Large Sub­son­ic Wind Tun­nel in Emmen, Switzer­land.

Below are the thoughts on the cam­paign from Bruno Bachinger — Direc­tor, Flight Sci­ences, Gio­van­ni Droan­di — Man­ag­er, Aero­dy­nam­ic and Mike Ker­ho — Senior Aero­dy­nam­ics Test Engi­neer.

This cam­paign fol­lows on to pre­vi­ous cam­paigns we’ve car­ried out at Politec­ni­co di Milan (pro­peller per­for­mance and pro­peller air­frame inter­ac­tions), Uni­ver­si­ty of Flori­da (acoustic tool­chain val­i­da­tion) and the Uni­ver­si­ty of Notre Dame (wing, tail and pro­peller air­foil per­for­mance val­i­da­tion).

This test at RUAG allowed us to gath­er valu­able data to fur­ther val­i­date the Mid­night vehi­cle con­fig­u­ra­tion, its aero­dy­nam­ic mod­els, exter­nal load pre­dic­tions, air­craft per­for­mance, sta­bil­i­ty and con­trol char­ac­ter­is­tics, and per­for­mance degra­da­tion in icing con­di­tions — keep­ing us on pace for Mid­night’s upcom­ing flight test pro­gram.

RUAG runs one of the largest and most capa­ble wind tun­nels in the world. They also have exten­sive test­ing expe­ri­ence that is trust­ed by the aero­space indus­try glob­al­ly, mak­ing RUAG an ide­al part­ner for this impor­tant cam­paign. 

DeHarde, a Ger­man com­pa­ny known for its high-qual­i­ty, pre­ci­sion wind tun­nel mod­els, designed and fab­ri­cat­ed the 27.6% scale, unpow­ered mod­el of Mid­night for this test cam­paign that fea­tures a wingspan of over 13 feet. This size was cho­sen very intentionally–to max­i­mize test data simil­i­tude and cor­re­la­tion to full-scale, while fit­ting with­in the 5m x 7m test sec­tion. The mod­el was unpow­ered, mean­ing the pro­pellers did not spin dur­ing test­ing, to allow focus on the aero­dy­nam­ics of the air­frame.  

The pur­pose of this test cam­paign was to devel­op a com­pre­hen­sive dataset to val­i­date our engi­neer­ing design tools and to con­firm the aero­dy­nam­ic pre­dic­tions of our Mid­night design. Over­all, 878 mea­sure­ment sweeps were made by vary­ing mod­el angle of attack and sideslip and cap­tur­ing force and moment trends for over 400 dif­fer­ent mod­el con­fig­u­ra­tions.

These mod­el changes includ­ed vary­ing con­trol sur­face deflec­tions, adjust­ing for­ward boom tilt angles, and step­ping through an air­frame com­po­nent build-up. This build-up start­ed with a bare wing/fuselage and then suc­ces­sive­ly added booms, land­ing gear, tail, and stopped lifter pro­pellers. This build-up approach allowed us to inves­ti­gate and clear­ly under­stand the incre­men­tal effects of each air­frame com­po­nent. 

Spe­cif­ic empha­sis was placed on col­lect­ing data to val­i­date our state-of-the-art com­pu­ta­tion­al flu­id dynam­ics (CFD) pre­dic­tions of the over­all Mid­night air­craft per­for­mance in cruise, includ­ing vehi­cle drag by air­frame com­po­nent, high-lift capa­bil­i­ty, and stall speeds.

Of par­tic­u­lar inter­est was explor­ing those con­di­tions where air­flows are unsteady and dif­fi­cult to pre­dict ana­lyt­i­cal­ly with high accu­ra­cy. We col­lect­ed exten­sive data on vehi­cle sta­bil­i­ty and con­trol deriv­a­tives, which relate changes in forces and moments with air­craft states (e.g., angle of attack or sideslip) and con­trol inputs (e.g., con­trol sur­face deflec­tions).

This data val­i­dates the cruise sta­bil­i­ty char­ac­ter­is­tics of Mid­night and is a major input to the design of the airplane’s con­trol log­ic and anchors flight sim­u­la­tion mod­els to accu­rate­ly rep­re­sent the true airplane’s behav­ior. Final­ly, data nec­es­sary to cor­re­late and sub­stan­ti­ate exter­nal loads pre­dic­tions were col­lect­ed and will be used to val­i­date the loads that size the air­craft struc­ture. 

This wind tun­nel test rep­re­sents a sig­nif­i­cant risk-reduc­tion and val­i­da­tion mile­stone for Archer. The results fur­ther rein­force our con­fi­dence in the capa­bil­i­ty and per­for­mance of the Mid­night design. For exam­ple, cruise drag pre­dic­tions trend extreme­ly well with test data, which in turn fur­ther val­i­dates our con­fi­dence in Midnight’s range.

The max­i­mum lift capa­bil­i­ty of the air­craft even came in slight­ly bet­ter than we pre­dict­ed, which assures us that we’ve cor­rect­ly set the stall and approach speeds for the air­plane. The mea­sured sta­bil­i­ty char­ac­ter­is­tics rein­force that the V‑tail, flap­er­ons and rud­der­va­tors are cor­rect­ly sized and that they have ade­quate aero­dy­nam­ic con­trol pow­er, while also con­firm­ing that con­trol sur­face hinge moments meet require­ments and that our actu­a­tors are prop­er­ly sized.

Test­ing for sim­u­lat­ed ice accre­tion has giv­en the team a good under­stand­ing of the speed addi­tives nec­es­sary to pro­tect the air­craft from per­for­mance degra­da­tions that we would expect to see dur­ing an inad­ver­tent icing encounter. 

In sum­ma­ry, this lat­est wind tun­nel test cam­paign was a huge suc­cess, fur­ther val­i­dat­ing our mod­els and pre­dic­tions about the Mid­night vehi­cle con­fig­u­ra­tion and expect­ed per­for­mance. All togeth­er, these learn­ings, and those still being mined from the rich dataset we col­lect­ed, allow us to take a sig­nif­i­cant step for­ward in the devel­op­ment and cer­ti­fi­ca­tion of Mid­night.

The next step is an upcom­ing wind tun­nel test that will inves­ti­gate the effects of the propul­sion sys­tem on aero­dy­nam­ic per­for­mance as we remain on pace for Mid­night’s upcom­ing flight test pro­gram.

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Jason Pritchard

Jason Pritchard is the Editor of eVTOL Insights. He holds a BA from Leicester's De Montfort University and has worked in Journalism and Public Relations for more than a decade. Outside of work, Jason enjoys playing and watching football and golf. He also has a keen interest in Ancient Egypt.

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