The speed increment between Vr and V2 in terms of TAS is proportional to thrust minus drag. Below the flat-rated temperature thrust is constant and V2-Vr is approximately constant in terms of TAS, but reduces with increasing temperature in terms of IAS.
Above the flat-rated temperature thrust reduces with temperature and hence acceleration. V2-Vr then reduces with increasing temperature in terms of TAS as well as in IAS.
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